Legionnaires' Disease: causes, effects, and a lot of questions

For the past several months New York City news outlets have closely covered the outbreak of Legionnaires' disease in the South Bronx.  This outbreak, though claiming few lives, has sickened a large number of people, and the causative agent of this disease has been detected in the cooling systems of apartment complexes, hospitals, a mall, and even a high school.   This is, in effect, a disease of summer.  The bacteria that causes Legionnaires' live in warm aquatic reservoirs (like industrial cooling towers and hot tubs), and are dispersed by aerosolization.  In other words – those a/c vents to which you want to cling during a heatwave might actually be blowing a cool mist of disease-causing bacteria in your face.   While not directly related to chemical ecology, we figured discussion of this disease – its causative agent, environmental requirements, risk-factors and current treatments – is certainly topical – and hopefully even thought-provoking.

The history of Legionnaires' disease is actually quite interesting.  It was first discovered after the outbreak of a mysterious pneumonia-like illness at a meeting of the American Legion in Philadelphia in 1976.  At first this outbreak did not seem notable - as the symptoms resembled those of pneumonia or influenza – diseases at which it seemed obvious to point a finger.  This disease, however, did not respond to the frequently prescribed beta-lactam antibiotics (like penicillin) – and ultimately 29 of the 182 patients at the American Legion Meeting died.   It took several months for scientists to determine that the causative agent of this disease was a genus of bacteria called Legionella (named after the conference that it terrorized).  In fact, it turned out that this bacteria had even been isolated before but it had just never been characterized or studied, and was certainly not suspected of being a human threat.

While new bacterial species are likely discovered daily, it’s not that often that we come across a completely unknown pathogenic agent that can cause such mayhem.  So maybe it’s worth it to think, just for a second, about how terrifying it would be to combat an organism about which absolutely nothing is known.    Determining effective treatments for infectious diseases generally requires the ability to grow the organism in a lab - but this process can take months of optimization to establish appropriate growth conditions (on a related side-note, the causative agent of leprosy – Mycobacterium leprae – has never been successfully cultivated in a lab).   Additionally, in order to deal with an outbreak, the source of the infectious agent must be identified.   So how do you secure a treatment when you can’t even grow the organism in the lab?  and how do you prevent further infections if you don’t even know the source?  These were obviously pressing questions for the CDC in 1976, and luckily it was not too long before researchers were able to culture Legionella, figure out which antibiotics were somewhat effective in fighting Legionella infections, and even determine that it was aerosolization of bacteria, rather than human-human contact,  that mediated the rapid spread of the disease. 

Legionella pneumophila, which is probably the best-studied species of this bacterial family, infects animal macrophages – immune cells that basically hunt and degrade foreign agents in the body.  Legionella, however, like Mycobacteria (the causative agent of tuberculosis and leprosy), manage to evade degradation by these cells, and even replicate and thrive within them.   Infections of this nature can be difficult to treat because not only do these bacteria successfully evade their host immune system, they can utilize the host cell as a sort of secondary armor making them less susceptible (or accessible) to antibiotic treatment.  Fascinatingly, Legionella are know to infect other types of cells besides animal cells and are believed to often reside and replicate within amoeba – a eukaryotic single-celled organism that are often abundant in the same types of aqueous environments as Legionella.  This observation about the environmental lifestyle of Legionella may help explain why this bacteria has adapted to colonize human macrophages – which in many ways are highly similar to amoebal cells – and also indicates a method by which this organism can evade water treatments by sequestering themselves within the cell body of the much heartier amoeba.  

In their non-human habitat, Legionella have also been known to form expansive biofilms, which are large aggregations of bacterial cells that are stuck together in a gooey matrix.  In short, biofilms are a real menace; they can form on many surfaces – including in the lung (cystic fibrosis) or on teeth (plaque), on the hulls of ships (apparently this slows the ship down?!?!), on hospital equipment (bad for obvious reasons) and a whole slew of other stuff.   Biofilm formation is yet another mechanism by which bacteria can become more resistant to antibiotics and the ability to break up or penetrate biofilms is an important focus in antibiotic research.  The propensity of Legionella to form these structures has made treatment of these infections more difficult and has likely made them much less susceptible to standard treatments (chlorine etc.) of our water supply. 

Outbreaks of Legionnaires' disease are unlikely to go away.  While Legionella are present in most fresh water sources, they are usually low in number and thus do not pose a threat.  Reservoirs such as hot tubs, humidifiers, cooling towers, those mist-sprayers they have over the vegetables at the grocery store, etc., present these bacteria with environmental factors that increase their growth and dissemination:  warm temperatures and aerosolization.  This sort of terrifying fact implies that urban infrastructure is actually selecting for the rapid growth and efficient dissemination of this bacteria.  That being said - it seems sort of strange that Legionnaires' outbreaks seem particularly common this summer seeing as air conditioners, hot tubs, cooling towers etc. have been around for quite a while.  Is it merely increased media coverage?  Or perhaps many previous outbreaks have been attributed to influenza or pneumonia?  The low mortality rate and unremarkable symptoms make this an infection that is probably commonly over-looked or not even reported.  

Luckily Legionnaires' is a treatable disease and a low percentage of infected people are likely to die from an infection.  However, while many strides have been made in understanding Legionella infections since the Legionnaires' outbreak in the late 1970s, there are certainly weaknesses in how we diagnose and deal with this disease.  As with almost all bacterial diseases, antibiotic resistance has presented a problem in the treatment of Legionella infections, so we must continue to be vigilant in discovering new ways to kill these bacteria.  Additionally, currently-used methods for detecting Legionella in a patient or in a water source lack either efficiency or sensitivity.  The gold standard of detection requires cultivation of the bacteria, which can be quite slow, whereas DNA-amplification-based methods, which have the potential to be both sensitive and efficient are not currently in use in the United States.  The development of a cheap diagnostic for detecting Legionella in the environment could be a critical advance  in preventing future outbreaks.  

