Understanding the Impact of Intra-Species Bacterial Competition on Native Probiotic Engraftment in the Gut

BACKGROUND: The microbiology field currently assumes that the presence of a specific species in the gut microbiome excludes the subsequent introduction of the same species. Likewise, two different species will not compete for the same niche. However, these assumptions are based on studies in mice with depleted microbiomes. We hypothesize that these findings do not translate to conventional mice.

METHODS: A murine-isolated Escherichia coli strain was engineered to express green fluorescent protein (GFP) and kanamycin resistance. The strain was further engineered to express spectinomycin resistance (SpecR) or bile salt hydrolase (BSH) with chloramphenicol resistance. We administered these strains to conventional C57BL/6 mice via oral gavage, varying the timing and order of delivery. We also assessed the influence of different diets (normal chow, high fat, or atherogenic) on colonization outcomes.

RESULTS: Order of administration of the two strains was not the sole predictor of engraftment; prior presence of one did not inherently preclude the latter from colonizing. Additionally, different diets altered the colonization outcomes. Interestingly, colonization by EcAZ-2^SpecR or EcAZ-2^BSH+ is prevented if a mouse is already colonized with a distantly related Enterococcus spp.

CONCLUSIONS: We discovered that (a) adding a transgene does not innately reduce native bacterial fitness, (b) the order of bacterial introduction does not solely determine colonization success, (c) diet impacts bacterial competition, and (d) different species can indeed compete for the same niche. These results highlight the complexity of gut microbiome dynamics and emphasize the need for a more nuanced understanding of these interactions.