The spatiotemporal dynamics of Aeromonas spp. and fecal coliforms in the sewage treatment ponds of an urban wastewater center were studied after 20 months of sampling from five stations in these ponds. Isolation and identification of 247 Aeromonas strains were undertaken over four seasons at the inflow and outflow of this pond system. The hemolytic activity of these strains was determined. The Aeromonas spp. and the fecal coliform distributions showed seasonal cycles, the amplitude of which increased at distances further from the wastewater source, so that in the last pond there was an inversion of the Aeromonas spp. cycle in comparison with that of fecal coliforms. The main patterns in these cycles occurred simultaneously at all stations, indicating control of these bacterial populations by seasonal factors (temperature, solar radiation, phytoplankton), the effects of which were different on each bacterial group. The analysis of the Aeromonas spp. population structure showed that, regardless of the season, Aeromonas caviae was the dominant species at the pond system inflow. However at the outflow the Aeromonas spp. population was dominated by A. caviae in winter, whereas Aeromonas sobria was the dominant species in the treated effluent from spring to fall. Among the Aeromonas hydrophila and A. sobria strains, 100% produced hemolysin; whereas among the A. caviae strains, 96% were nonhemolytic.

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Flow cytometry was used to count bac-of this study showed that flow cytometry terial cells from diverse origins: one was a reliable technique for counting a strain of E. coli, one sample of lake water, mixture of bacteria in samples from and 18 samples of estuary water. To verify the accuracy and the precision of this technique, total bacteria counts made by flow cytometry were compared with counts by direct observation using epifluorescence microscopy. The results aquatic ecosystems.Key terms: Bacteria, enumeration, flow cytometry, epifluorescence microscopy, aquatic ecosystems Bacterial abundances in water samples are usually estimated by the indirect method of growing the bacterial cells in culture medium. The selective specificity of culture media can permit both the counting and the qualification of a certain population of bacteria (e.g., Escherichia coli) among a bacterial community. Even the least specific culture media is still selective with respect to total bacterial cell numbers in a sample. For example, a nutrient agar allows only the growth of heterotrophic bacteria. However, the selective pressure of the culture media (e.g., the presence of 2-3-5 triphenyl tetrazolium chloride towards E . coli) can prohibit the development of a certain number of cells that are stressed but not dead, and that should develop in the medium. Thus there is a n underestimation of bacterial abundance. A second, more direct method to estimate bacterial populations is the technique of epifluorescence microscopy. Coupled with a fluorochrome dye to stain cells, epifluorescence microscopy allows the enumeration of all bacterial cells in a water sample. This direct counting method is especially important in bacterial ecology for determining total bacterial numbers in aquatic ecosystems. However, this method is both time consuming and tedious if one is to have a high level of precision in the estimation of bacterial abundances (12). The technique of flow cytometry permits the quantification of physical and biochemical aspects of individual cells. This technique has been verified for the analysis of reference bacterial populations using fluorochrome dyes (ethydium bromide and mithramycin) (10, 11) and for the detection of pathogenic bacteria in milk, blood, and cooling tower waters using fluorescent antibodies and fluorochrome dyes (1,3,13). Recently, flow cytometry has been used to characterize bacterial communities in aquatic ecosystems according to cell size and relative DNA content (6).

A retrospective follow-up study was conducted during the summer of 1986 in the French Ardèche basin in order to assess the relationship between swimming-related morbidity and the bacteriological quality of the recreational water. 5737 tourists in eight holiday camps were questioned as to the occurrence of illness and their bathing habits during the week preceding the interviews. The rate-ratio contrasting swimmers and non-swimmers for total morbidity is 2.1 (1.8-2.4) = 95% confidence interval); gastrointestinal illness is the major type of morbidity and differs significantly between the two groups (RR = 2.4 (1.9-3.0) for total gastrointestinal cases; RR = 2.3 (1.7-3.2) for 'objective' gastrointestinal cases). Faecal streptococci (FS) are best correlated to gastrointestinal morbidity, using direct linear regression models. Faecal coliforms (FC) are not as good predictors of the risk. The concentration of faecal streptococci above which the 'objective' gastrointestinal morbidity among bathers is significantly greater than among non-bathers is 20 FS/100 ml. Swimmers suffer skin ailments much more frequently than non-swimmers (RR = 3.7 (2.4-5.7]; although the relationship may be artefactual, this type of morbidity is well correlated with the concentration of faecal coliforms, aeromonas and pseudomonas. This study provides epidemiological data on which to base microbiological standards for river recreational waters dependent on what might be considered as an 'acceptable' risk.