Antimicrobial-Resistant Listeria Species from Retail Meat in Metro Detroit

Abstract

A total of 138 Listeria isolates from retail meat, including 58 Listeria welshimeri, 44 Listeria monocytogenes, and 36 Listeria innocua isolates, were characterized by antimicrobial susceptibility tests against nine antimicrobials. In addition, the 44 L. monocytogenes isolates were analyzed by serotype identification using PCR and genotyping using pulsed-field gel electrophoresis. Resistance to one or two antimicrobials was observed in 32 Listeria isolates (23.2%). No multidrug resistance was identified. Tetracycline resistance was the most common resistance phenotype and was identified in 22 Listeria isolates. A low prevalence of resistance to ciprofloxacin, erythromycin, gentamicin, and vancomycin was also detected. L. innocua isolates demonstrated the highest overall prevalence of antimicrobial resistance, 36.1%, followed by 34.1% in L. monocytogenes isolates and 6.9% in L. welshimeri isolates. Serotypes 1/2a, 1/2b, and 4b were identified in 19, 23, and 1 L. monocytogenes isolate, respectively. One isolate was untypeable. Fifteen L. monocytogenes isolates were antimicrobial resistant (12 were serotype 1/2b, 2 were 1/2a, and 1 was untypeable). A diverse population of L. monocytogenes isolates was identified, as evidenced by multiple pulsed-field gel electrophoresis patterns in the 44 isolates. The data indicate that Listeria contamination is common in retail meat. Although antimicrobial resistance still occurs at a low prevalence, multiple Listeria species can serve as reservoirs of antimicrobial resistance. Various antimicrobial susceptibilities may exist in L. monocytogenes isolates of different serotypes.