Diversity of Listeria monocytogenes isolates of human and food origin studied by serotyping, automated ribotyping and pulsed-field gel electrophoresis

Automated ribotyping, pulsed-field gel electrophoresis (PFGE) and serotyping were evaluated for the epidemiological study of isolates of Listeria monocytogenes collected in Finland in 1997-1999 from human blood (n = 116) and the food industry (n = 72). The isolates divided into six serotypes, 23 EcoRI ribotypes, 54 AscI PFGE types, and 57 final subtypes if all results were combined. The discrimination index of ribotyping was lower (0.873) than that of PFGE (0.946). Two final subtypes dominated among human isolates, and identical subtypes were also found among food industry isolates. All PFGE types were serotype-specific, whereas two ribotypes included isolates of two serotypes. Isolates of serotype 3a, involved in an outbreak in Finland in 1999, matched one of these ribotypes, which also included some food industry isolates of serotype 1/2a. Ribotyping with EcoRI would not have been sufficient to define the outbreak in Finland caused by serotype 3a isolates. Although ribotyping is applicable as the first method in outbreak situations, human and food isolates with identical ribotypes should be investigated further by PFGE.