Characterization of CRISPR Loci and Antimicrobial Resistance in Foodborne Listeria monocytogenes Isolates

Clustered regularly interspaced short palindromic repeats (CRISPR) are widespread in bacterial and archaeal genomes as an adaptive immune system against invading mobile genetic elements. This study investigated the distribution of CRISPR loci and their potential association with antimicrobial resistance (AMR) in 40 foodborne Listeria monocytogenes isolates. CRISPR analysis showed that 18 isolates harbored CRISPR Locus 1, five carried Locus 2, and five possessed both loci. Antimicrobial susceptibility testing against seven antimicrobial agents indicated that most isolates were highly susceptible to the tested agents. Specifically, all isolates were susceptible to gentamicin, ampicillin, penicillin, and tetracycline, whereas resistance was observed in a small subset of isolates: four were resistant to chloramphenicol, five to levofloxacin, and four to ciprofloxacin. Statistical analysis showed no statistically significant association between the presence of CRISPR loci and antimicrobial susceptibility phenotypes. These findings provide baseline information on CRISPR locus distribution and antimicrobial susceptibility profiles in foodborne L. monocytogenes isolates. Further studies based on larger isolate collections, whole-genome sequencing, and characterization of associated cas genes are needed to clarify the potential role of CRISPR-Cas systems in AMR evolution in this species.