Two hundred strains ofListeria monoctyogenespreviously isolated from 19 soft and semi-soft cheeses by enrichment were characterized by serotyping and pulsed-field gel electrophoresis (PFGE). Strains of serogroup 1/2 predominated, with 33.5% of all strains belonging to serovar 1/2a, 58.5% to serovar 1/2b and 5% to serovar 1/2c. By using a PFGE method, 16 different clonal types were obtained. All 10 isolates ofL. monocytogenesrecovered from one white mould cheese were serovar 1/2a, but displayed two different PFGE profiles. Another 10L. monocytogenesisolates from white mould cheese recovered after a 48-h enrichment broth procedure revealed two clonal types, only one of which could be detected after 5 additional days of incubation in the same enrichment broth. Cross-contamination in dairy plants and/or retail stores may play an important role in the incidence and diversity ofL. monocytogenesclonal types recovered from soft and semi-soft cheeses. Therefore, it is necessary to characterize several isolates from the same sample when conducting epidemiological investigations.
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