Date of this Version
Frese, SA. (2012). The Evolution of Host Specificity in the Vertebrate Gut Symbiont, Lactobacillus reuteri. Doctoral dissertation, University of Nebraska-Lincoln.
The vertebrate gut is home to one of the densest populations of life on Earth. This microbial community has a profound effect on host health, nutrition, development, behavior, and evolution. However, very little is known about how these microbes have evolved with their vertebrate hosts, how and whether they select hosts or how they remain associated with their hosts. Recent work identified Lactobacillus reuteri as an organism that is composed of host-specific sub-populations, each population associated with a different host animal. Representatives from each host-associated population were tested for their ability to colonize gnotobiotic mice, which only rodent strains could accomplish. To determine the genetic source of these differences, genome sequences from representative strains (from humans, pigs, chickens, and rodents) and genomic microarrays were used to identify host-specific genes tied to ecological success in vivo. Many of the rodents pecific genes were essential to the ecological success of this strain. Overall, the types of genes which were essential suggested that they were related to biofilm formation. Using a novel method of biofilm quantification and a germ-free mouse model, the ability of L. reuteri strains to form biofilms on the forestomach of germ-free mice was examined. In vivo biofilm formation was exclusive to the rodent associated strains. Using genome comparisons, in vitro biofilm formation, and related models of biofilm formation, genes essential to biofilm formation were identified. Genes responsible for adherence, aggregation, and gene regulation were all critical to biofilm formation. To determine whether the observed host specificity of rodent associated L. reuteri in mice extended to other host-associated lineages, a crossover human experimental trial was conducted. Three species of Lactobacillus were tested for their ability to persist in the human gastrointestinal tract. A human-associated strain of L. reuteri, as well as a related species isolated from humans reached higher populations than a non-associated strain, suggesting that host-specificity of L. reuteri extends to other lineages as well. Together, these experiments demonstrated the host-specificity of the species and identified the molecular mechanisms by which rodent-associated L. reuteri colonize their host.
Advisor: Dr. Jens Walter