Low-Level Tolerance to Fluoroquinolone Antibiotic Ciprofloxacin in QAC-Adapted Subpopulations of Listeria monocytogenes

There was a development of low-level tolerance to fluoroquinolone antibiotic ciprofloxacin in Listeria monocytogenes after sublethal adaptation to quaternary ammonium compound (QAC). Using eight L. monocytogenes strains, we determined the changes in short-range MIC, growth rate, and survival for heterologous stress response to ciprofloxacin, after sublethal exposure to daily cycles of fixed or gradually increasing concentration of QAC. Three main findings were observed. (1) MIC increase—QAC-adapted subpopulations exhibited a significant increase in short-range MIC of ciprofloxacin, by 1.5 to 2.9 fold, as compared to non-adapted control for 4/8 strains (p < 0.05). (2) Growth rate increase—QAC-adapted subpopulations exhibited significant 2.1- to 6.8- fold increase in growth rate (OD(600) at 10 h) in ciprofloxacin-containing broth, as compared to non-adapted control for 5/8 strains (p < 0.05). (3) Survival increase—QAC-adapted subpopulations of L. monocytogenes yielded significantly higher survival in ciprofloxacin-containing agar by 2.2 to 4.3 log CFU/mL for 4/8 strains, as compared to non-adapted control (p ˂ 0.05). However, for other 4/8 strains of L. monocytogenes, there was no increase in survival of QAC-adapted subpopulations, as compared to non-adapted control in ciprofloxacin. These findings suggest the potential formation of low-level ciprofloxacin-tolerant subpopulations in some L. monocytogenes strains when exposed to residual QAC concentrations (where QAC might be used widely) and such cells if not inactivated might create food safety risk.