The recent Legionnaires' outbreak in New York City and subsequent outbreaks in San Quentin Prison and western Illinois, have certainly captured the attention of the public and hopefully has highlighted the need for improvement in how to manage and prevent these outbreaks in major cities.   For obvious reasons, further identifying the underlying biological processes mediating Legionella infections in humans will improve treatments for this disease in patients.  However, understanding the ecological behavior of this organism (like biofilm formation or amoebal infection) could unveil new drug targets or diagnostic approaches that would minimize dissemination of this resourceful (and summer-ruining) bacterium.


Welcome to the plastisphere

We are a one-time-use kind of world. I often see labmates grab a plastic plate in our lab’s kitchen rather than a “real” plate because, well, who has time to wash a dish? In our “grab and go” state of mind we legitimize our rampant use of plastic with the palliative thought that we’ll recycle these items and so, it’s like we never even used them, right?

The worldwide use of plastic is increasing, and today about 78 pounds of plastic are produced per person per year to meet an insatiable demand for some of the most annoying things on the planet, like individually wrapped bananas, clam-shell packaging, and the very worst of them all, microbead-containing cosmetics (more on these below). While some of these plastic items make it to the recycling plant –perhaps more aptly named the down-cycling plant when it comes to plastics– there is a percentage of all plastics that simply get washed away from landfills or beaches and into the wide-open sea. Due to the sheer amount of it and its impressive longevity, plastic is now the dominant debris in the ocean. This debris largely accumulates in five gyres, or areas of circulating water pushed together by larger ocean currents. The Great Pacific Garbage Patch, the gyre off of the coast of the northwest US, is the size of Texas. A study from late 2014 makes an estimate from extensive sampling and modeling that there are 5.25 trillion plastic particles that cumulatively weigh about 270 thousand tons floating in the ocean. That’s a lot of plastic.

Various ideas have been put forth to deal with this garbage. One plan, which may soon be deployed off the coast of Japan, involves the use of giant net to gather the debris floating on the water, followed by centrifugation of this water to pellet the plastics, and finally disposal and recycling of this material back on land. However, there’s one major catch (ha!) that this proposal and others like it fail to address: 92% of all marine plastic trash is less than 5 mm in size, which is way smaller than the plastic trash we toss away. These so-called microplastics come from various sources, including the tiny pellets used as raw materials in plastic manufacturing as well as physical fragmentation of larger pieces of trash. Oh, and the majorly-evil microbeads in exfoliating facewash or sparkly toothpaste or a whole cadre of gimmicky products (see this list here), which aren’t caught in sewage systems and are released into the environment to float away and persist forever. Bye-bye, see you in a million years.

The most interesting consequence (at least in the eyes of yours truly at Chemical Intuition) is the ecosystem that has sprung up in and around the ocean’s microplastics. This ecosystem has been coined the plastisphere, and we like that name a lot.

 So, what’s shakin’ in the plastisphere?

First, there is the obvious consumption of microplastics by animals. Birds ingest plastics both accidentally and because some plastic items look like their food. Smaller organisms like zooplankton passively ingest microplastics from filtering seawater (see the image below). While plastics are essentially inert, and plastic ingestion may not be so harmful in and of itself, plastic is a chemical magnet for toxins. Many pollutants, like polychlorinated biphenyls (PCBs), are hydrophobic. Plastic is also hydrophobic and so these chemicals stick to plastic much more readily than they are soluble in water. A recent report found that plastics concentrate PCBs from the surrounding seawater by up to six orders of magnitude. This means that organisms ingesting microplastics may have much higher exposures to toxins than we might think given those compounds’ concentrations in the seawater. This higher exposure could lead to various deleterious consequences for the organisms, like endocrine disruption or other sublethal effects, which can disrupt ecosystem functioning.

Images from a recent  study  examining ingestion of microplastics by zooplankton. The plastic pieces are visualized using fluorescence microscropy in iii) a bivalve larvae and iv) a Brachyuran larvae. Reprinted with permission from Cole,  et al. Environ. Sci. Technol. , 2013,  47  (12), pp 6646–66. Copyright (2013) American Chemical Society

Images from a recent study examining ingestion of microplastics by zooplankton. The plastic pieces are visualized using fluorescence microscropy in iii) a bivalve larvae and iv) a Brachyuran larvae. Reprinted with permission from Cole, et al. Environ. Sci. Technol., 2013, 47 (12), pp 6646–66. Copyright (2013) American Chemical Society

The second big thing happening in the plastisphere is the microbial ecosystem supported by plastics. Microplastic is an excellent substrate for microbial growth: its high surface area and hydrophobicity promote the formation of bacterial biofilms. A paper from 2013 from labs at the Woods Hole Oceanographic institute (WHOI) focuses on the bacterial communities present on marine microplastic. The authors hypothesized that due to the distinct physical and chemical environment presented by the microplastics, that these debris could harbor a microbial community distinct from that in the surrounding ocean water. They examined pieces of polyethylene and polypropylene plastic from several areas in the North Atlantic Subtropical Gyre. Using DNA sequencing and scanning electron-microscopy (SEM) techniques, they compared the bacterial communities present on the collected plastic bits to the surrounding seawater. From these genetic and phenotypic data, the authors could begin to construct the ecological relationships and roles of the plastisphere microbial community, which included both microbe-microbe and microbe-plastic interactions. They discovered that the microbes on both types of plastic were distinct from those in the water; interestingly, the plastic widely differed from each other, too. The SEM images in the paper show various eukaryotic microbes, like diatoms and ciliates, coexisting with bacteria. The image below shows an ectosymbiotic relationship, in which the symbiont lives on the surface of its host. Here, rod-shaped bacteria completely cover a protozoan that attaches itself to surfaces, i.e. a suctorian (the unofficial name of the lowest ranked student at graduation). 

