Gut microbiota in the burying beetle, , provide colonization resistance against larval bacterial pathogens.

Carrion beetles, are reared on decomposing carrion where larvae are exposed to high populations of carcass-derived bacteria. Larvae do not become colonized with these bacteria but instead are colonized with the gut microbiome of their parents, suggesting that bacteria in the beetle microbiome outcompete the carcass-derived species for larval colonization. Here, we test this hypothesis and quantify the fitness consequences of colonization with different bacterial symbionts. First, we show that beetles colonized by their endogenous microbiome produce heavier broods than those colonized with carcass-bacteria. Next, we show that bacteria from the endogenous microbiome, including and , are better colonizers of the beetle gut and can outcompete nonendogenous species, including and , during in vivo competition. Finally, we find that and provide beetles with colonization resistance against and thereby reduce -induced larval mortality. This effect is eliminated in larvae first colonized by , suggesting that while competition within the larval gut is determined by priority effects, these effects are less important for -induced mortality. Our work suggests that an unappreciated benefit of parental care in is the social transmission of the microbiome from parents to offspring.