Sessions

The healthy development of the gut microbiome in broiler chickens is of major economic importance. Antimicrobial agents — particularly broad-spectrum antibiotics such as enrofloxacin — can affect the proportion of beneficial microbial taxa in the gut, potentially leading to dysbiosis and, indirectly, to the emergence of antimicrobial resistance genes (ARGs). Increasing attention is being paid to alternative approaches, such as antioxidant-based feed additives, which may reduce the adverse microbiome-altering effects of antibiotic use.

The aim of our study was to assess the effects of a feed additive with antioxidant complex, both alone and in combination with enrofloxacin, on the cecal microbiome and resistome of broiler chickens. Six treatment groups were established, and cecal samples were collected at three different time points (days 14, 20, and 42) for microbiome and resistome analysis.

The results indicated that microbiome composition varied in a treatment- and time-dependent manner. Enrofloxacin treatment alone led to a reduction in the proportion of Bacteroidota and an increase in Bacillota of caecal content collected on day 20, a shift that may indicate dysbiosis. In contrast, the antioxidant treatment applied from day one, particularly when combined with enrofloxacin, resulted in a more favorable microbiome profile, with higher levels of Bacteroidota and lower levels of Pseudomonadota.

Resistome analysis revealed that the qnrB gene — associated with fluoroquinolone resistance — was detected exclusively in the enrofloxacin-only group on day 42. Glycopeptide resistance genes (vanA, vanB) were primarily associated with the combined treatment groups. Peptide and polymyxin resistance genes appeared transiently during the mid-phase of the rearing period. The presence of resistance genes was associated with microbiome status: gene emergence was more frequent under dysbiotic conditions, whereas more stable microbial profiles were linked to lower levels of ARG detection.

In conclusion, the application of an antioxidant complex — particularly when initiated from the first day — may positively influence microbiome balance and reduce the likelihood of resistance gene emergence against critical antibiotics in broiler chickens. These findings may support the development of more sustainable feeding strategies aimed at improving animal health and food chain safety.