This scanning electron microscopy image from the  2013 study out of WHOI shows a stalked predatory suctorian ciliate covered with symbiotic bacteria (magnified in the inset). Diatoms, bacteria and other microbes are seen in the background of this image. Reprinted with permission from Zettler,  et al. Environ. Sci. Technol. , 2013,  47 ( 13), pp 7137–7146. Copyright (2013) American Chemical Society

This scanning electron microscopy image from the  2013 study out of WHOI shows a stalked predatory suctorian ciliate covered with symbiotic bacteria (magnified in the inset). Diatoms, bacteria and other microbes are seen in the background of this image. Reprinted with permission from Zettler, et al. Environ. Sci. Technol., 2013, 47 (13), pp 7137–7146. Copyright (2013) American Chemical Society

When I see these images of microplastics teeming with microbes all I can think of is that amazing moment in Jurassic Park when Jeff Goldblum's character half-mumbles the prophetic words, "Life, uh, finds a way." Even though we may dislike the fact that our oceans are polluted, our trash is now the happy home of microbial communities. If we dismiss these communities as uninteresting because they're found on unnatural substrates, we may miss opportunities to understand how human activity impacts other organisms and ecosystems. For instance, can we find microbes on these plastics that could serve as effective bioremediation agents? Do certain types of plastic degrade faster than others due to microbial catabolism? Are the chemicals found in plastics mediating any ecological interactions happening on the plastic? Do marine microplastics harbor any novel symbioses unique to that synthetic surface? 

Finally, we tend to prioritize the conservation and study of ecosystems that we perceive as pure, or "natural."  What does "natural" mean on our tiny planet that hosts more than 7 billion people? Can we truly find any ecosystem free from some mark of humankind? If we effectively remove the trillions of pieces of microplastics from the ocean in a dramatic gesture to clean up our trash, what remaining ecosystems will be affected by the sudden loss of the plastisphere? In a thousand years, will scientists regard the ecology of the plastisphere as compelling an ecosystem to conserve as, say, that which is built around the wolves of Yellowstone?  We're all for avoiding pollution, but we should be wary of practicing ecological relativism.




Relevant or not, here it comes

Interesting and exciting papers related to chemical ecology are getting churned out week after week.  In this semi-regular segment ("relevant or not") we will concisely summarize a small selection of interesting (relevant) studies that have come out in the last couple weeks.   

   Images not drawn to scale


Images not drawn to scale

Sigillin A is a newly discovered natural product produced by the snow flea (Certaophysella sigillata).  This little guy is a winter-active form of springtail – flightless fleas that live in the soil.  Schmidt et al. show that snow fleas produce a chemical repellant (sigillin A, shown above) that deters spiders, ants, millipedes, mites, and other common snow flea predators that are abundant in the soil.  This novel molecule is interesting from a chemical perspective because it contains multiple chlorine atoms - an unusual feature for a terrestrially produced natural product.  Next steps for this study will undoubtedly involve determining how this molecule is synthesized by the snow flea (or more likely by a symbiotic bacteria residing on the snow flea).

This week in Cell, Liu et al. published a study on the success of a previously discovered natural product, Celastrol, in the treatment of obesity in mice.  Celastrol is produced by a bad-ass plant called thunder god vine (Tripterygium wilfordii), which is used in traditional Chinese medicine for treatment of basically all diseases, syndromes, and afflictions (well, almost). In this study, researchers showed that treatment with celastrol re-sensitized diet-induced obese mice to leptin – a hormone that basically tells your central nervous system when you are satiated.   Much like insulin-resistance in type 2 diabetes, certain cases of obesity can be caused by (or exacerbated by) resistance to leptin;  finding a way to re-sensitize the body to this hormone could be a viable weight-loss option.  While celastrol's mechanism of action is still unclear, a number of biological activities have been ascribed to this natural product (anti-inflammatory, antioxidant) as well as to the producing plant (anti-cancer, contraceptive).  It's sort of interesting to contemplate how this newly exposed bioactivity of celastrol could relate to its natural function.


Two papers were recently published on the role of the mammalian hormone oxytocin and its effects on inter and intra-species bonding.  Oxytocin is a peptide consisting of 9 amino acids linked together.  It has long been known to be an important neuro-signaling molecule that plays a significant role in social behaviors  - most notably pair-bonding (aka love and monogamy and all that). 

Marlin et al. demonstrate the role of oxytocin in mediating maternal behaviors in mice by exploring the relationship between oxytocin and the tuning of a mother’s auditory perception of her pup’s distress call.  In a study too complex to sum up adequately, researchers demonstrate that oxytocin signaling may facilitate learning in new mothers.  Read a nice summary here.

Nagasawa et al. explore how oxytocin signaling may indicate co-evolution in inter-species interactions through bonding between humans and their pets.  Experiments conducted in this paper literally involve dog and human gazing into each other's eyes for prolonged periods of time.  Ultimately these researchers demonstrate that oxytocin mediates bonding between a pet and its owner much in the same way that it mediates bonding between individuals of the same species. Nagasawa et al. also explore whether the role of oxytocin in dog-human bonding is reflected in wolves, providing some food for thought on the evolutionary implications of domestication.



