Miljøprojekt, 934 – Hurtigmetoder til screening for coliforme bakterier og E .coli i drikkevand

Rapid Methods for Qualitative Screening of Coliform Bacteria, including E. coli, in Potable Water

Summary and conclusions

Having experienced a series of protracted incidents of polluted potable water the Danish EPA has called for more simple and rapid methods for qualitative screening (P/A) of coliform bacteria, including E. coli in potable water.

The tested methods are not intended to replace the quantitative method(s) approved at all times by the Danish EPA but meant to be used in connection with a specific pollution incident, with a view to tracking its source/cause.

Commercial rapid methods are available on the market which have not been approved by the Danish EPA. The intention of the present project was therefore to evaluate the suitability of the methods for Danish potable water and at the same time to evaluate a modification of the method approved by the Danish EPA (DS 2255) in terms of a lowered detection limit.

The testing included 10 samples of Danish potable water of varying quality. All samples were analysed as double determination for coliform bacteria as well as for E. coli. Five of the samples were furthermore analysed for both parameters after inoculation with E. coli, whereas the other five samples were analysed after inoculation with pure cultures of Aeromonas hydrophila, Klebsiella pneumoniae and Citrobacter freundii.

The tested rapid methods gave positive findings for all the natural water samples (100 ml, 500 ml, 1000 ml) where the reference method detected coliforms and E. coli.

For a few samples the rapid methods gave positive results where the reference method showed negative findings. This means that the rapid methods do not fail to detect contaminated water.

The samples inoculated with E. coli, A. hydrophila (non-coliform) and K. pneumoniae (coliform) showed full agreement between the tested methods.

For samples inoculated with Citrobacter freundii, however, the results were inconsistent. This may be explained partly by the fact that Citrobacter is a slow lactose fermenter, and partly by the interference between the low concentration of coliforms and the other microorganisms in the water. Atypical reactions are known to appear for Citrobacter in MacConkey-broth.

On the basis of the results the Danish EPA will permit the use of the tested alternative methods for screening of Danish potable water for coliform bacteria, including E. coli. However, final negative findings with these qualitative methods in cases with polluted drinking water must be verified by quantitative analysis with the method approved at all times by the Danish EPA. Verification can be done on the same sample within 24 hours from sampling or by testing a new sample from the same source.

The methods which are, thus validated and approved and can be used for qualitative screening, are listed below. The methods cannot be used for the legally required routine public control of potable water, but may form part of tracking the source in case of contamination. Apart from the statutory control, operational control of potable water is not under the auspices of the authorities, and the choice of method is free.

The methods are:

Modified DS 2255 with analysis of greater volumes (500 or 1000 ml) by membrane filtration, at which the sensitivity is improved (lowering of the detection limit) and pollution, if any, may be observed at an earlier stage.
DS 2255 supplemented with direct incubation of MacConkey tubes at 44°C.
The Colilert® systems (18 or 24 hours)
ReadyCULT®.