The presence of Aeromonas in drinking water supplies in The Netherlands.

The occurrence of Aeromonas in raw, treated and distributed waters of 20 different treatment stations in the Netherlands was studied over a one-and-a-half year period. River water yielded highest numbers (greater than 10,000 cfu/100 ml) of predominantly anaerogenic strains. In open storage reservoirs for river water or dune infiltrate, numbers were usually between 1,000 and 10,000/100 ml with a majority of aerogenic strains, in particular A. sobria. River water after dune infiltration or pure dune water, collected in closed systems, as well as aerobic or anaerobic groundwater, were usually free of Aeromonas in 100 ml. Treated waters showed low counts (usually less than 10 cfu/100 ml), irrespective of raw water source. Regrowth of aeromonads occurred in 16 out of 20 distribution systems examined. Geometric means counts (2nd half of 1986) in these systems varied between 1 and 440 cfu/100 ml and maximum counts between 10 and 3300 cfu/100 ml. Aeromonas-densities were related to temperature and residence time, but not to total organic matter or heterotrophic plate counts. Regrowth occurred particularly in drinking water derived from anaerobic groundwater containing methane. A. hydrophila was the most frequently isolated species from distribution waters, but A. caviae and A. sobria were predominant in a few systems.