Smells like hyena spirit


Have you ever found yourself walking through the African savannah and wondering what the odorous light brown goop found on various strands of grass is? Me neither, but that’s probably just because I’ve never been in the African savannah, let alone traveled to sub-Saharan Africa. If I had had that opportunity, however, I may have observed (or smelled) this rather-nasty-sounding substance, referred to by scientists as “paste”. This paste, quite different from the tooth, adhesive, and caviar varieties, is the foul-smelling calling-card of the gregarious, savannah-dwelling hyenas. Excreted from a specialized scent gland under hyenas’ tails and rubbed onto grass stalks throughout the hyena’s habitat, paste is a vehicle for chemical communication between one hyena and another. This behavior (referred to as “pasting”) is a form of scent marking, a chemical signaling behavior widely used by mammalian species to establish territory or communicate complex information such as identity, sex, reproductive status, social status, kinship, and/or group membership. Each individual hyena’s paste has a unique chemical signature, a complex mixture of volatile fatty acids, esters, hydrocarbons, alcohols, and/or aldehydes, whose specific chemical composition and varying concentrations of these components can communicate identity. Indeed, the individual chemicals present and concentrations in wild hyena paste have been shown to vary with hyena identity, group membership, sex, and reproductive state. Although the paste overall exudes a fermenting mulch-like smell (to us, at least), the slight variations in the individual odors appear to have differential effects on hyena behavior, possibly influencing the complex social structure of these species.

This convoluted chemical signature of identity, as well as the apparent ability of hyenas to interpret and differentiate the presence and concentration of certain chemicals, is in itself an amazing chemical ecology story. In my opinion, however, this chemical communication narrative has become even more fascinating as scientists have begun to investigate the molecular source of the pungent “chemical bouquets” found in hyena paste. Like many mammalian specialized scent glands, the glands found underneath the hyenas’ tails are warm, moist, and nutrient-rich- basically, ideal environments for prolific microbial growth. Although it historically has been difficult to identity and characterize the symbiotic microbial communities that reside in mammalian scent glands (the vast majority of microbes found on the planet cannot be cultured in typical lab settings!), scientists have long suspected that these bacterial residents may play a role in generating the social odors of their host. This hypothesis, formally called the “fermentation hypothesis,” posits that as the microbial symbionts metabolize the nutrient-rich substrates found in the scent glands, they generate odorous metabolites that can be hijacked by a mammalian host for communication. Variations in the microbial community found in hyena scent glands (i.e. the presence and abundance of certain species) may thus correlate with variations in metabolite production, creating a unique paste scent for the host. If this hypothesis is true, when hyenas are spreading their paste throughout the savannah, they are actually depositing part of a unique scent-gland microbial community and relying on these microscopic organisms and their metabolism to communicate their identity.

Recently, this hypothesis was explored for the first time in wild hyena populations by a biologist at Michigan State, Kevin Theis. Prof. Theis and co-workers traveled to Kenya’s Masai Mara National Reserve to track different spotted and striped hyena populations and occasionally dart and tranquilize individuals from these communities. While the hyenas were peacefully sleeping, the scientists used a scapel to remove the paste from their posterior scent glands. They then flash froze the paste samples and sent them back to the US for analysis. Taking advantage of modern DNA sequencing technology that can be used to identify bacterial strains by the sequence of a species-specific characteristic gene (termed 16s sequencing), Theis and co-workers were able to identity the presence and abundance of certain bacterial strains in each individual hyena’s paste. Additionally, they were able to use standard analytical methods to characterize and quantify the chemicals odors produced in each individual paste. Their results, published in PNAS, indicated that microbial communities present in paste are dominated by fermentative anaerobic species (such as Clostridial species) and that the bacterial communities, as well as the odors produced, differed between the two species of hyenas, as well as signaled the identity of the hyena as a female or male, and if the females were pregnant or lactating.

Although the data collected by Theis and co-workers strongly suggested that hyenas use bacteria to mediate social scent-based communication, it does not provide conclusive proof and many questions inevitably remain. What genetic or biochemical links can be established between certain microbial species present in the paste and the odors produced? Can hyena communication systems be disrupted by antibiotics? What correlations exist between these microbes and metabolites and hyena behavior? Do specific microbes or the molecules they produce trigger specific behaviors?  

Perhaps the most intriguing question, however, is: how can these bacterial communities and the metabolites they produce actually communicate any information about the hyena they inhabit? It may not be as far-fetched as it may seem. Many factors are known to play a role in establishing and modifying bacterial communities in an animal host- including genotype, physiology, diet, social relationships, and the environment. The microbes that live on and within a young mammal are typically derived from the mother’s birth canal, as well as its environmental surroundings. One could thus imagine a correlation between microbial communities and familial relations, as well as the sharing of bacteria between hyenas that live together as a clan. Additionally, as the host ages, changes in physiology could also influence the structure of its microbial community- for example, the differences in physiology and hormonal levels between a pregnant and a non-pregnant female may change the bacterial environment, favoring the growth of one type of bacteria over another. Together through these sorts of correlations/hypotheses, one may begin to appreciate how bacterial community structure may be indeed be predictive of host identity, and that perhaps its not surprising that bacterial-derived odors may communicate this type of information.

Unfortunately, further exploration of the role bacteria play in hyena scent communication may be limited, considering the test subjects are large, wild (and scary?) carnivores. Fortunately, numerous other organisms are hypothesized to rely on bacteria for chemical communication, and scientists- biologists, microbiologists, and chemists alike- are very interested in studying the characteristics and mechanisms of these types of animal-microbe interactions. Hopefully the next few decades of research will be fruitful in expanding our understanding of (and appreciation for!) microbial-mediated animal communication. In the meantime, however, we’ll just try to wrap our heads around the fact that these teeny, tiny, unseen microbial species are microscopic agents influencing the complex social behavior of charismatic megafauna like the hyena! 


What's in a handshake?

Remember when “firm” or “limp” were the only metrics by which a handshake could be evaluated?  Well, those simple days might be over.   Have you ever considered the molecules that are being transferred from you to your new acquaintance during this handshake?  Or, more disturbingly, from your new acquaintance to you?  And what if these molecules served some purpose in controlling how we respond or relate to this person?

A recent paper published in eLife poses a few of these questions through the investigation of handshaking as a behavior that facilitates chemosignaling between humans.  While shaking hands is considered a cultural norm, it could also be a way for us to convey information about ourselves to others without the hassle of using words.

