Persistence of Listeria monocytogenes strains in a frozen vegetables processing plant determined by serotyping and REP-PCR

Seventy presumptive Listeria monocytogenes strains isolated from frozen vegetable products in a processing plant during a period of six months were characterized by serology, biochemical tests (API system), multiplex PCR, virulence characteristics and REP-PCR analysis. Most isolates belonged to 1/2a (62 strains) and 1/2b (3 strains) serotypes, the most common isolated from foods. Amplification of virulence gene lmo2821 was positive in 51 of the PCR-confirmed L. monocytogenes strains. Automated REP-PCR of L. monocytogenes isolates yielded 17 different patterns, formed by 70 to 80 different bands ranging from 150 to 7000 bp. A total of three genetic groups were defined at 82% homology degree. At this level, serovars 4b and 1/2b strains were discriminated from the rest of strains. Two REP-PCR patterns were frequently found for isolates sharing the same serotype and biochemical profile. These isolates were obtained from different vegetables during the sampling period, what clearly suggests the persistence of some L. monocytogenes strains in the processing plant. While the multiplex PCR assay applied in this work provided an accurate and rapid method for species identification, REP-PCR was confirmed as a rapid and reliable method for L. monocytogenes subtyping, providing useful information for epidemiological or risk assessment studies, as well as for tracking surveys in food processing plants.
This work was supported by grant AGL2008-05275-C03-02 from Ministerio de Ciencia e Innovacion, Spain.