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

THE COLIFORM TEST

• The bacteriological examination of water supplies for organisms of the coliform group comprises the most suitable and the most generally applicable test for evidence of pollution.
• The complete examination comprises a number of separate procedures by which it is possible not only to demonstrate the presence of coliform bacilli, but also to assign them to the sub-groups already mentioned.
• The complete test for coliform bacilli comprises the following procedures (i) the presumptive coliform count, (ii) the differential coliform test, and plate counts at 22⁰C and 37⁰C.
• The particular test or combination of tests required for any given sample submitted for examination will be decided by the bacteriologist after consideration of the relevant details pertaining to the circumstances under which examination of the water supply in question became necessary.
• The importance of supplying as full particulars as possible when forwarding samples to a laboratory for examination can, therefore, be readily understood.
• By the plate counts at 22⁰C and 37⁰C, the total number of viable bacteria per unit volume of water is determined. Most of the bacteria growing at 22⁰C , are saprophytic types which are non-pathogenic to human beings or animals.
• It may be thought that the plate count at 22⁰C, is immaterial; to come extent this is true, but it may be pointed out that this count affords some indication of firstly the amount of food substance available for bacterial nutrition, and secondly the amount of soil, dust and other extraneous material carrying bacteria with it that has gained access to the water.
• On general grounds the greater the number of organisms developing at this temperature, the larger is the amount of available organic matter present, and the less suitable is the water for human consumption.
• The bacteria developing at 37⁰C on the other hand, are mainly parasitic or potentially parasitic, though not necessarily pathogenic, types derived from soil, sewage or excretal material , and must therefore be regarded as evidence of the possible presence of pathogenic types such as members of the typhoid , paratyphoid group.
• The presumptive coliform count, together with the differential coliform test, is by far the most delicate index of excretal pollution. These tests are carried out on a quantitative basis and the results reported, according to the most recent recommendations, in the following manner
• Probable number of coliform bacilli present in 100 ml.
• Probable number of faecal coli present in 100 ml.