The occurrence of heterotrophic bacteria, Pseudomonas aeruginosa, Aeromonas, total coliforms, faecal streptococci, molds and yeasts in distribution systems of two surface water, two artificial groundwater and two groundwater supplies was examined three times during one year. All water samples studied were free of Pseudomonas aeruginosa and faecal streptococci in 100 ml. Total coliforms were detected in water distributed from one groundwater supply. Aeromonas hydrophila/caviae in the concentration range <1 - 32 cfu/100 ml also occurred in distributed water from the same supply. Molds and yeasts were frequently found in piped water from all watenvorks. The concentration of molds exceeded 100 cfu/100 ml in two sampling points. The differences in heterotrophic plate counts were greater between different sampling points of the same surface water supplies than between waterworks with different raw water origins. Over 80 % of the isolated strains of heterotrophic bacteria from different sampling locations were gram-negative and 57 % were oxidase-negative. The identified gram-negative strains belonged to the genera Pseudomonas, Moraxella, Alcaiigenes and Flavobacterium. The majority of strains (64 %) remained unidentified with the biochemical tests used.

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