Report No FR/DW0003

HEALTH SIGNIFICANCE OF HETEROTROPHIC BACTERIA IN DRINKING WATER

FR/DW0003

April 1998

 

EXECUTIVE SUMMARY

Objectives:

  1. To determine whether HPC organisms may be of significance for health
  2. to determine whether it might be appropriate to develop standards for individual species or genera.

 

Methods

The techniques used for the detection of heterotrophic bacteria (HPC) in water were reviewed to establish if they are appropriate for the study of the health significance of heterotrophic bacteria. After reviewing the passbook ways of assessing the potential virulence of HPC that commonly occur in water a battery of tests was selected and set up in order to apply it to the virulence assessment of fresh isolates. The tests selected included assays for: Verotoxins using Vero cells: LT or CT type toxins using Y1 cells; adhesive and invasive factors in Hep-2 cells; and haemolysins using horse, human and sheep erythrocytes.

After a preliminary investigation the method chosen for the isolation of HPC for further study was the inoculation of volumes (0.1 and 0.5ml) of water onto R2A medium and incubation at 30ºC and 37ºC for seven days to maximise the recovery of as wide a range of heterotrophs as possible. HPC were isolated from 18 drinking water distribution systems in England and Waless and tested for the presence of potential virulence factors

Over 1000 isolates were collected and attempts were made to identify them by basic biochemical tests in combination with commercial identification kits. A peliminary trial was performed of a PCR based method for the detection of specific genetic markers in water using Campylobacter as a model.

Conclusions

The population of HPC organisms in normal tap water includes small numbers of organisms that possess activities on tissue cultures (markers) similar to those associated with the virulence markers of some recognised enteropthogenic species a bacteria.

These activities are not associated with any particular species or phenotypic group of bacteria or recognised enteropathogenic species

If these organisms can produce disease they would probably need to be present in water in high numbers in excess of the guideline values for the HPC at 37ºC after 48 hours incubation.

More extensive trials should be undertaken to compare the use of R2A medium and YEA spread versus pour plates for the determination of the HPC count at 37ºC and the effect of using chelating agents to improve recovery of HPC bacteria.

There is insufficient evidence at present to indicate a need for the development of standards for individual species but this should be continually reviewed in the light of new epidemiological information.

There is an urgent need for taxonomic studies on HPC bacteria in order to improve the ability to identify them and further our understanding of any role, if any, they may have in the epidemiology of gastrointestinal disease. The strains collected in this study could form the basis of such studies.

Although appropriate for research projects the tissue culture based assays are too laborious for the application as routine test methods in water laboratories

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