Clindamycin susceptibility and virulence characterization of Listeria monocytogenes strains isolated from meat and meat-processing environments

Listeria monocytogenes is a major foodborne pathogen widely distributed in food-processing environments and is considered intrinsically resistant to clindamycin. The aim of this study was to investigate the relationship between clindamycin susceptibility and virulence in Listeria monocytogenes strains isolated from meat and meat-processing environments belonging to different clonal complexes (CCs). Clindamycin susceptibility in L. monocytogenes strains was phenotypically and genotypically characterized by disk diffusion method, minimum inhibitory concentration (MIC) determination, and detection of resistance-associated genes. Whole-genome sequencing was used to compare strains with different susceptibility profiles and to explore the relationship between clindamycin resistance and virulence factors. Among 62 strains analyzed, 22.6% were phenotypically susceptible, showing MIC values between 0.25 and 0.5 μg/mL. The clindamycin resistance gene vgaG was detected in both resistant and susceptible strains, with a potentially deleterious mutation identified in susceptible CC155 strains. All genomes harbored the additional intrinsic antibiotic resistance genes fosX, norB, mprF, and sul. The clindamycin susceptible strains also harbored truncated or impaired virulence genes, including prfA, inlA, inlB, actA, and plcA, which correlated with the absence of phosphatidylinositol-specific phospholipase C activity on CHROMagar™ Listeria. Premature stop codons were detected in inlA in strains belonging to CC31 and CC155, as well as a truncated variant of prfA in CC31 strains, both alterations associated with hypovirulent phenotypes. Overall, these findings provide new insights into the interplay between antimicrobial resistance and virulence in L. monocytogenes, and highlight the relevance of genomic surveillance in food-processing environments.