Characterization of plasmids from Listeria monocytogenes and Listeria innocua strains isolated from short-ripened cheeses.

The plasmid content of 30 isolates of Listeria monocytogenes and 18 isolates of Listeria innocua obtained from short-ripened cheeses was analysed. The isolates of L. monocytogenes serogroup 1 harboured a single plasmid, pLM33 (33.2 kbp), whereas the serogroup 4 isolates did not contain plasmids. One group of L. innocua strains harboured the plasmid pLI71 (71 kbp) and another one contained two plasmids: pLI59 (59.5 kbp) and pLI56 (56.5 kbp). These plasmid groups were in accordance with clusters previously defined by pulsed-field gel electrophoresis analysis of the chromosomal DNA of Listeria isolates. Plasmids pLM33, pLI71 and pLI59 shared homology regions of at least 20 kbp. Plasmid pLI56 did not encode genes for any known character (such as carbohydrate fermentation, resistance to antibiotics, heavy metals or disinfectants, growth at low pH, NaCl tolerance or thermal inactivation by pasteurisation) and displayed different characteristics to the other three plasmids. It was also the only one cured from the parent strain and the sole plasmid not digested by the restriction enzyme PstI. In addition, its lack of homology with pLM33, pLI71 and pLI59 enhanced the possibility of a different origin for plasmid pLI56.