Surveillance of stewed meat products in Shanghai, China: Importance of controlling processing environment contamination for Listeria monocytogenes

Human listeriosis is a foodborne disease caused by Listeria monocytogenes contamination. Most cases occur due to consumption of ready-to-eat (RTE) foods. Stewed meat in seasoning, a type of pre-packaged ready-to-eat (RTE) food, is a popular traditional product in China. We used whole genome sequencing to analyze core genome multilocus sequence typing (cgMLST), serogroups and multilocus typing sequence typing (ST) of 67 L. monocytogenes isolates. The L. monocytogenes serogroup IIb-ST5 was predominantly isolated from different sources and linked to stewed meat in seasoning of Brand A, B, and C with <7 allelic differences by cgMLST, suggesting that this kind of food may be a high-risk food causing sporadic cases of listeriosis. Furthermore, L. monocytogenes isolates from different batches of stewed meat in seasoning products processed in the same processing plant had <5 five allelic differences, confirming that contamination occurred during manufacture. Additionally, 16 L. monocytogenes isolates from Plant A were the same clone (0 allelic difference), which confirmed the strong ability of L. monocytogenes to survive in the processing environment. Thus, increased and frequent monitoring, as well as constant technical support for the control of L. monocytogenes in food processing plants is necessary.