The greatest threat to water supplies is contamination by raw sewage which contains pathogens capable of causing a number of diseases. The examitanion of drinking water for the presence of specific pathogens, whether they are viral, bacterial or protozoal, is time consuming, impractical, expensive, and requires a fairly hogh level of technical expertise. It is not necessary for routine testing and control.
Pollution may occur at irregular intervals or at short notice, due to problems such as damage to distribution systems or heavy rain following a long drought. One satisfactory examination does not mean therefore that the water supply will remain safe over a long period of time.
Monitoring for the presence of specific pathogens is unnecessary. Pathogens present tend to die out more quickly and are more difficult to detect than the normal human or animal gut microflora. Therefore the use of relatively rapid, simple, and cheap tests to determine the presence or absence of commensal indicator organisms is adequate.
The bacterial indicator used for faecal pollution are Escherichia coli , the coliform group, faecal streptococci, and Clostridium perfringens. These indicator organisms persist much longer than pathogens and are realtively easy to isolate and identify as they are present in faeces in large numbers. Their presence in water shows that the water has been polluted by sewage at some time in tis part history, and is therefore a potential health hazard. The tests for coliforms and E. coli are the most important of these routine tests.
The characteristics features of the coliforms are non-sporing, Gram-negative rods, which are vile tolerant and able to ferment lactose at 37 graus Celsius producing, acid and gas within 48 hrs. The term faecal coliform was used to describe coliforms capable to fermenting lactose to acid within 24 hrs at 44 graus Celsius.
The presence of coliforms in water indicates that pahogens could be present, and that the supply is potentially dangerous, although there is no correlation between numbers of coliforms and pathogens. The absence of coliforms indicates that pathogens are probably absent.
E.coli is a thermotolerant coliform. It is very rarely found in water when faecal pollution is absent, and testing for E.coli and coliforms is the most sensitive method of demonstrating faecal pollution. Confirmation of the presence of E.coli indicates faecal pollution and prossible presence of intestinal pathogens. High counts suggest recent or heavy pollution, whilst low counts suggest slight pollution or pollution at some time in the distant past.
The absence of E.coli combined with the presence of coliforms is more difficult to interpret. Whilst faecal pollution is the most probable explanation of contamination by coliforms, there are also other sources of coliforms which are generally innocuous, e.g., decaying vegetation and other organic matter such as washers and grease used in pipe joints.
This problem be solved by examining the water for faecal streptococci. Faecal streptococci survive longer in water than E.coli , and are more resistant to chlorination. The presence of faecal streptococci is always indicative of faecal pollution and therfore they are a useful method of determining the significance of a result in whch E.coli is absent, but coliforms are present
Streptococcus faecalis is the species found mainly in humans.
This indicator organism is an anaerobic Gram-positive rod, usually present in faeces in low numbers. It produces spores which resist boiling, survive for long periods in water, and show considerable resistance to chlorination. The presence os C. perfringens in water samples on several succesive occasions suggests that the frequency of sampling should be increased. As Clostridium spores are resistant to chlorination, the presence of spres in contaminated water which has been treated, combined with an absence of coliforms, show that the treatment process has been successful.
Large quantities of water are udes in the food, drink, and pharmaceutical industries. This is frequently subjected to further treatment on entering the plant and is usually of very high quality. Plate counts are used widely in these idustries to assess the effectiveness of this extra treatment.
The industries also frequently test for the genus Pseudomonas especially the fluorescent pseudomonads, which are found widely in dust, air and water, frequently occcuring in spoilage situation. The pseudomonasds are a diverse, poorly defined group and in this context tests for Pseudomonas aeruginosa are usually carried out.
REF: Microbiology for the Analytical Chemist, R. K. Dart, Loughborough University, The Royal Society of Chemistry, Information Services, 1996.