Abstract
BACKGROUND: The widespread increase of antibiotic resistant bacteria (ARB) in agriculture has led to an interest in improving the competitive exclusion ability of the microbiota in livestock through the application of probiotics. In this study, we examined a collection of gram-negative bacteria isolated from healthy chickens for their ability to inhibit the growth of Salmonella enterica serovar Typhimurium. METHODS: Twenty-five bacteria isolated from healthy chickens were mixed at a 1:1 or a 10:1 ratio with S. enterica ser. Typhimurium and grown overnight in co-culture on a solid media that contained bile salt. The S. enterica in each co-culture was quantified using on Salmonella Shigella Agar (SSA) to determine which isolates can inhibit the growth of S. enterica in vitro. Isolates that showed inhibitory action against S. enterica ser. Typhimurium were further analyzed by whole genome sequencing (WGS) to identify potential mechanisms. The WGS analysis, included using a variety of tools to identify potential mechanisms of S. enterica ser. Typhimurium antagonism. RESULT: Three bacterial isolates: Alcaligenes faecalis Ae-14, Citrobacter braakii J-D0-20, and Escherichia ruysiae J-D0-44 significantly reduced the growth of S. enterica ser. Typhimurium in co-culture. Growth inhibition was most pronounced during the exponential growth phase. Sequence analysis revealed that the potential mechanisms of S. enterica ser. Typhimurium inhibition by C. braakii J-D0-20 and E.ruysiae J-D0-44 could be through T6SS-mediated competition, bacteriocin (carocin D) production, having a similar quorum sensing system, or by competition for similar nutrients. The potential antagonistic mechanisms for A. faecalis Ae-14 were unclear. CONCLUSION: Some bacteria, found naturally in the fecal material of healthy chickens, are capable inhibiting the growth of S. enterica ser. Typhimurium in co-culture and should be further investigated as potential probiotic prophylactics to provide protection against S. enterica ser. Typhimurium infections.