Report No DWI0324


Final report to the Department of the Environment


Dec 1992


Recently concern has been expressed over the recovery of aeromonads from potable water supplies as certain strains are known to be potentially pathogenic. Similarly, pseudomonads, and in particular the fluorescent group, are considered as potential opportunistic pathogens. This study was initiated to examine the occurrence and significance of these bacteria in water distribution systems.

The main emphasis of the study was the examination of the significance of aeromonads. Initially an evaluation of suitable media to recover aeromonads was undertaken and ampicillin dextrin agar was selected for all routine investigations. Confirmation of the identity of aeromonads required a diagnostic test to distinguish them from vibrios. The reaction to the antibiotic 0/129 was used but did not prove satisfactory as some aeromonads were recovered which were sensitive at concentrations to which they would be expected to be resistant.

A survey of aeromonads in potable water distribution systems was conducted which revealed that aeromonads were found in the highest numbers at the dead ends of the network. In such environments the aeromonad population was responding to low chlorine concentrations and long residence times. Experiments conducted in support of this study demonstrated that aeromonads were susceptible to the effects of disinfection and that they were capable of growth in distribution system waters once the chlorine had been neutralised. A similar pattern in the occurrence of pseudomonads was found. Comparison of the total heterotrophic plate count and recovery of aeromonads and pseudomonads revealed that these bacteria comprised a low proportion of the total bacterial population.

This study was funded by the Department of the Environment and carried out under the supervision of the Drinking Water Inspectorate whose permission to publish is gratefully acknowledged.

The views expressed in this report are those of the author and not necessarily those of the Department of the Environment or any other government department of organization.This report can be obtained from the WRc Publication Unit, Tel: (0491) 571531, priced 25.00

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