Probiotic characteristics and protective effects of chicken-derived Enterococcus faecium against infection with Avian Pathogenic Escherichia coli and Salmonella in laying hens.

Avian Pathogenic Escherichia coli (APEC) and Salmonella Enteritidis (SE) cause high mortality and economic losses in poultry. This study identified chicken-derived probiotics that can effectively combat APEC O78 and SE infection in laying hens. 7-day-old Dawu Jinfeng chicks naturally resistant to APEC O78 and SE infection were pre-screened and subjected to antibiotic treatments, revealing that Enterococcus species are crucial for maintaining resistance, as their reduction led to susceptibility and diarrhea. A total of 34 Enterococcus faecium (E. faecium) strains were isolated from the feces of resistant chicks. Three isolates (P4, C7, and 2S4) displayed in vitro probiotic potential by tolerating low pH and bile salts, inhibiting the growth of APEC O78 and SE, and lacking hemolytic activity and antibiotic resistance. In vivo, 7-day-old chicks were orally administered three doses of P4, C7, and 2S4 isolates (1 × 104 CFU/mL, 1 × 108 CFU/mL, 3 × 108 CFU/mL) daily for 28 days. E. faecium administration significantly enhanced intestinal antioxidant capacity, superoxide dismutase (SOD) activity, and glutathione peroxidase (GPx) activity (all P < 0.05), while reducing malondialdehyde levels (P < 0.05). It also elevated secretory IgA levels in the intestinal fluid (P < 0.05) and altered the gut microbiota composition by promoting beneficial genera (Parabacteroides, Barnesiella) and reducing Desulfovibrio abundance. The treatment led to a notable reduction in diarrhea and intestinal hemorrhage in chicks challenged with APEC O78 and SE. Overall, E. faecium isolates P4, C7, and 2S4 have significant potential as probiotics for preventing chick diarrhea caused by APEC and Salmonella.