Bacteriological water analysis

Bacterial water analysis is a procudure for analsing represenataive samples of water to determine thair bacterilogcal concentration and from this draw inferences about the suitability of the water for use This proces is, for example used routinely to confirm that the concentration of potentially pathogenic bacteria in drinking water is sufficiently low that it is safe for human consumption Judgements as to suitability 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
Analysis is usually performed using culture biochemical and sometimes optical methods Specific pathogens can be quickly detected (where suspected) using molecular biology
In drinking water sampling the colilert culture method is common A reagent is added to water samples of a uniform amount These are cultivated on a nutrient tray that is sealed and incubated at 22ºC and 37ºC for 24 hours Bacteria reproduces to form a colony and each one can be counted easily because they become fluorescent in the reagent Prior to this method each sample would be cultivated on a single nutrient pad such as agar and the colonies were then painstakingly counted under a microscope
The total number of colonies is referred to as the total viable count (TVC) The unit of measurement is cfu/ml (or colony forming units per millilitre) Typically three indicator bacteria are chosen; non-specific coliforms Escherichia coli and Pseudomonas aeruginosa The presence of Pseudomonas may indicate that the water is not as clean as it should be, although it may still be potable if the levels are sufficiently low and prompt action is taken Coliforms (and especially E coli) could suggest the possibility of fecal matter contamination of a water supply

Types of nutrient media used in analysis

ENDO medium: used for analysis of coliforms; contains fuchsin ensuring that only G- bacteria will grow Escherichia coli develops a specific metallous gloss on this medium
SB medium: stands for Slanetz-Bartley this medium contains sodium azide (NaN₃) which only enterococci are able to tolerate During analysis only red colonies are counted
mFC medium: cultivated at 44°c this medium is used to detemine freshness of enterobacterial contamination - recently contaminated water will be positive in this test
TYEA medium: usually the first of all test non-selective this medium is usually cultivated at two temperatures (22 and 36°c) to determine generous contamination