Low-level tolerance to fluoroquinolone antibiotic ciprofloxacin in QAC-adapted subpopulations of Listeria monocytogenes

Authors: KODE, D - Mississippi State University; NANNAPANENI, R - Mississippi State University; BANSA, M - Mississippi State University; CHANG, S - Mississippi State University; CHENG, W-H - Mississippi State University; SHARMA, C - Mississippi State University; KIESS, A - Mississippi State University

Submitted to: Microorganisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2021
Publication Date: 5/13/2021
Citation: Kode, D., Nannapaneni, R., Bansa, M., Chang, S.C., Cheng, W., Sharma, C.S., Kiess, A. 2021. Low-level tolerance to fluoroquinolone antibiotic ciprofloxacin in QAC-adapted subpopulations of Listeria monocytogenes. Microorganisms. 9(5):1052. https://doi.org/10.3390/microorganisms9051052.
DOI: https://doi.org/10.3390/microorganisms9051052

Interpretive Summary: In the food processing environments, even though sanitizers and disinfectants are routinely used at 50-100 times greater than that of their minimum bactericidal concentration (MBC) to kill foodborne bacterial pathogens, both planktonic cells and biofilms cells of these pathogens present in the crevices may be frequently exposed to a gradient of concentrations of biocides in the processing environments. Recent findings show that such gradual exposure to sublethal concentrations of biocides can co-select for bacterial cells that are tolerant to lethal concentrations of biocides (sanitizers). Therefore, it is important to understand the role of sublethal concentrations of biocides in the emergence of heterologous stress-response in L. monocytogenes and if it may lead to the antibiotic tolerance/resistance development. In this project, we tested three approaches for continuous exposure to sublethal concentrations of quaternary ammonium compounds (QAC, commonly used sanitizer to clean food processing equipment) against actively growing planktonic cells of L. monocytogenes and evaluated the subsequent changes in antibiotic susceptibility against ciprofloxacin by three different methods. Our findings show that there is a development of low-level tolerance to ciprofloxacin in L. monocytogenes strains after exposure to sublethal concentrations of QAC. These findings are useful in identifying the predisposing conditions for slow emergence of antibiotic (fluoroquinolone)-resistant strains of L. monocytogenes in various environments, which may create food safety risk.

Technical Abstract: 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 (OD600 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.