As described in our earlier post “Butterflies or Butterlies”, the production and sensing of pheromones (or chemosignals) is one of the most fascinating forms of chemical communication.  Almost all vertebrates and invertebrates are believed to have evolved to perceive volatile molecules – whether to locate an apt mate, track down food or sense danger.   While it is assumed that pheromones are likely an aspect of human chemo-communication (possibly a made-up term), studying human systems is complicated because of our acute awareness of our own biology.  In other words, it’s pretty confusing to analyze “natural” human behavior or to even define “natural” when it comes to humans.   

The Sobel lab, a research group at the Weitzmann Institute, is interested in human chemosignals and their effects on behavior, and they have previously published research investigating the presence of and response to chemosignals packaged in human emotive tears.   This study, which is absolutely worth a glance, explored the effects of female emotional tears on the sexual arousal of male.  In short, to induce “negative emotion” tears the donor women “watch sad films in isolation” and the tears are collected in a vial.  These tears are then soaked onto a pad, which is placed under the nose of the male subject.  Ultimately, the conclusion of this study was that compared to a saline control, these emotive tears caused a decrease in “self-rated” arousal, various physiological measures of arousal, as well as testosterone levels in the male subjects.   While it’s somewhat comforting to know that men are not aroused by crying, it’s generally important to consider the biological relevance of this study.  Does it make biological sense for a male to be so proximal to female tears?  What is the physical range of this chemosensation?  Do we think this is truly an evolutionary adaptation?  Or do these tear-secreted compounds have another function unrelated to human-human signaling?

The recent study on chemosignaling and the handshake is, in my opinion, even more thought provoking than the previous study.  The experimental design is the following:  The volunteers are aware that they are participating in a scientific experiment, though they do not know the nature of the research.  Upon arrival each subject is greeted by the scientists using a standard greeting – the handshake.   The volunteers, thinking that they are still waiting for the experiment to begin, are then observed for several minutes and the frequency and duration with which they draw their hands to their face is recorded.  Researchers observed that after the handshake the subjects were significantly more likely to bring the hand-shaking hand close to their face, specifically in the vicinity of their nose.   Interestingly, this behavior was only true for interactions between people of the same gender.  Additionally, researchers attempted to confirm the olfactory (smell-related) nature of this interaction by “tainting” the experiment using perfume or putative male or female chemosignals.   These additives altered the sniffing behavior of the subjects suggesting that this behavior is indeed a result of olfactory sampling.

So basically, in shaking hands with a new acquaintance, we are receiving and sensing molecules from their skin surface, a behavior these researchers imply may be related to how we perceive or sense or respond to other human beings.  

This article really raises more questions than it answers.   What’s the deal with the gender specific behavior?  Are these molecules produced by us?  Or microbes on our skin?  What sort of physiological or chemical response do we have to these molecules? While clearly much (or all) remains mysterious, raising interesting questions and generating hypotheses is a great way to generate more research in the field.  

When I describe this study to my friends, acquaintances, colleagues, almost everyone recoils with disgust.  “Wtf! I would never do that!” is the general response.  And, honestly, I kind of feel that way too.   But maybe that’s what makes this behavior so interesting.  While it is always important to note that such human studies have significant error bars, and generally inadequate controls (despite an honest effort by the researcher), it is truly interesting to ponder how chemo-signaling can drive subconscious behavior.   Ultimately we are all slaves to chemistry in some form or another.  


Mountains made of tiny skeletons

One of the most eerie zoom-ins I’ve ever seen (it was probably the eeriest, because how many zoom-ins can one recall) was a zoom-in during a research presentation from a picture of the white cliffs of Dover- imposing, beautiful, grand– to hundreds of tiny skeletons –chalky, white, lifeless. I was quite disturbed to learn that such a huge geological feature was actually made from dead things preserved in astonishing detail. The slow accumulation of the detritus of life in layer upon layer over millions of years is how many geological features are formed. It’s one of those facts that I know, but don’t really like to think about.  

These tiny skeletons were actually the calcium carbonate shells, or coccoliths, of one of the most important organisms in the ocean, Emiliania huxleyi. E. huxleyi is an abundant and wide-spread single-celled photosynthetic algae, a microalgae, that forms the basis of many food webs in the ocean. It’s known best for forming extensive blooms in the ocean, up to the size of England, that can be seen from space, due to the differential refraction of light by the calcium carbonate coccoliths compared to the surrounding water. In addition to its space-visible blooms, E. huxleyi is fascinating for its role in the study of the ancient earth as well as its interactions with other microbes.

E. huxleyi not only makes a beautiful shell but also produces very long-chain lipids containing  37-39 carbon atoms, double-bonds (unsaturation) and a ketone or an ester moiety. These molecules, named alkenones, are a unique chemical signature of E. huxleyi and related coccolithophores (which literally means “bearers of coccoliths”, a group which includes microalgae related to E. huxleyi). The uniqueness of this chemical signature comes from the fact that most lipids have way fewer carbon atoms, usually in the range of 13-21 carbon atoms. For instance, fish oil, a popular supplement made from the fat of fish like sardines, is composed of long-chain fatty acids (which contain carboxylic acids) with 20-22 carbons.

The alkenones of E. huxleyi also happen to be preserved remarkably well over long periods of time. Whenever molecules are well-preserved, scientists find ingenious ways to use them to study the earth millions of years ago. In this case, the alkenones can be used to infer the temperature at which that E. huxleyi cell was growing at the time it produced the alkenones. Specifically, the degree of unsaturation in the alkenones is correlated with the sea-surface temperature where that coccolith was made. So, in this manner, scientists can extract the alkenones in a geological sample and infer what temperature the sea was at that moment in time. In combination with other measurements, paleoclimatologists (researchers who study the climate in the past) can reconstruct the paleoclimate and use this information to aid our understanding of modern-day climate change, which, I’ve heard is real (!). 

