Aeromonas hydrophila is a valuable indicator of the quality of water polluted by sewage and pathogens that pose a risk for humans and cold-blooded animals, including fi sh. The main aim of this research was to evaluate anthropogenic pollution of river water based on genetic diversity of 82 A. hydrophila strains by means of RAPD, semi-random AP-PCR (ISJ) and the rep-BOX conservative repeats test. Genetic diversity of A. hydrophila was HT = 0.28 (SD = 0.02) for all DNA markers (RAPD, semi random and rep-BOX). None of the analyzed electrophoretic patterns was identical, implying that there were many sources of strain transmission. The presence of genes for aerolysin (aerA), hemolysin (ahh1) and the cytotoxic enzyme complex (AHCYTOGEN) was verifi ed for all tested strains, and drug resistance patterns for tetracycline, enrofl oxacin and erythromycin were determined. The most diverse A. hydrophila strains isolated from river water were susceptible to enrofl oxacine (HS = 0.27), whereas less diverse strains were susceptible to erythromycin (HS = 0.24). The presence of the multidrug resistance marker (ISJ4-25; 1100 bp locus) in the examined strains (resistant to three analyzed drugs) indicates that intensive fi sh cultivation affects the microbiological quality of river water.

The research covered the determination: of the numbers or hcicrorrophic bacteria: psychrophil ic, psychrctolcrant, mcsophilic and percentage participation or hemolytic bacteria and .ieromonas hydrophila (with acrolysinc and hcmolysine genes) in the waters of the Drwęca River depending on environmental Ilictors and fishery management. The mean quantities 01· hclcrotrophic bacteria (Ht'C) at 4, 14 and 2~°C ranged: O. 78-7.57-101, 1.40-6.65-101 and 1.93-16.23- 103 efuen -3, respectively. The percentage participation 01· hemolytic heterotrophic bacteria (HemPC) and A. hvdrophila among psychrophilic, psychrotolcrant, mcsophilic microorganisms determined at 4, 14, 28°C, ranged: 7.9-10.4, 6.8-12.2, 8.6-22.0 ,111d 1.1-6.4%, respectively. Statistically significant correlation between examined bacteria and temperature values, flows and O2 saturations confirm that the occurrence of those microorganisms depends on the degree of microbiological contamination of that ecosystem, resulting from the fishery management and environmental factors.