The host-centric view of the numerous symbiotic, commensal, and pathogenic bacteria living on and in us is that they’re merely microbial hitchhikers and roommates. They’re small organisms that might make us sick or exchange nutrients with us–but they’re also just that–separate organisms. However, as researchers look closer at the relationship between host and microbe, it turns out that things may not be as straightforward as one might learn in a grade school science class. Instead, the crosstalk between bacterial and eukaryotic cells is a complicated web that influences host immunity and even behavior. In the hologenome theory, we might consider our microbes as just another part of us since the microbe-host interactions ultimately influence our fitness in the environment1. So bacteria may not only be their hosts’ roommates but their most intimate confidants, dictating not only when they get sick or how food is metabolized but also more complex social interactions–like their love lives.
In a fascinating paper by Sharon et al. in PNAS2, several experiments demonstrate that mating preference in the fruit fly, Drosophila melanogaster, is mediated by the fly’s gut microbiota. In the initial experiment, lab flies were divided into two groups. One group was fed with starch and the other was fed with a cornmeal-molasses-yeast (CMY) mixture. Then the flies were given a multiple choice mating test where one male and one female fly from each group were placed in a plastic dish and observed over a one hour period for mating behavior. The researchers observed that even after the second generation, the flies preferred to mate with flies reared with the same food source. Preference persisted up to the 37th generation, which was as far as the researchers had tested.
But why on earth would food source affect mating preferences? The gut microbiota was analyzed by 16S rRNA gene sequencing. Flies fed on CMY had a diverse collection of microbes living in their gut. Flies that consumed starch, however, had a collection of gut microbes in which over a quarter of the microbial population consisted of Lactobacillus plantarum. In the next experiment, the flies were fed antibiotics. This abolished the preference and matings became random. Since the antibiotic killed the microbes in the fly gut, it must mean that the microbes were in some way influencing fly mating behavior. In the subsequent test, the flies that had already been treated with antibiotic were fed Lactobacillus plantarum. And lo and behold, the mating preference reestablished itself. Other bacteria, such as Providencia rettgeri and 41 other species, were also tested, but they had no influence on mating preference. So Lactobacillus might be solely responsible for preference.
While analysis of cuticular hydrocarbon levels in the flies show that Lactobacillus may be influencing fly sex hormones, exactly how the bacteria is modulating hormone levels is still unknown. One possible clue might be gleaned from another process that is regulated by hormones–sleep. Trypanosoma brucei is one sleep-altering parasite that infects humans. During an infection, the immune system is triggered. The immune system is known cross-react with hormones. For example, in the early 1990s, a slew of research showed that the immunomodulatory cytokine IL-1 interacts directly with the sleep hormone serotonin3.
Well, what about humans and their gut microbes? No one has yet looked at the relationship of the bugs in human stomachs and human hormone levels, it is interesting that there is variation of gut microbiota between humans due to diet and lifestyle4,5. And while there is the adage that like attracts like that was proven with the Drosophila experiment, we cannot say whether this is a cause or an effect in humans. Or if this even is a significant contributor to human interactions in the first place.
Gut microbes, however, may not be the only ones playing matchmaker. It has already been known for several decades that bacteria cause body odor6. According to Rachel Herz, a scientist at Brown University:
These bacteria feed on the protein on our skin, and as they digest they release gases reflecting their meal. The specific proteins that populate your underarm are genetically determined and reflect your individual MHC gene profile, which is why each of us has a unique smell.
Herz points out that the stuff on our skin includes the chemicals–“quasi-pheromones” such as androstadienone–secreted by our apocrine sweat glands. As the skin bacteria munch and process these chemicals, the gases given off will make us smell either female or male7.
So will the futuristic versions of the little blue pill or love potion number nine be found in the aisles of your local grocery store? Should people pick up a pint of yogurt instead of querying the neighborhood yenta if they want to find their soul mate? I have no idea. On the surface, this seems a bit silly. What we know today is suggestive yet speculative. Your friendly gut microbes probably care less about your love life than your next meal. But it is not inconceivable to think that the human microbiome might become just another one of those dimensions of compatibility an internet dating site might exploit.
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1. Zilber-Rosenberg, Ilana and Rosenberg, Eugene. “Role of microorganisms in the evolution of animals and plants: the hologenome theory of evolution.” FEMS Microbiol Rev. (2008) 32: 723-735. (PubMed)
2. Sharon, Gil et al. “Commensal bacteria play a role in mating preference of Drosophila melanogaster.” PNAS. (2010) 107: 20051-20056. (PubMed)
3. Opp, Mark R and Imeri, Luca. “Sleep as a behavioral model of neuro-immune interactions.” Acta Neurobiol Exp. (1999) 59: 45-53. (PubMed)
4. Ley, Ruth E et al. “Worlds within worlds: evolution of the vertebrate gut microbiota.” Nature Reviews Microbiology. (2008) 6: 776-788. (PubMed)
5. Ley, Ruth E et al. “Evolution of Mammals and Their Gut Microbes.” Science. (2008) 320: 1647-1651. (PubMed)
6. Leyden, James J et al. “The Microbiology of the Human Axilla and Its Relationship to Axillary Odor.” J Invest Derm (1981) 77: 413-416. (PubMed)
7. Herz, Rachel. The Scent of Desire. Harper Perennial: New York, NY. (2007) p 150.