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

Authors: KODE, DIVYA - Mississippi State University; NANNAPANENI, RAMAKRISHNA - Mississippi State University; CHANG, SAM - Mississippi State University

Submitted to: Foods
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2021
Publication Date: 8/4/2021
Citation: Kode, D., Nannapaneni, R., Chang, S. 2021. Low-level tolerance to antibiotic trimethoprim in QAC-adapted subpopulations of Listeria monocytogenes. Food and Agricultural Immunology. https://doi.org/10.3390/foods10081800.
DOI: https://doi.org/10.3390/foods10081800

Interpretive Summary: Antibiotic resistance in foodborne isolates of Listeria monocytogenes is slowly increasing in many countries. To combat the proliferation of antibiotic resistance, which is a growing public health threat, this work is needed to enhance the understanding the potential underlying driving mechanisms of the development of antibiotic resistance of pathogens. Our specific objective for this work was to deduce any changes against antibiotic trimethoprim resistance in Listeria monocytogenes. Our work presented evidence for low-level tolerance to one of the alternate antibiotics used for listeriosis treatment in some L. monocytogenes strains when exposed to sublethal doses of Quaternary Ammonium Compound (QAC), a widely used bacteria sanitizer in the food industries and hospitals. While we did not find the clinical level of resistance to trimethoprim to be high enough to compromise the treatment of human listeriosis, a low-level tolerance to trimethoprim in QAC-adapted subpopulations may raise the possibility of a future acquisition of resistance by L. monocytogenes. Therefore, our work may lead to early detection of low-level antibiotic tolerance in food isolates of L. monocytogenes induced by QAC-adaptation based on early growth dynamics. Also, these experimental models are useful in developing early detection methods for tracking the slow emergence of antibiotic tolerant strains through the food supply chains. This information is useful for improving the sanitation steps and to eliminate safety concerns by preventing and mitigating the formation of low-level antibiotic tolerant strains if persisting in some food production and food processing environments.

Technical Abstract: Between January and July 2021, there were as many as 30 recalls in the U.S. due to potential Listeria monocytogenes contamination from a variety of food products including muffins, kimchi, chicken salad, ready-to-eat chicken, smoked fish, mushrooms, queso fresco cheese, ice cream, turkey sandwiches, squash, and other foods. A contaminated food chain can serve as a potential vehicle for transmitting antibiotic resistant bacteria since there is a slow emergence of multi-drug antibiotic resistance in L. monocytogenes. Biocides are essential for safe food processing, but they may also induce unintended selective pressure at sublethal doses for the expression of antibiotic resistance in L. monocytogenes. To better understand the sources of such slow emergence of antibiotic resistance through biocide residues present in the food environments, we are working on the role of sublethal doses of commonly used biocides in defined broth and water models for understanding L. monocytogenes adaptation. We recently published the development of low-level tolerance to fluoroquinolone antibiotic ciprofloxacin in quaternary ammonium compound (QAC) adapted subpopulations of L. monocytogenes (Microorganisms 9, 1052). Of the six different antibiotics tested to determine heterologous stress adaptation in eight strains of L. monocytogenes, trimethoprim was the second one that exhibited low-level tolerance development after continuous exposure (by three approaches) to sublethal concentrations of QAC against actively growing planktonic cells of L. monocytogenes. When adapted to daily cycles of fixed or gradually increasing sublethal concentrations of QAC, we observed three main findings in eight L. monocytogenes strains against trimethoprim: (a) 3 of the 8 strains exhibited significant increase in short-range minimum inhibitory concentration (MIC) of trimethoprim by 1.7 to 2.5 fold in QAC-adapted subpopulations compared to non-adapted cells (p < 0.05); (b) 2 of the 8 strains exhibited significant increase in growth rate in trimethoprim (optical density (OD) by 600 nm at 12 h) by 1.4 to 4.8 fold in QAC-adapted subpopulations compared to non-adapted cells (p < 0.05); and (c) 5 of the 8 strains yielded significantly higher survival by 1.3-to-3.1 log CFU/mL in trimethoprim in QAC-adapted subpopulations compared to the non-adapted control (p < 0.05). However, for 3/8 strains of L. monocytogenes, there was no increase in the survival of QAC-adapted subpopulations compared to non-adapted control in trimethoprim. These findings suggest the potential formation of low-level trimethoprim tolerant subpopulations in some L. monocytogenes strains where QAC may be used widely. These experimental models are useful in developing early detection methods for tracking the slow emergence of antibiotic tolerant strains through food chain. Also, these findings are useful in understanding the predisposing conditions leading to slow emergence of antibiotic resistant strains of L. monocytogenes in various food production and food processing environments