The talk that featured the mind-blowing zoom-in was concerned with some discrepancies between the actual, recorded measurement of sea-surface temperature and temperature calculated using the the ratio of the different alkenones made by E. huxleyi. The research suggested that the symbiont of E. huxleyiPhaeobacter inhibens could actually influence the alkenone ratios. There is ample research that suggests the relationship between E. huxleyi and P. inhibens might garner the Facebook status of “complicated.”

E. huxleyi and P. inhibens exist in symbiosis when both members of the relationship benefit by getting a nutrient or another resource from the other partner. In this case, the bacteria eat algal–made dimethylsulfoniopropionate, which serves as a carbon and nitrogen source, and the algae are protected from pathogenic bacteria by P. inhibens-derived tropodithietic acid, a broad-spectrum antibiotic. In addition to the production of tropodithietic acid, the bacteria also produce phenylacetic acid, which is an algal auxin, or growth hormone. More algae equals more food in the form of dimethylsulfoniopropionate for the bacteria. 

When the algae start aging, their cell walls break down, and as part of this aging process the algae secrete p-coumaric acid (pCA). The bacteria, like a superficial man in a mid-life crisis getting ready to dump his wife, sense pCA and betray their long-time partner. In the presence of pCA, P. inhibens turns on the production of a set of small molecules called the roseobacticides. These compounds  are anti-algal compounds that cause lysis, or bursting, of the algal cells. The bursting of the algal cells allows the bacteria to get more nutrients than they would from the intact, aging algae cells. Once their host is dead and they’ve eaten the nutrients that have become available upon algae-cell lysis, the bacteria move on to new, healthy members of the algae community, and the process of symbiosis begins again. The most twisted thing about this situation is that P. inhibens use pCA as a building block in the synthesis of the roseobacticides. The only analogy I can think of for this is that it’s as if the mid-life-crisis-man used a piece of his wife’s jewelry to fashion a dagger that he used to kill her, so he could quickly move on to a new host, er, wife. It’s ugly, but that’s nature for you. 


Red Hot Chili Peppers


Most of us are quite familiar with the intense pain and profuse sweating that occurs when we consume a particularly spicy pepper. This discomfort, often accompanied by a runny nose and exclamations of pain, is avoided by some (Midwesterners) and sought out by others (most everyone else). Whether the consumer is accidentally or purposefully consuming the chili pepper, it has been well established that the resulting physical reactions are due to our body’s response to a particular alkaloid compound found in these peppers. This molecule, capsaicin, is known to bind to a non-specific, heat-activated ion channel in our tongues (TRPVR1). This binding event initiates a signaling pathway that ultimately results in sensations of burning and pain, as well as a bit of a natural high, thanks to the release of endorphins. This special combination of pain and pleasure, so often at the core of “things humans like,” has fueled a fascination and worldwide demand for spice (see: growth of the underground “chilihead” movement to breed and eat super-hot chilies). 

Although these spicy chilies provide us with flavorful food, their purpose on earth is not to entertain and challenge human taste buds. So, then, why are some peppers spicy?

Before we answer this question of spice, we need to first ask, why do plants produce fruit? Since plants are stationary organisms, they rely on a variety of methods to disperse their seeds, including wind, gravity, water, ballistics, and animals. Plants that use animals often produce juicy and delicious fruit to attract animal consumers and provide a reward to dispersing their seeds. However, not all consumers are created equally: fruit not only attracts organisms that can effectively disperse their seeds, but also organisms that destroy their seeds. To avoid consumption by these seed predators, some plants use directed deterrence, often chemically-mediated, to discourage these predators without deterring seed dispersers. 

After observing in both labs and natural populations that peppers with high levels of capsaicin were readily consumed by birds and not mammals, it was hypothesized that capsaicin may be involved in such a mechanism of directed deterrence. In a paper published in Nature in 2001, researchers studied a population of chilies in Arizona and found that avoidance of chili peppers by small mammals was directly related to the capsaicin content of the chilies. Birds, however, readily consumed the peppers regardless of how spicy they were. While we might hypothesize that birds just love the spice, it turns out the spice taste doesn’t even register with birds: the bird homolog of the receptor TRPVR1 is not activated by capsaicin and thus birds don’t feel the heat. The researchers in Arizona also found that consumption of peppers by the small mammals resulted in zero germination of the consumed seeds (the seeds get destroyed in mammalian guts). Consumption by birds, however, resulted in germination rates similar to control seeds directly planted from the fruit. Thus, by producing a chemical that targets mammalian heat receptors, capsaicin plays a role in deterring mammalian seed predators (such as us humans) while not deterring beneficial seed dispersers. So, in other words, the pain we feel when consuming hot chilies maybe tied to the fact that we, like most mammals, don’t s&#$ out viable pepper seeds that can grow into new plants. 

At this point, you may think that macro-fauna with seed-destroying-guts are the main threats to chili peppers. However, the most widespread enemy to all fruits and the seeds within them may actually be microscopic. Many fruits are thought to contain bitter, distasteful, and sometimes toxic chemicals as a method of microbial deterrence. Indeed, capsaicin has been found to be broadly antimicrobial in addition to selectively deterring mammalian consumers.  

Interestingly, this anti-microbial activity of capsaicin may be connected to our culinary obsession with spicy foods: food scientists, ethnopharmacologists, and evolutionary biologists have all postulated that humans may have domesticated chili plants to harness capsaicin’s antimicrobial activity for food preservation. Microbial contamination was a major source of illness and death before the advent of refrigeration and many traditional methods of food preservation are intimately tied to efforts to control our microbial friends and foes. An study published in 1998 in the Quarterly Review of Biology called “Antimicrobial functions of spices: why some like it hot” quantified what many of us who are familiar with world cuisine know: the use of chili as a spice is more prevalent in warmer climates (India, Thailand, Central America) than in colder climates (Scandinavia, Ireland, other areas with noticeably spice-lacking food cultures), where the chance of spoilage is less likely. Humans thus may have learned to accept (and even love!) the burning and pain that comes with consuming peppers in order to reap benefits of health and longevity that comes with killing microbial pathogens in our foods. 

