Molecular characterization and antimicrobial susceptibility of Listeria monocytogenes isolated from foods and humans

• A total of 120 Listeria monocytogenes isolates in Shanghai, China, were obtained.
• 13 isolates were resistant to ceftriaxone and seven to tetracycline.
• Three clinical isolates shared the same PFGE patterns with retail meet isolates.
• Food of animal origin serves as a major vehicle in transmitting human listeriosis.

A total of 120 Listeria monocytogenes isolates (107 from foods and 13 from humans) in Shanghai, China, were characterized by serogroup typing and virulence genes detection with PCR, antimicrobial susceptibility testing, and molecular subtyping using pulsed-field Gel Electrophoresis (PFGE). Isolates belonged to three L. monocytogenes serogroups, 1/2c, 3c (n = 47, 39.1%) was the most prevalent, followed by 1/2a, 3a (n = 44, 36.7%) and 1/2b, 3b, 7 (n = 29, 24.2%). Although the isolates were all susceptible to most antibiotics tested, 13 isolates were resistant to ceftriaxone and seven to tetracycline. None of the isolates were resistant to multiple antibiotics. The tet (M) genes were detected among tetracycline-resistant isolates. The isolates all harbored virulence genes hly, prfA, plcA, plcB, inlA, inlB, inlI; 116 isolates (96.7%) were positive for inlC; 78 (65.0%) for inlG; 119 (99.2%) for inlJ; 8 (6.67%) for llsX. A total of 74 distinct patterns were generated for the 119 isolates tested using ApaI and AscI with Pulsed Field Gel Electrophoresis (PFGE). Three clinical isolates shared the same PFGE patterns with retail meet (chicken, duck, and beef) isolates. These findings indicated that food of animal origin likely serve as a major vehicle in transmitting human listeriosis in Shanghai. Active surveillance for L. monocytogenes is needed for better understanding the epidemiology of the pathogen, and for effective control and prevention of human listeriosis.