Impact of a defined bacterial community including and excluding <i>Megamonas hypermegale</i> on broiler cecal microbiota and resistance to <i>Salmonella</i> infection

Intensive broiler production practices impair the transmission of commensal microbes from hens to offspring, resulting in a lower abundance of non-spore-forming strict anaerobic bacteria. We evaluated the effects of introducing a defined community (DC) of bacteria including and excluding <i>Megamonas hypermegale</i> to chicks prior to a <i>Salmonella</i> challenge. Inoculation with DC resulted in higher phylogenetic diversity (PD) and the dominance of Bacteroidetes species in the cecal microbiota, with a decrease in the relative abundance of <i>Salmonella</i> and <i>Escherichia/Shigella</i>, as well as a lower <i>Enterobacteriaceae</i> load. Substantial shifts in microbiota composition were coupled with subtle changes in metabolites and host responses, including changes in interferon-γ, macrophage colony-stimulating factor, propionate, valerate, and isovalerate concentrations in the ceca. We identified bacterial isolates included in the DC that were able to establish and persist after a single exposure, many of which were members of Bacteroidetes phyla. Although co-culture with <i>M. hypermegale</i> was observed to reduce <i>Salmonella</i> counts <i>in vitro</i>, <i>in vivo</i> inoculation of <i>M. hypermegale</i> without an accompanying DC was shown to increase splenic <i>Salmonella</i> counts. Inoculation with DC containing <i>M. hypermegale</i> increased cecal concentration of valerate and propionate. Our findings provide further insight into how DC might be selected to impact infection resistance. The comparison of DC differing by one single isolate is useful to aid our understanding on the role of individual strains in the broiler cecal community, guiding the formulation of DCs that can potentially favor poultry health and production.IMPORTANCEIntensive production practices can reduce beneficial gut bacteria in broiler chickens, potentially leading to higher disease risk. We investigated whether introducing a defined community (DC) of commensal bacteria, with or without <i>M. hypermegale</i>, could improve gut health and resistance to <i>Salmonella</i> in broiler chicks. Our findings show that DC increases microbial diversity and reduces the relative abundance of potential pathogens, like <i>Salmonella</i> and <i>Escherichia/Shigella</i> in the ceca, which was coupled with subtle changes in the immune responses of the birds and higher concentration of volatile fatty acids in the ceca. This study suggests that inoculation with DC in early-life can alter microbiota composition, providing a potential strategy to be employed in broiler production. However, further research is needed to understand the role of individual bacteria and refine these bacterial communities for practical use in farming, thus enabling the development of natural methods to enhance poultry health and safety.