Population diversity affects the ability of a species to invade a community

One of the most exciting concepts generated from studying host-microbe systems has been the idea of the individual as a community. For example, the human body supports the growth of hundreds of microbial species within us and on us that compete and cooperate with us and each other. This concept has allowed us to connect two previously disparate fields of thought and study: infection biology and community invasion. Most people are familiar with infection biology. A virus, bacterium, or eukaryotic pathogen physically intrudes into our body, co-opting and disrupting host cell function. To prevent this, our bodies employ a multifaceted immune system that acts as a border patrol and home defense.

Most people are also familiar with “invasive species”, which can disrupt a community’s function and result in species extinction events. However, invasion is simply the establishment of a new species within a community, and can have benign, beneficial, or detrimental effects on the diversity and function of the community. And here is where our two ideas meet. As we have learned from studying host-microbe systems, pathogens do not only have to infect the host, they often have to invade the microbial community associated with the host.

To date, there has been much focus on the role of community diversity in preventing community invasion by pathogens. But Anders Forsman’s meta-analysis,“Effects of genotypic and phenotypic variation on establishment are important for conservation, invasion, and infection biology”, focuses on a different aspect that is often overlooked: the role of variation in the invading population. Forsman concentrates on plant and animal studies (18 in total, 9 focusing on plants, 8 on invertebrates, and 1 on vertebrates). It was found that success of community invasion is positively correlated with the genotypic and phenotypic diversity of the invading population. Key reasons Forsman hypothesizes that population diversity of invading species might matter include:

• Higher probability that the population harbors preadapted phenotypes
• Reduced intra-population competition via differential resource exploitation
• Facilitation (cooperation) between different phenotypes

While Forsman’s selection of studies is relatively small, the studies seem to be of high quality. First, he only chose studies that involved actual experimental manipulation. Furthermore, in his meta-analyses, he found no evidence of bias toward small studies with a large effect size, the effect size actually increased in studies performed in natural communities with fewer controls (usually the reverse is true), and the studies all generally agreed (even the ones that did not detect statistical significance trended in a similar direction). One drawback to the cited studies was that they were all relatively short in duration, spanning no more than 8 generations for the study organisms.

Obviously, these studies have implications for conservation and community invasion biology, and Forsman draws a link to infectious disease as well. These results also have implications for something at the convergence of conservation and infection biology: probiotics. Currently, there is very little evidence for their efficacy, and it has been very difficult to demonstrate that probiotic species can successfully establish within the gut community. Moreover, probiotic research often involves the introduction of a specific bacterial strain or a cocktail of strains. Forsman’s review would suggest that the genetic diversity within certain species, as opposed to multiple phenotypically distinct species, might be an important aspect to consider.

Zebrafish and other highly amenable host organisms serve as powerful experimental models with multiple methods available for further testing the hypothesis that increased population diversity positively correlates with community invasion success. Species of bacteria isolated from zebrafish, but not present in the study facility, could be introduced to conventionally raised zebrafish and the gut contents of different individuals in the population. Alternatively, light-sheet and confocal microscopy could be used to monitor the success of different mixtures of fluorescently tagged strains of particular species. Combined with what is already known about the importance of life history traits, understanding the role of population diversity in community invasion and host infection could lead to novel models of infectious disease and solutions to improving probiotic efficacy.

References

Forsman, A. (2014). Effects of genotypic and phenotypic variation on establishment are important for conservation, invasion, and infection biology. Proceedings of the National Academy of Sciences of the United States of America, 111(1), 302–7. doi:10.1073/pnas.1317745111

Sanders, M. E., & Klaenhammer, T. R. (2001). Invited Review: The Scientific Basis of Lactobacillus acidophilus NCFM Functionality as a Probiotic. Journal of Dairy Science, 84(2), 319–331. Retrieved from http://www.sciencedirect.com/science/article/pii/S0022030201744815

Picard, C., Fioramonti, J., Francois, A., Robinson, T., Neant, F., & Matuchansky, C. (2005). Review article: bifidobacteria as probiotic agents — physiological effects and clinical benefits. Alimentary pharmacology & therapeutics, 22(6), 495–512. doi:10.1111/j.1365-2036.2005.02615.x

Stecher, B., Robbiani, R., Walker, a W., Westendorf, a M., Barthel, M., Kremer, M., … Hardt, W. D. (2007). Salmonella enterica serovar Typhimurium exploits host inflammation to compete with the intestinal microbiota. PLoS Biology, 5(10), e244. doi:10.1371/journal.pbio.0050244

March 12, 2014