Characterization of Listeria monocytogenes from an ice cream plant by serotyping and pulsed-field gel electrophoresis.

One dominating strain of serotype 1/2b was found when serotyping and pulsed-field gel electrophoresis (PFGE) patterns were used for the characterization of 41 Listeria monocytogenes isolates originating from an ice cream plant. Samples were taken from the production environment, equipment and ice cream during the years 1990-1997. Serotyping divided the isolates into two serovars, 1/2b and 4b. Three rare-cutting enzymes (ApaI, AscI and SmaI) were used in the creation of PFGE patterns. AscI resulted in the best restriction enzyme digestion patterns (REDPs) for visual comparison. Eight different AscI REDPs were obtained, whereas ApaI produced six and SmaI seven banding patterns. When one-band differences are taken into account, 12 different PFGE types were distinguished based on information obtained with all three enzymes. The dominant PFGE type was found to have persisted in the ice cream plant for seven years. Improved and precisely targeted cleaning and disinfection practices combined with structural changes making for easier cleaning of the packaging machine, resulted in eradication of L. monocytogenes from this plant.