Cross-resistance between halquinol and antibiotics of importance in human and animal health.

The objective of this research was to assess whether halquinol, a zootechnical antimicrobial, can lead to cross-resistance to antibiotics of importance in human and animal health. The minimum inhibitory and bactericidal concentrations of halquinol against 37 Salmonella and Escherichia coli strains/isolates were determined, along with their resistance profiles against 14 antibiotics used in human and veterinary medicine. Sublethal exposure to halquinol was performed to evaluate the acquisition of resistance to the compound by determining new minimum inhibitory and bactericidal concentrations. Cross-resistance to other antibiotics was examined by establishing a new resistance profile after sublethal exposure and comparing it with the previous results. Resistance to halquinol was successfully induced; prior to sublethal exposure, concentrations ranging from 18.25 to 300 µg/mL were required for bacterial inhibition, whereas following sublethal exposure, these values increased to 75 to 1200 µg/mL. The induction of halquinol resistance also impacted resistance to human and veterinary antibiotics. For example, before sublethal exposure to halquinol, 71.4% of Salmonella isolates were resistant to cephalexin; after sublethal exposure, 100% of the isolates exhibited resistance. In E. coli, the percentage of azithromycin-resistant isolates increased from 66.7 to 100% following sublethal exposure. These findings indicate that halquinol, in addition to inducing resistance to itself, may also promote resistance to essential drugs in both human and veterinary medicine. Further studies, particularly molecular investigations, are necessary for a comprehensive characterization of the observed in vitro effects and to determine whether these findings are replicated in vivo.