Characterization and evaluation of some virulence markers of Listeria monocytogenes strains isolated from Brazilian cheeses using molecular, biochemical and serotyping techniques.

A total of 207 L. monocytogenes strains isolated from different types of cheeses commercialized in the city of Rio de Janeiro, Brazil, were serotyped and evaluated for their ability to produce beta-haemolysin and lecithinase and to adsorb Congo red dye. Of the 207 strains, 59.9, 27.5 and 12.6% belonged to serotypes 1/2a, 1/2b and 4b, respectively. In addition, 175 strains of L. monocytogenes produced lecithinase while strains of the other species did not. Some of the non-L. monocytogenes strains adsorbed the dye Congo red, while some L. monocytogenes did not. Statistical analysis of the results showed significant differences (P < 0.05) amongst the virulence tests and the three serotypes found. In the present study, 32 L. monocytogenes strains were also analyzed by RAPD (randomly amplified polymorphic DNA). RAPD analysis allowed the discrimination among strains of different serotypes, as well as among strains of the same serotype. It is important to emphasize that the use of more than one primer is needed for characterization of L. monocytogenes strains. With RAPD the strains were grouped into six different profiles, some of them common for strains belonging to different serotypes. The results also indicated a close genetic relationship among strains of different serotypes.