Properties of aeromonads and their occurrence and hygienic significance in drinking water.

In the Netherlands, aeromonads in drinking water have attracted much attention in recent years. This development was caused by a sudden increase of the Aeromonas density in the drinking water of the municipal Dune Waterworks of The Hague and reports about the possible health significance of these organisms in drinking water. Literature data indicate that representatives of the motile Aeromonas species A. hydrophila, A. sobria and A. caviae generally have been observed in larger percentages of diarrheal feces than in normal stools, with isolation rates ranging from less than 1% to more than 20%. These data and the virulence properties of the aeromonads, viz. hemolytic activity, cytotoxicity and enterotoxicity, as tested in the suckling mouse assay or the rabbit ileal loop, strongly suggest that these aeromonads are potential enteric pathogens for susceptible hosts, including young children and immunocompromised persons. Aeromonads are ubiquitously present in fresh water environments, with densities depending on pollution with sewage, trophic state and temperature. About 100 years ago, bacteria identical with Aeromonas spp. have already been isolated from drinking water. Depletion of free chlorine residuals in drinking water generally results in increasing Aeromonas densities, particularly in the summer months. Investigations in the Netherlands have shown that Aeromonas densities in drinking water increase with increasing residence time. Furthermore, the aeromonads constitute a minor fraction of the heterotrophic bacterial population in drinking water. Growth measurements with pure cultures of A. hydrophila revealed that certain compounds, e.g. oleate as present in soft soap, promote the growth of the organism at substrate concentrations of a few micrograms per liter. Based on a number of surveys on the presence of aeromonads in drinking water, the health authorities in the Netherlands have defined so-called indicative maximum values for Aeromonas densities in drinking water i.e. 20 CFU/100 ml in drinking water at the production plant and 200 CFU/100 ml in drinking water during distribution. Further research is necessary (i) to elucidate the health significance of aeromonads in drinking water and (ii) to define measures for limiting Aeromonas densities in drinking water.