Prevalence, Genotypic Characteristics, and Antibiotic Resistance of Listeria monocytogenes From Retail Foods in Huzhou, China.

Listeria monocytogenes are considered to be the major foodborne pathogen worldwide. To understand the prevalence and potential risk of L. monocytogenes in retail foods, a total of 1243 retail foods in 12 food categories were sampled and screened for L. monocytogenes from 2020 to 2022 in Huzhou, China. A total of 46 out of 1234 samples were confirmed to be L. monocytogenes positive with a total rate of 3.7%. The contamination rate of seasoned raw meat (15.2%) was the highest, followed by raw poultry meat and raw livestock meat (9.9%) and salmon sashimi (9.5%). The L. monocytogenes isolates belonged to four serotypes, 1/2a,1/2b, 1/2c, and 4b, with the most prevalent serotype being 1/2a (47.9%). All isolates were grouped into 15 sequence types (STs) belonging to 14 clonal complexes (CCs) via multilocus sequence typing (MLST). The most prevalent ST was ST9/CC9 (23.9%), followed by ST3/CC3 (19.6%) and ST121/CC121 (17.4%). Notably, 11 STs were detected from ready-to-eat (RTE) foods, some of them have been verified to be strongly associated with clinical origin listeriosis cases, such as ST3, ST2, ST5, ST8, and ST87. Listeria pathogenicity islands 1 (LIPI-1) and LIPI-2 were detected in approximately all L. monocytogenes isolates, whereas the distribution of both LIPI-3 genes and LIPI-4 genes exhibited association with specific ST, with LIPI-3 in ST3 and ST288, and LIPI-4 in ST87. The strains carrying LIPI-3 and LIPI-4 virulence genes in this study were all isolated from RTE foods. Antimicrobial susceptibility tests showed that >90% of isolates were susceptible to PEN, AMP, ERY, CIP, SXT, VAN, CHL, and GEN, indicating the antibiotic treatment might be still efficient for most of the L. monocytogenes strains. However, for the three clinical first-line antibiotics (PEN, AMP, and GEN), we also observed three and four strains showing MIC values greater than the susceptibility standards for PEN and AMP, respectively, and one strain showing resistance to GEN.