Bacteria determining our love life?
I came across a fascinating article this morning, while randomly searching other WordPress blogs. Here is a blog by Syaffolee, a PhD student at the University of Idaho, who wrote a stirring Valentine’s Day posting about the bacteria we have living in and on us. The blog posting pointed to this article from one of the premier scientific journals in the United States. We will unfortunately have to wait until May for the article embargo to go away and we can access the full text via Schmidt Library.
Let’s see what we can glean from the published abstract though. The study, from Tel Aviv University, examined the mating preference of Drosophila melanogaster. The experiment was set up by dividing the population into two groups, one of which was raised on molasses media and the other was raised on starch media. Once adult flies were obtained, the researchers found that the “molasses” flies preferred to mate with “molasses” flies, and the “starch” flies also preferred their own kind. The abstract further states that:
(t)he mating preference appeared after only one generation and was maintained for at least 37 generations. Antibiotic treatment abolished mating preference, suggesting that the fly microbiota was responsible for the phenomenon.
This last point is the key one. We already know that we are walking Petri dishes, with microorganisms covering every exposed surface of our bodies, and most internal surfaces (respiratory system, gastrointestinal tract, genitourinary tract) as well. This normal flora resides on us in the absence of disease, and as we’ll see after we get back from Winter Break, actually can confer benefits to us. Consider though how these organisms grow: then need a source of nutrition, just like an organism that we might culture in the lab, and different organisms have a variety of nutritional requirements. Therefore, if the source of nutrition is changed (and if these are gut bacteria, that comes from what you eat,) the organism populations may alter when growth conditions are altered. The researchers changed the bacteria via the diet, and appeared to change mating preference in the flies.
So what are the bacteria that appear to be responsible for this? Flies have a mixture of Lactobacillus in their gut flora, and the “starch” flies had an isolate of Lactobacillus plantarum that was recovered from them in pure culture. When the “molasses” flies were infected with L. plantarum, they then preferred the “starch” flies as mates. The authors provide further evidence that small molecule sex hormones may be made by the different bacterial species are influencing sexual selection in the flies by modifying their behavior. L. plantarum is a common environmental organism that actually finds its way into numerous human fermented foods, including sauerkraut, pickles, Korean kimchi, and sourdough bread. A recent study found that L. plantarum significantly reduced the antigenicity of soybean flour, suggesting that the organism would be a useful agent for probiotics, or the use of dietary microbes to modulate human health.
We have an example here of a microorganism that appears to be modulating the behavior of its host, and in an exquisitely subtle way. There are other examples of infectious agents modifying behavior; the cognitive alterations that occur with rabies infection comes immediately to mind, although these appear to occur due to the neurological damage the infectious agent causes. In the instance of L. plantarum, the behavioral modification is more global in nature, with an organism in the gastrointestinal tract producing chemical messengers that shift behavior in a very specific manner. We are well aware of the role of pheromones acting as chemical messengers between animals, but here is an intriguing possibility that the pheromores are possibly not made by the host itself, but instead are made by the microorganisms that live on us. What is the advantage of this? It’s rather easy to see from the point of view of the microbe: they are able to ensure that their population (that is the total population of Lactobacillus in all of the flies that carry the strain) is able to be in continued genetic contact with one another by bringing the carriers into contact with one another.
Does this have any relevance to human behavioral biology? Should you offer some sauerkraut to someone you’d like to get to know better? We’ll have to wait for some more studies on this matter, but I have no doubt that small molecule pheromones have played and continue to have important influences on human interactions; we are animals after all! I would in the time being advise all to stick with the tried and true: flowers and chocolate, and save the sauerkraut for next Oktoberfest.
BONUS: In the comment thread, suggest a benefit that a host receives from a microorganism that lives on it. Search Wikipedia or another source; I want to see examples that have not been discussed in class.