Administration route and trial repetition shape the effects of a commercial synbiotic on broiler production performance, cecal microbiota and pathogen colonization.

Campylobacter and Salmonella are leading causes of foodborne bacterial enteritis, with poultry meat being an important source. This longitudinal field study evaluated the impact of the commercial synbiotic PoultryStar, administered via water or feed, on broiler production performance, gut colonization with Campylobacter jejuni and Salmonella Infantis, and cecal microbiota. Two large food business operators were included in the study, with two independent trials conducted at each. A significantly higher European efficiency factor was observed when synbiotic was supplemented in water rather than in feed, although the effect did not differ from the control group. Body weight also tended to increase with synbiotic supplementation in water, although the effect was inconsistent across farms. Feed supplementation significantly reduced colonization of broilers with S. Infantis, whereas no effect was observed regarding C. jejuni. However, two bacterial taxa potentially contributing to the colonization resistance against C. jejuni were identified, belonging to the genus Lactobacillus and an unclassified representative of Bacteroidota. Administration of the synbiotic significantly influenced cecal microbiota, with outcomes depending on the administration route but, importantly, varying significantly between food business operators and even trial repetitions. Overall, PoultryStar exerted limited effects on pathogen control and production performance, while external factors such as administration route and trial repetition strongly modulated outcomes. Our findings highlight the need for repeated, farm-level evaluations and tailored probiotic strategies to optimize broiler health.