Stress resistance, motility and biofilm formation mediated by a 25kb plasmid pLMSZ08 in Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen that can form biofilm and tolerate severe environment condition and thus persist in food product for a prolonged period, which poses a significant risk to food safety. While some chromosomal genetic determinants associated with stress resistance and biofilm formation have been characterized, plasmid-mediated stress resistance in particular L. monocytogenes strains are less known. In this study, we showed a L. monocytogenes strain SZ08 (serovar 1/2b) isolated from a raw meat processing environment harbored a 25kb plasmid, designated as pLMSZ08. After incubation under the selective condition containing novobiocin, pLMSZ08 was successfully cured from SZ08. Compared to its parental strain SZ08, the cured strain SZ08* was not able to grow well under the high concentration of sodium chloride (5.5%). SZ08* displayed higher efficiency of plaque formation compared with its parental strain SZ08. There were no significant growth difference under different temperature and pH conditions between SZ08 and SZ08*. Biofilm formation of SZ08* reduced significantly compared with SZ08 after 48hr incubation. Furthermore, the relative expression levels of flagella-associated genes in SZ08* were up-regulated comparing to those in SZ08. We have sequenced the pLMSZ08 and the sequencing results showed that pLMSZ08 harbors genes associated with transcriptional regulation, metal/drug transport, histidine kinase and ATPase. These results suggested that these genes encoded by pLMSZ08 are associated with tolerance, motility and biofilm formation in L. monocytogenes SZ08.