Animal-Use Antibiotics Induce Cross-Resistance in Bacterial Pathogens to Human Therapeutic Antibiotics

Exposure of bacteria to a sub-lethal dosage of antibiotic is one the major causes for the onset of antibiotic resistance. Therefore, we aimed to assess the emergence of antibiotic cross-resistance in bacteria after exposure to a sub-lethal dose of veterinary feed directive (VFD) antibiotics, tilmicosin, and florfenicol. The minimum inhibitory concentrations (MICs) of tilmicosin and florfenicol against Salmonella enterica serovar Enteritidis, Klebsiella pneumoniae, Staphylococcus aureus, and Listeria monocytogenes were determined. Next, the pathogens were exposed to a sub-inhibitory concentration of tilmicosin (0.5, 5, 20 µg/ml) and florfenicol (1, 20 µg/ml) for 24 h and 48 h, and acquired cross-resistance to human therapeutic antibiotics was measured by determining the increase in MIC values. MICs of ampicillin, tetracycline, nalidixic acid, and meropenem against Salmonella and Klebsiella were in the range of 20-1000 µg/ml, 5-62.5 µg/ml, 5-125 µg/ml, and 0.05-0.1 µg/ml, respectively, whereas MICs against Staphylococcus and Listeria were 2.5-10 µg/ml, 2.5 µg/ml, 62.5-500 µg/ml, and 0.1-0.2 µg/ml, respectively. Pre-exposure of these bacteria to a sub-inhibitory concentration of tilmicosin and florfenicol, increased cross-resistance against ampicillin, tetracycline, and nalidixic acid from 1.25- to 40-fold compared to the antibiotic unexposed bacteria with the exception of meropenem, which did not show increased resistance. This study could serve as a foundation to understand the mechanisms of acquired cross-resistance to traditional therapeutic antibiotics, and to develop strategies to alleviate such problem by using alternative antimicrobials.