Our story of capsaicin’s role in peppers extends from the chemical ecology of plant seed-dispersal strategies to the clever use of chilies in food. The human use of a secondary compound to serve a purpose thats mirrors the evolutionary role of this compound in nature, however, is not unique to capsaicin. We’re excited to explore the many compounds have we co-opted from the environment for our own culinary, medicinal, or otherwise “other” uses! 


Butterflies, or butterlies?

Most of us rely on our sense of sight to identify the people around us. So, if your roommate were to try and fool you into thinking that she was your boyfriend, she’d have to go to fairly epic, award-winning-special-effects-makeup-level lengths to fool you. In nature, however, organisms can often fool members of entirely different species into thinking they are related using methods that are analogous to simply putting on your boyfriend’s cologne.

The vast majority of organisms use chemical signals to identify – and trick– each other. Chemical mimicry is the process of synthesizing the pheromones of another species so that members of that species will think that you are one of them.  Indeed, while we have little experience with non-humans dressing up and fooling us into thinking they are our neighbor or family member, chemical mimicry is a very common phenomenon in nature. And, due to our long tradition of using mistaken identities as a comedic device, and our general impulse to view the actions of organisms as willful choices instead of non-emotional instincts produced by millennia of evolution, the stories of chemical mimicry are often hilarious.

One story about chemical mimicry involves an endangered species of butterfly, the rare and beautiful Phengaris rebeli (formerly Maculinea rebeli, which someone thought sounded too much like “masculine rebel” and needed changing). While sure to make you laugh, P. rebeli’s chemical mimicry story also helps us to understand why this butterfly is so threatened and informs how we think about conserving its habitat.

P. rebeli, native to the Alps and other areas in southern Europe, is a social parasite of red ants named Myrmica schencki.  The story of its deception of these unwitting ants begins with the flowering plant Gentiana cruciata. P. rebelifemales lay their eggs specifically on the leaves of this plant. When the larvae hatch, they get their first meal from the plant. After a period of feeding on these leaves, the larvae fall to the ground (cue strangely loud  plopping noise of the wiggly larvae falling a few inches onto the soil below). This happens during the early evening, which is exactly the time of night that Myrmica red worker ants are out-and-about looking for food and, as it happens, any misplaced ant larvae.

When the ants  encounter the P. rebeli larvae, from here-on called caterpillars, they ask “who is this?” by swabbing the caterpillars’ grubby bodies with their antennae, which is the way the ants can get a whiff of the chemicals there.Researchers have studied the process of Myrmica ants interacting with the caterpillars to understand this key interaction more deeply. They analyzed the surface chemicals the larvae produce, and whether these chemicals closely mimic the preferred host–Myrmica schencki– or mimic pheromones found in most Myrmica species. They discovered that the chemicals secreted by the caterpillars are highly similar to those present on Myrmica schencki ant larvae and workers. Rather than synthesizing one or a just a handful of important pheromone compounds, the caterpillars’ chemicals are instead a complex mixture, a subtle perfume built from many different volatile and non-volatile hydrocarbons, including limonene, which is a terpene that smells like citrus fruits. Upon encountering this lovely perfume of ant larvae, the Myrmica ants are fairlyconvinced: it takes the ants much longer to actually pick up the caterpillars compared to larvae of their own species, and many ants even place their caterpillars on the rubbish heap, before changing their minds and removing the caterpillars from the pile of ant trash. Eventually, most of the caterpillars end up safe and sound amongst the ant larvae, or “brood.”

Once amongst the ant brood, things get even stranger. Having secreted their own fairly convincing ant-larvae perfume, the caterpillars become even more convincingly “ant-like” through the process of chemical camouflage. Through physical  contact with ants and their built environment, the caterpillars pick up even more ant scents. The researchers used a scoring system, named “Nei’s distance,”  to measure the degree of similarity between the chemical profiles of extracts of caterpillars and ants, where 1 is identical and 0 is no chemicals in common. The score for the caterpillars freshly plopped onto the ground from their eggs compared to the adult ant workers was 0.3. After adoption by the ants, the score jumps to 0.85. The treatment of the caterpillars by the ants reflects this change: once fully ingrained into ant society and camouflaged by ant compounds, the caterpillars are treated so well that in times of stress, the ants will actually feed the caterpillars the chopped up bodies of ant larvae and eggs. If you think about it, this is pretty messed up.  

Living amongst the ants over the course of one to two years, the caterpillars grow from 1 mg to about 100 mg in size, which is about one-hundred times larger than an ant, making us wonder how the gig isn’t up for the caterpillars once they’ve grown so remarkably giant compared to their hosts. After their long co-habitation with the ants, the caterpillars emerge as butterflies, their chemical charade a success. The strategy of P. rebeli to parasitize ants in such a lengthy and integrated manner is a strategy to gain resources, including food and safety from predators, and deal with ants as competitors for those resources at the same time. Other species of butterflies have altogether different ideas for how to deal with the ants, including agitating ants or simply throwing in the towel and feeding and appeasing them with gifts of sugar and amino acids.

Chemical mimicry is found throughout nature, and can involve insect-insect interactions (such as that between P. rebeli and M. schencki) as well as plant-insect interactions. For example, orchids produce the female sex hormones of pollinators, and these chemicals are carried by the wind to seduce male pollinators to the flower. Finally, our review of the chemical mimicry of the large blue butterflies P. rebeli points to the specificity of its lifestyle: the eggs are deposited on specific flowers, and the larvae fall to the ground at the peak foraging time of red ants, one species of which will be fooled into taking the caterpillars into their colony. This high degree of specificity during multiple stages of its lifetime points to the fragility of its habitat, and suggests that protecting the butterfly means protecting Gentiana cruciata and M. schencki, too.


