Occurrence and antimicrobial susceptibility of Listeria monocytogenes isolates from retail raw foods

The occurrence and counts of Listeria monocytogenes were investigated in a total of 526 retail raw food samples. All L. monocytogenes isolates were further analyzed by serotyping and antimicrobial susceptibility assays. The molecular basis of tetracycline resistance of each isolate and the genetic relatedness were determined. L. monocytogenes isolates were found in 12.4% (65/526) of the samples, with counts below 102 CFU/g. L. monocytogenes was most commonly isolated from pork (20%, 20/100), seafood (13.8%, 15/109), chicken (13.2%, 14/106), and beef (10.3%, 11/107). In addition, L. monocytogenes was also detected in 4.8% (5/104) of raw mutton samples. Four serogroups were identified among the 65 L. monocytogenes isolates, with serogroups 1/2a-3a (60%) and 4b-4d-4e (24.6%) being dominant. Most L. monocytogenes isolates were resistant to cefotaxime (54.6%), fosfomycin (51.5%), and clarithromycin (36.4%). Some isolates showed intermediate resistance to streptomycin (12.1%), norfloxacin (13.6%), ciprofloxacin (13.6%), and nitrofurantoin (9.1%). Multiple resistances were observed in 72.3% of isolates. Genetic relatedness analysis revealed that there were no prominent associations between specific food types, serotypes, antimicrobial susceptibility profiles and Pulsed-field gel electrophoresis (PFGE) patterns. In addition, these isolates were multiresistant and belonged to the epidemiologically important serotypes 1/2a and 4b, implying a potential public health risk.

► Listeria monocytogenes were detected in each food categories tested. ► All isolates were subjected to serotyping and antimicrobial susceptibility. ► Genetic relatedness of these isolates was analyzed. ► Epidemiologically important serotypes and multiresistances were found. ► No associations were between food types, serotypes and PFGE patterns.