Quantitative bacteriological examinations of the water in Lake Oświn, mucus from the skin surface and gastric contents of roach and pike in this water body were conducted in spring, summer and autumn. The general number of heterotrophic bacteria (TVC 20ºC and TVC 37ºC) and the number of potentially pathogenic ones, such as P. fluorescens, P. aeruginosa and Staphylococcus sp., were determined. The number of TVC 20ºC and TVC 37ºC bacteria was 102-103 cfu·cm^-3 in water, 105-107 cfu·cm^-2 in mucus from the skin and 106-107 cfu·g^-1 in gastric contents. In the material under study, among the potentially pathogenic bacteria, Staphylococcus sp. constituted the smallest proportion, whereas P. fluorescens and P. aeruginosa were much more numerous.

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