Bees on cocaine

A few years ago researchers published a study in the Journal of Experimental Biology investigating the effects of cocaine on bees.  In all honesty this research article caught my attention because of an amusing “Shouts & Murmurs” in The New Yorker clearly inspired by the seemingly wacky set of experiments outlined in the paper.    So what exactly was the logical basis behind this article?  What were these researchers setting out to understand?    What does this have to do with chemical ecology?  And then the obvious question:  Do bees abuse cocaine?  

This “bees on cocaine” study, in a roundabout way, gets at a central question in the field of chemical ecology:  What are the natural roles of chemicals produced in the environment?  As humans we have an understandably anthropocentric take on why certain molecules are useful.  Lovastatin, for example, is a natural product produced by the fungus Aspergillus terreus.  This molecule, which inhibits a human enzyme involved in cholesterol biosynthesis, ultimately paved the way for the development of the statin class of drugs (Lipitor), which have become a major treatment for heart disease.  Yet, while we know quite a bit about the biology of lovastatin in the human body, we have very little understanding of what this molecule is actually doing for the fungus that produces it.   Is it essential for the survival of the fungus?  Does it target another organism involved in a symbiosis with Aspergillus?   Most likely this natural function has nothing to do with chronic heart disease in humans.

Cocaine, which is produced by the plant Erythroxylon coca, is an oddly similar example.  It has had some success in the clinic, but has been remarkably popular for more recreational purposes.  Yet, much like the production of lovastatin byAspergillus, the production of cocaine is not naturally intended for human use.  In 1993, Nathanson et al. published a study showing that cocaine may serve as an insecticide – a way for the coca plant to deter its insect predators.  Researchers measured “leaf protection” by placing insect larvae on tomato leaves (which obviously do not produce cocaine) and then treating them with increasing doses of cocaine.  These experiments revealed that larvae exhibited impaired motor function, decreased time spent on the leaf, and with further increased concentrations, would ultimately die.  For understandable reasons, these physiological effects minimized the larvae’s desire, or ability, to chow down.  This study revealed a potential natural role for cocaine as a defense mechanism for the coca plant – and ultimately raised a lot of questions about the difference between insect and human responsiveness to this drug.

So this is where the bees come in.  In a further exploration of the effects of cocaine on insects, Barron et al. administered cocaine to bees and observed its effects on a number of behaviors.  Most notably, these researchers made the thrilling observation that cocaine had a profound effect on the bees’ dancing. Upon the discovery of an exciting resource (pollen, for example), forager honeybees return to the hive and perform a “waggle dance” that indicates to their hive mates the location of this desirable resources.  Treatment with cocaine increased the rate and likelihood of dancing after foraging without obviously affecting other behaviors or general motor function.  Further, after multiple treatments with the drug, the bees exhibited signs of withdrawal if the drug was removed.   So, to answer the question posed at the beginning of this post, apparently bees would abuse cocaine…? If given the opportunity of course.

These two studies taken together seem sort of contradictory.  One asserts that cocaine is basically toxic to insects while the other suggests that cocaine not only triggers reward processing in insects, but is also addicting.   The difference in findings between these two papers is highly interesting for a number of reasons.  One is that the dosage of cocaine (which is not clearly consistent between these two studies) likely has a pretty significant effect on how it affects invertebrates.  More interestingly, it perfectly exemplifies the “paradox of drug reward” - in other words, the question of why an organism, like the coca plant, would evolve to produce an insecticide that has a potentially self-defeating side effect, like addictiveness (nicotine is another example).  This question remains wide open.  Ultimately, studying the effects of cocaine on bees gained widespread attention – in part because of its novelty (clearly the motivation for the “Shouts & Murmurs” piece), but also because the similarities between bee responsiveness to cocaine and the well-documented effects of cocaine on mammals suggested the potential for using insects as a model system for understanding the convoluted neurological effects of cocaine and perhaps even untangling the perplexing biology behind drug addiction.

Chemical ecology is amazingly complex but can be far-reaching in its implications.  Acknowledging the difference in how we, as humans, use natural products, versus how these molecules are actually used by the organisms that produce them, can increase our understanding of the world around us and cause us to wonder how, or why, these organisms evolved to produce such diverse molecules.   Pursuing such studies can ultimately lead to the discovery of new molecules with diverse biological activity, and the development of model systems for understanding human biology and treating disease.  



Humans like to assume everything is made for human use. Consider the following list of drugs:






Salicylic acid




These drugs that we think of as antibiotics, anti-cancer agents, hallucinogens, painkillers, etc. are actually chemicals produced in nature for completely non-human related uses. For millennia, bacteria, plants, fungi have been making drug-like molecules for their own purposes. So what are their natural functions in the environment? For the most part, we don’t really know.

We will show you why organisms make molecules that so often end up in the clinic. The why of natural products is explained through the science of chemical ecology.

The field of chemical ecology seeks to understand how organisms mediate interactions through space and time with both their abiotic environment and other organisms using chemicals. We’ll define chemicals here as small molecules, which we think excludes large biomolecules like DNA (deoxyribonucleic acid) and proteins. Other words we might use to refer to chemicals are natural products, molecules, compounds, and secondary metabolites.

In the search to understand these interactions, chemical ecologists will discover new bioactive molecules and the mechanisms through which molecules have their biological effects. In this way, chemical ecology can be an explanatory science, an especially useful attribute in today’s world of big data that often collects massive amounts of information but fails to connect the information to a mechanistic understanding.

Finally, chemical ecology is especially relevant today as we try to engineer nature to suit our needs as well as save fragile ecosystems that have been negatively affected by modern life.

Hopefully this blog will provide you with cool information and “chemical intuition” that you’ll enjoy and share with your friends.