Sessions

Antimicrobial resistance (AMR) is one of the most serious animal and public health threats of our time, spreading rapidly worldwide and directly endangering the future effectiveness of antibiotics. Resistance development also affects the efficacy of fluoroquinolones, widely used in poultry farming, including enrofloxacin, which not only causes economic losses but also poses a zoonotic risk through the food chain. Some studies have pointed out that bacterial adaptation and resistance mechanisms are closely linked to oxidative stress and cellular stress responses. This recognition has opened new opportunities for the application of naturally derived compounds, such as antioxidants, which may influence the development of resistance. However, the presence of antioxidants can affect the effectiveness of antibiotics in both positive and negative ways, and thus their application requires careful consideration.The aim of our research was to investigate the extent to which two naturally derived antioxidants - quercetin and fumaric acid - can influence the development of resistance to enrofloxacin in Escherichia coli strains isolated from poultry. Furthermore, we also examined whether the efficacy of enrofloxacin changes when applied in combination with the individual antioxidants.For our studies, we used an Escherichia coli strain derived from broiler chickens. Using the microdilution method, we determined the minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC) values of the antioxidants, as well as evaluated the interaction of the antioxidants with enrofloxacin by calculating the fractional inhibitory concentration index (FIC-index). Resistance development was modelled through ten consecutive passages, after which DNA was extracted from the samples, subjected to whole-genome sequencing, and mutation analysis was performed to identify genetic changes compared to the pre-passage state.Our results show that the antioxidants did not negatively influence the effect of enrofloxacin. Next-generation sequencing results highlighted that, in the presence of antioxidants, bacteria may employ alternative strategies, such as stress response and membrane changes, instead of classical target enzyme modifications. Overall, our findings support that quercetin and fumaric acid may offer promising complementary strategies for preserving the efficacy of enrofloxacin in the poultry sector. While antioxidants do not replace antibiotics, they can contribute to slowing the spread of resistance as complementary therapy or feed additives, thereby enhancing food chain safety. Our results open new perspectives in exploring the combined application of natural molecules and antibiotics and may contribute to the development of more sustainable veterinary practices.