The bacteriological examination of water supplies.

Date:
1939
Catalogue details

Licence: Public Domain Mark

Credit: The bacteriological examination of water supplies. Source: Wellcome Collection.

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    of being misleading. If, for instance, coliform organisms of an unusual type appear in a large water supply, and it is doubttul whether they are or are not indicative ot excretal pollution, a search for taecal streptococci and for Cl. welchit may be of considerable assistance in reaching a decision. Both of these organisms are of excretal origin, and their presence denotes con- tamination of the water. The value of the faecal streptococcal test is estimated variously by different workers. Some regard it as of consider- able value, whereas others are of the opinion that it adds little to the information yielded by the test for coliform bacilli. This discrepancy of opinion is largely due to the fact that very few studies of faecal streptococci in relation to water supplies have been carried out during recent years. Many of the older obser- vations were made under somewhat artificial conditions and at a time when the differentiation of faecal from non-faecal types of streptococci was less satistactory than it is to-day. It is not definitely known whether faecal streptococci survive on the average for a shorter or a longer time in water than coliform bacilli. Observations by modern methods are required on this point. Until they are completed, the interpretation of the results of the faecal streptococcal test, particularly when negative, must remain doubtful. The Cl. welchii test stands on much firmer ground. This organism forms spores, which survive for a considerably longer time than coliform bacilli, and usually resist chlorination. Its presence in water indicates that faecal contamination has occurred, and the presence of Cl. welchii in the absence of coliform bacilli indicates that the contamination is not of very recent date. The chief value of the test would seem to lie in the detection of remote or intermittent pollution, especially in shallow well waters. Supplies of this type are often distributed to only a small population, and their frequent examination by the coliform test is not usually practicable. An occasional examination for coliform bacilli may yield misleading results, since these organisms may have died out since the last access of pollution. In such instances the demonstration of Cl. welchi will show that the water is subject to contamination, and will point to the desirability of taking precautions to safeguard the health of the consumer. The technique of carrying out these tests is described on pp. 40-43. Adaptation to special purposes. These three suggested methods of examination of increasing degrees of completeness may of course be varied by the bacteriologist to suit local conditions or special needs. In the control of filtration or chlorination, for example, a test should always be carried out on the water before and after treatment.
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