Molecular Characterization of Extended-Spectrum ß-Lactamases-Producing Escherichia coli Isolated from a Greek Food Testing Laboratory

Background/Objectives: Over the past decade, extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli have emerged as a significant public health concern due to their potential to spread beyond clinical settings and healthy carriers, in various environments, including in animal source foods. This study seeks to investigate the molecular characteristics of ESBL-producing E. coli strains isolated from food of animal origin, with a focus on chromosomal typing, plasmid typing, and the description of the associated resistance genes’ genetic environment. Methods: Ninety-seven food of animal origin samples were tested for E. coli isolates resistant to cefotaxime. The resulting isolates were then subjected to antimicrobial susceptibility testing and PCR analysis to detect beta-lactamase genes. Additional assays, encompassing mating-out procedures, molecular typing utilizing Pulsed-Field Gel Electrophoresis, Multilocus Sequence Typing Analysis, and Oxford Nanopore Technology Lite whole plasmid sequencing, were also conducted. Results: E. coli was detected in 26 raw food specimens, generating a percentage of 27%. Fourteen of the current isolates (14%) were resistant to third generation cephalosporins, producing CTX-M-1, CTX-M-15, CTX-M-55, and SHV-12 beta-lactamases. The respective genes were accompanied by Insertion Sequences ISEcp1 and IS26, facilitating their transfer. Among plasmids harboring ESBL genes, representatives belonging to incI1 incompatibility group prevailed (5/8), followed by IncY and IncX3. Most plasmids proved conjugative. Diversity of molecular fingerprints of ESBL producing E. coli was revealed. Conclusions: To the best our knowledge, this study is the first to describe the molecular characteristics of E. coli isolates producing ESBLs sourced from foods of animal origin in Greece. The prevalence of ESBLs in our confined food collection is primarily associated with the very successful IncI1 plasmids, which were not linked to a specific E. coli genetic background. This lack of association confirms that horizontal plasmid transfer plays a more significant role than clonal dissemination in the spread of ESBL-mediated cephalosporin resistance.