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 environmental origin) to survive and/or grow in tap water microcosms supplemented with sodium thiosulphate was tested. After bottling, the autochthonous microflora reached 6×105 cfu ml−1 after a 5-day incubation period in tap water unfiltered and which was non-autoclaved. In filtered tap water, “ultramicrocells” were detected and final populations of ca. 106 cfu ml−1 after 7 days were obtained. Aeromonas was inoculated at an initial cell concentration of ca. 104 cfu ml−1. All strains were able to grow in tap water samples, which were filtered and autoclaved, and a final concentration of 105–106 cfu ml−1 was observed. Any inherent capability of Aeromonas to grow in tap water was eliminated by the presence of autochthonous microflora and “ultramicrocells” bacteria. Survival rates were strain- and microcosm-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 microcosm. Although inoculation ratios (100/1 in unfiltered-non-autoclaved and 1000/1 in filtered microcosms) were favourable for aeromonads, at least for 1 to 3 days, the organisms disappeared in these microcosms. Thus, competition for nutrients was an unlikely cause of the limitation of aeromonads. The bacteriolytic effect of enzymes released by membrane vesicles from the autochthonous 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.