Phenotypic and Genotypic Analysis of Antibiotic Resistance of Listeria monocytogenes Isolated from Food Products

Summary

Introduction: The gram-positive bacterium Listeria monocytogenes is considered as an important foodborne pathogen. Although the antibiotic resistance of L. monocytogenes is not as widespread as in some other food-borne pathogens it is important to monitor the antimicrobial susceptibility and presence of resistance genes during surveillance. In addition, little is known about L. monocytogenes antimicrobial resistance from food isolates.
Purpose: The purpose of this study was to determine the phenotypic antibiotics resistance pattern of L. monocytogenes isolates from various food products and the presence of antibiotic resistance genes by whole genome sequencing.

Methods: A total of 391 L. monocytogenes isolates was isolated from from a wide range of food products from 2015 to 2017 using ISO 11290-1. Susceptibility testing was performed with broth microdilution according to ISO 20776-1:2006 in Sensititre panel EUVENC. Whole genome sequencing was performed using Nextseq sequencing. Reads were assembled using ABYSS, and Blast was used in combination with the Resfinder database to identify the acquired antimicrobial resistance genes.

Results: Only seven different isolates displayed phenotypic and/or genotypic antibiotic resistance (two percent). In three isolates the displayed phenotypic antibiotic resistance to tetracycline and/or erythromycin matched with the presence of resistance genes erm(B), tet(L), tet(M) revealed by WGS. In two isolates only phenotypic resistance and in three isolates only genotypic resistance was observed. Whole genome sequencing showed to be a useful tool to analyze antibiotic resistance genes in Listeria monocytogenes.

Significance: These data show that antibiotic resistance in L. monocytogenes is not commonly present. However, regular surveillance should be maintained for detecting any shift in the antimicrobial resistance of L. monocytogenes isolated from food.