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 is olates of Listeria innocua obtained from short-ripened cheeses was ana lysed. The isolates of L. monocytogenes serogroup 1 harboured a single plasmid, pLM33 (33.2 kbp), whereas the serogroup 4 isolates did not c ontain plasmids. One group of L. innocua strains harboured the plasmid pLI71 (71 kbp) and another one contained two plasmids: pLI59 (59.5 kb p) and pLI56 (56.5 kbp). These plasmid groups were in accordance with clusters previously defined by pulsed-held gel electrophoresis analysi s of the chromosomal DNA of Listeria isolates. Plasmids pLM33, pLI71 a nd pLI59 shared homology regions of at least 20 kbp. Plasmid pLI56 did not encode genes for any known character (such as carbohydrate fermen tation, resistance to antibiotics, heavy metals or disinfectants, grow th 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 pl asmid not digested by the restriction enzyme PstI. In addition, its la ck of homology with pLM33, pLI71 and pLI59 enhanced the possibility of a different origin for plasmid pLI56. (C) 1998 Elsevier Science B.V.