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Subtyping Listeria monocytogenes isolates genetically related to the Swiss epidemic clone

Macrorestriction analysis by pulsed-field gel electrophoresis was used to assess the diversity of strains within the epidemic-associated electrophoretic type 1 (ET1) clone of Listeria monocytogenes. For this purpose, a total of 144 isolates from Switzerland shown by multilocus enzyme electrophoresis to belong to the ET1 were examined. These isolates were subtyped by macrorestriction analysis using the enzymes ApaI and SmaI and field inversion gel electrophoresis. Among these 144 isolates, 45 were isolated in human listeriosis cases of the postepidemic period of 1988 to 1993 and 44 were isolated in animal listeriosis cases of the same period. Forty-seven isolates were from the epidemic period of 1983 to 1987, and eight additional isolates were from cattle from two different farms. Twenty-nine different subtypes could be identified among the 144 isolates tested. Five major subtypes were found more frequently than the others during the postepidemic period, both in humans and in animals. Two of these subtypes had been previously implicated in outbreaks of listeriosis, thus suggesting that particular pulsed-field gel electrophoresis subtypes may be frequently associated with disease in humans and animals. Two of these frequent subtypes were also suspected to be related to small clusters of listeriosis cases during the postepidemic period. The results obtained by typing epidemiologically related isolates from different animals within the same farms and from different body sites of a given patient confirmed the potential of macrorestriction analysis for epidemiological studies restricted to short periods of time and to small number of isolates. The analysis of 47 isolates related to the Swiss listeriosis epidemic period of 1983 to 1987 and the use of Southern blotting and hybridization experiments show that the interpretation of relatedness between isolates presenting slightly different macrorestriction patterns may be more complex than commonly accepted. In such cases, careful interpretation of the potential molecular mechanisms leading to the differences observed between patterns is necessary.