VPE 511: Environment and Environmental Hygiene (2+1)

INTERPRETATION OF BACTERIOLOGICAL EXAMINATION

• The interpretation of the results of the bacteriological examination for coliform bacilli, as well as those for the other intestinal bacteria, calls not only for careful consideration of all the relevant factors, but also for considerable experience in this work.
• Final decisions have yet to be reached as to the meaning of all the data obtainable in the laboratory, and any conclusions reached must still be to some extent reflections of individual experience and , therefore , variable.
• The bacteriological condition of a water, too , has to be considered in relation to many other factors such as season, nature and topography of the source of supply, the frequency of examination, etc.
• The following quotations give the most authoritative information available at the present time regarding the interpretation of results and the hygienic classification of water supplies
•  E. coli is essentially an index of recent excretal pollution. The finding of this organism in water in more than minimal numbers can never be safely ignored.
•  The presence of organisms of the saprophytic group in water in the absence of faecal coli may be due either to (a) contamination of the water with soil; (b) contamination of the water with excretal material at a time sufficiently distant to allow faecal coli to die out; (c) Contamination of the water with the excreta of a person who is discharging the saprophytic organisms in almost pure culture; this must be relatively uncommon; (d) inadequate treatment of an initially polluted water with chlorine; which has only succeeded in killing off the more susceptible faecal coli. Which of these explanations is correct can be determined only by inquiry into the source and history of the water.
•  The finding of a high proportion of faecal coli among the total coliform organisms is indicative of heavy or recent excretal pollution. On the other hand, a result showing that the majority of the coliform organisms appear to belong to saprophytic group , or to irregular types may be regarded as indicative of a slight, infrequent, or remote excretal pollution; or perhaps if no faecal coli are present at all, of simple contamination with soil that may or may not have been excretally polluted some time previously.
•  In practice it is unwise to neglect completely the presence of organisms of the saprophytic group. Even though no faecal coli can be found , their presence may indicate a minor degree of pollution which at any time might become serious. Their appearance in a water, particularly a deep well water, from which they are normally absent, sometimes heralds the advent of pollution and enables steps to be taken in time to stop further pollution or , if this is impossible, to protect the consumer by suitable treatment of the water.
•  In general terms, the presence of faecal coli denotes recent and possibly dangerous excretal contamination, which must be urgently attended to. The presence of saprophytes in an untreated water suggests less recent contamination, which though not immediately dangerous is nevertheless sufficient to call for further steps towards obtaining greater purity of the supply. The presence of saprophytes in a treated water suggests either inadequate treatment or the access of undesirable material to the water after treatment.