SEASONAL OCCURRENCE OF MESOPHILIC AEROMONAS SPP AS A FUNCTION OF BIOTYPE AND WATER-QUALITY IN TEMPERATE FRESH-WATER LAKES

Densities of mesophilic aeromonads were compared in Virginia freshwater lakes with trophic classifications ranging from mesotrophic to hypereutrophic. Aeromonad concentrations were independent of trophic status and did not correlate statistically with water quality parameters used to measure trophic state (total phosphorus, chlorophyll a and Secchi depth). Bacterial indicators (fecal coliforms and Escherichia coli) used to assess the public health safety of recreational waters correlated with mesophilic aeromonads only when sewage pollution occurred. Overall (n = 101-107) there were small negative correlations of aeromonad densities with dissolved oxygen (r = -0.30, P = <0.05), and total dissolved nitrogen (r = -0.30, P = 0.05). During frequent sampling from April through October 1991, mean mesophilic aeromonad concentrations ranged from 10(3-4) cells 100 ml(-1) with the lowest values measured during mid-summer. Over the course of twelve months, 273 aeromonad isolates were identified as belonging to the following biotypes: A, sobria (54%), A. hydrophila (29%), A. caviae (10%), and undetermined (7%). Seasonally, the frequency of isolation of A. sobria, considered the most pathogenic biotype, increased from 17 to 69% when water temperatures exceeded 20 degrees C in a hypereutrophic lake. These temperatures typically occur during periods of peak recreational usage. Phenotypic characteristics associated with virulence were positive in 70% (hemolysis) and 42% (autoagglutination) of glycerol-stored strains (n = 85), and in 15% (autoagglutination) of strains immediately processed (n = 73). S-layer proteins associated with invasiveness were not found in 14 strains which autoagglutinated. To evaluate the public health significance of aeromonads in nutrient enriched lakes, our results demonstrate a need to couple investigations of mesophilic aeromonad ecology with virulence and biotype assessments.