Epidemiological investigation and antimicrobial resistance profiling of Salmonella in broiler chickens with assessment of Lactococcus lactis probiotic therapy

Salmonella infection in broiler chickens poses a significant threat to poultry health and food safety, especially with the rise of multidrug-resistant strains. This work assessed the prevalence, antimicrobial resistance profiles, and resistance genes of Salmonella isolates from 30 broiler farms, and evaluated the protective effects of probiotics against Salmonella challenge. Salmonella spp. were isolated from 30% (9/30) of farms, with Salmonella Enteritidis accounting for 60% (15/25) and Salmonella Give for 40% (10/25) of the recovered isolates. Mortality rates were 10.13±0.89% in Salmonella Enteritidis-infected flocks and 7.13±1.44% in those infected with Salmonella Give. Antimicrobial susceptibility testing revealed 100% resistance to spectinomycin, tigecycline, amoxicillin-clavulanic acid, cefoxitin, cefalexin, and cefazolin; 92% to ampicillin and cefotaxime; 80% to sulphatrimethoprim; 72% to chloramphenicol; 60% to nalidixic acid; 36% to colistin; and 40% to enrofloxacin. The most effective agents were ofloxacin (68% sensitive) and ciprofloxacin (60% sensitive). PCR analysis detected the blaDHA gene in 72% (18/25) and the mcr-1 gene in 44% (11/25) of isolates. Lactococcus lactis IL403 demonstrated strong adhesion to Caco-2 cells, intracellular internalization in macrophages, and significantly reduced LPS-induced TNF-α release. In vivo experiments showed that Lactococcus lactis IL403 provided partial protection, lowering mortality to 2/50 birds (4%) and liver colonization to 3/7 birds (42.85%) in treated, challenged birds. In contrast, the Salmonella Give challenged group without probiotic treatment exhibited higher mortality (7/50; 14%) and complete liver colonization (7/7; 100%). These results highlight the high prevalence of multidrug-resistant Salmonella in broilers and support the potential of probiotics as alternative preventive strategies in poultry production.