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Listeria species isolation frequency by season. Seasonal prevalence (average of 2001 and 2002 data) of each Listeria species expressed as the percentage of positive samples from natural (A) and urban (B) environments with the corresponding number of positives samples indicated. A total of 233, 338, and 336 samples from natural environments and 194, 376, and 328 samples from urban environments were collected during spring, summer, and autumn, respectively.
