Bacteriological water analysis

The common feature of all these routine screening procedures is that the primary analysis is for indicator organisms rather than the pathogens that might cause concern.

Indicator organisms are bacteria such as non-specific coliforms, Escherichia coli and Pseudomonas aeruginosa that are very commonly found in the human or animal gut and which, if detected, may suggest the presence of sewage.

Judgements as to suitability of water for use are based on very extensive precedents and relate to the probability of any sample population of bacteria being able to be infective at a reasonable statistical level of confidence.

Because the analysis is always based on a very small sample taken from a very large volume of water, all methods rely on statistical principles.

ATP is a molecule found only in and around living cells, and as such it gives a direct measure of biological concentration and health.

ATP is quantified by measuring the light produced through its reaction with the naturally occurring enzyme firefly luciferase using a luminometer.

To be effective, the dilution of the original sample must be arranged so that on average between 30 and 300 colonies of the target bacterium are grown.

This approach is widely utilised for the evaluation of the effectiveness of water treatment by the inactivation of representative microbial contaminants such as E. coli following ASTM D5465.

[citation needed] When the analysis is looking for bacterial species that grow poorly in air, the initial analysis is done by mixing serial dilutions of the sample in liquid nutrient agar which is then poured into bottles which are then sealed and laid on their sides to produce a sloping agar surface.

[citation needed] MacConkey agar is culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation.

Alfred Theodore MacConkey developed it while working as a bacteriologist for the Royal Commission on Sewage Disposal in the United Kingdom.

Non-lactose-fermenting organisms produce clear, colourless colonies against the faint pink background of the medium.

These include rosolic acid to inhibit bacterial growth in general, except for fecal coliforms, bile salts inhibit non-enteric bacteria and aniline blue indicates the ability of fecal coliforms to ferment lactose to acid that causes a pH change in the medium.