Growth and survival of clinical vs. environmental species of Aeromonas in tap water

The ability of four species of Aeromonas (two of clinical and two of enviro nmental origin) to survive and/or grow in tap water microcosms supplemented with sodium thiosulphate was tested. After bottling, the autochthonous mic roflora reached 6 x 10(5) cfu ml(-1) after a 5-day incubation period in tap water unfiltered and which was non-autoclaved. In filtered tap water, "ult ramicrocells" were detected and final populations of ca. 10(6) cfu ml(-1) a fter 7 days were obtained. Aeromonas was inoculated at an initial cell conc entration of ca. 10(4) cfu ml(-1). All strains were able to grow in tap wat er samples, which were filtered and autoclaved, and a final concentration o f 10(5)-10(6) cfu ml(-1) was observed. Any inherent capability of Aeromonas to grow in tap water was eliminated by the presence of autochthonous micro flora and "ultramicrocells" bacteria. Survival rates were strain- and micro cosm-dependent. In unfiltered-non-autoclaved water, viable counts declined to below the detection limit (i.e. 1 log cfu ml(-1)) in 1.5 to 20 days. The declines in viable counts were even more pronounced in the filtered microc osm. Although inoculation ratios (100/1 in unfiltered-non-autoclaved and 10 00/1 in filtered microcosms) were favourable for aeromonads, at least for 1 to 3 days, the organisms disappeared in these microcosms. Thus, competitio n for nutrients was an unlikely cause of the limitation of aeromonads. The bacteriolytic effect of enzymes released by membrane vesicles from the auto chthonous microflora. and of "tail phage-like particles" bacteriocins were suggested as an in situ control of aeromonad populations. The present study showed that environmental strains of Aeromonas had no ecological advantage over clinical isolates. Thus, waterborne infections and contaminations of foods by pathogenic Aeromonas species could not be discounted. (C) 2001 Els evier Science B.V. All rights reserved.