pLM33 provides tolerance of persistent Listeria monocytogenes ST5 to various stress conditions and also enhances its virulence.

Listeria monocytogenes is a major foodborne pathogen. In our previous study, we found that L. monocytogenes ST5 and ST121 strains were dominant in two food plants of Shanghai. Genetic characterization revealed that the environmental tolerance of these strains was attributable to the plasmids pLM33 and pLM5578. To further evaluate the function of L. monocytogenes plasmids, we selected ST5 and ST121 wild-type strains, and used their plasmid-cured strains as controls to conduct tolerance tests. In addition, we analyzed 108 L. monocytogenes strains isolated from four major food categories in Shanghai. Our results showed that compared with the plasmid-cured strain, the ST5 strain carrying pLM33 showed higher tolerance to environmental stress conditions, including low acid, high salt, oxidizing, and high-temperature conditions; as well as higher virulence. Furthermore, we found that the plasmid carriage rate of food isolates was 97.22%, with the highest carriage rate of 68.57% for pLM5578, followed by 24.76% for pLM33. Notably, all L. monocytogenes ST5 isolates from ready-to-eat food products (n = 11) carried plasmids, suggesting that contamination of these food products may pose a serious risk to human health. In summary, the results of this study broaden our understanding regarding the role of L. monocytogenes plasmids in stress responses.