Microbial ecology, and its relation to the development of infectious disease, is an ever-growing field of study. Of course, there are a vast number of bacterial species living amongst us, most of which do not cause us any harm. Others may infect us only when, so to speak, the stars align in a certain manner: when a number of factors collide that result in the development of a diseased state. For instance, we may already be immunocompromised due to the presence of another infection–something minor, such as a rhinovirus, or something more serious, such as HIV–and this chink in our armor allows another organism to more easily infect, and potentially damage, us.
Other agents in the environment also play a key role in the ecology of potentially pathogenic microorganisms. A recent study in Science highlighted one of these that appears to play an important role in the ecology and evolution of Vibrio cholerae, a major human pathogen of the past several centuries.
V. cholerae is the bacterial agent of cholera, a deadly water-borne disease. The bacterium itself is somewhat of a boomerang or kidney bean shape, and can be found in a number of aquatic environments of varying salinity. Cholera has killed millions over the past 200-odd years, frequently re-appearing in pandemic form after initially emerging from India in the early 1800s. Infection with the bacterium can lead to severe gastrointestinal problems, and the production of copious amounts of “ricewater stool.” Death is generally due to severe dehydration. It’s also a bacterium that has played a key role in the development of the very science of epidemiology. John Snow, considered the “grandfather” of epidemiology, became famous for tracing a 1854 outbreak of cholera in London to a contaminated well, introducing the basic principles of epidemiology along the way.
More recent research has shown that in nature, the bacterium uses the polymer chitin as both a food source and an anchor. Chitin is the second most common polymer on earth (beaten only by cellulose), and is the most abundant in the marine environment, where V. cholerae thrives. Chitin can be found in a number of diatoms, in the exoskeletons and fecal material of arthropods, and in fungi, just to name a few sources.
Why does this matter? V. cholerae that are associated with chitin have been found to be more highly resistant to acid–a primary defense mechanism against food (including water)-borne pathogens. Chitin surfaces can also serve to concentrate these bacteria. Biofilms of V. cholerae on a single chitin-containing plankton, for example, may be enough to constitute an infectious dose of the organism—meaning you’d have to ingest an incredibly small amount of contaminated water in order to develop disease. If this wasn’t all bad enough, the new study by Meibom et al. shows that chitin causes V. cholerae to become naturally competent—it makes it take up DNA.
Previously, V. cholerae wasn’t thought to be naturally competent (also referred to as “naturally transformable”). Though it was known that there was a large amount of genetic diversity within the species, it was thought this was largely due to transduction–movement of genetic material between bacteria by viruses, since in laboratory culture, V. cholerae didn’t readily take up DNA. Meibom et al. showed that when V. cholerae were grown in the presence of chitin polymers, they took up a Kanamycin resistance gene at much higher rates than isolates grown in the same medium without chitin–the chitin caused them to become competent.
Now, to return to microbial ecology and evolution. I already mentioned that chitin is the most abundant polymer in the aquatic environment, and that the results of this new study show that chitin can greatly increase the possibility of horizontal gene transfer in V. cholerae. Imagine, now, what can happen when there’s a copepod bloom (literally “oar foot;” this is simply a general name for a number of aquatic crustaceans)—a giant increase in the population of these (chitin-containing) animals, in water that’s contaminated with V. cholerae. Under these conditions, the potential for rapid evolution of these populations of bacteria–and hence, the transmission of novel strains to humans–may be immense, if the laboratory findings hold up under natural conditions.
Indeed, it was already known that weather conditions that can lead to these copepod blooms played a role in cholera outbreaks–and scientists have been working on modelling the conditions that may lead to cholera outbreaks, as well as testing additional potential environmental conditions that play a role in disease. Will this help prevent–or at least provide advance warning of–cholera outbreaks in the future? Time will tell. In the meantime, I wonder how many other bacteria that aren’t considered to be naturally transformable (such as Group B strep, which I discussed here) would be if we only found the right set of conditions.