Typing of Listeria monocytogenes Isolates Originating from the Food Processing Industry with Automated Ribotyping and Pulsed-Field Gel Electrophoresis

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Abstract
A total of 486 Listeria monocytogenes isolates originating from 17 Finnish food processing plants (representing meat, poultry, fish, and dairy production) were collected and typed by automated ribotyping using EcoRI as the restriction enzyme. The isolates were divided into 16 different ribotypes (RTs). Some of these isolates (121), representing all EcoRI types and 16 food plants, were subjected to ribotyping with the PvuII enzyme, to pulsed-field gel electrophoresis (PFGE) typing with AscI and SmaI restriction enzymes, and to serotyping with O-antigen antisera. Nineteen ribotypes were generated with PvuII, 42 macrorestriction patterns were generated with AscI and 24 with SmaI, and three serotypes were generated with antisera. When the results were combined, the overall number of RTs was 23, and that of the PFGE types was 46. Thus, the overall discrimination power of PFGE was higher (discrimination index [DI] 0.966) than that of ribotyping (DI 0.906). The most common serotype (90.1% of the isolates) was 1/2, and isolates of serotype 4 (3.3%) were rare. There was no connection between food sectors and RTs or PFGE types, but PFGE indicated the single plants (78.3% of the types) better than ribotyping (56.5%). On the basis of its automation and on the availability of identification databases, automated ribotyping had some advantages over PFGE. Overall, automated ribotyping can be considered a practical and rapid tool when Listeria contamination is suspected and when screening a large number of isolates is necessary, e.g., when tracing contamination sources. However, in cases of outbreaks, the identical patterns must be confirmed by PFGE, which is a more discriminatory method.