The bacteriological examination of water supplies.

Date:
1939
Catalogue details

Licence: Public Domain Mark

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

    13/68 (page 5)
    of recent excretal pollution of human or animal origin. Since Bact. coli can be detected in numbers as small as one in 100 ml. of water, the demonstration of this organism affords far and away the most delicate and specific test at our disposal for proving that excretal contamination has recently occurred. For this reason very careful estimation has to be made of the num- bers of Bact. colt betore deciding whether the pollution has been severe enough to render the water potentially dangerous, i.e. the whole essence of the test is quantitative. | It will be seen that the tendency of water analysis has been to pass from the more crude and less discriminative methods of chemical examination to the more delicate and more specific methods of bacteriological examination. At the present time, the trend of practice among bacteriologists in this country is to concentrate on the demonstration of faecal coli in the water. Recently methods have been developed which enable a rapid enumeration of this organism, as apart from other organisms of the coliform group, to be made and it is anticipated that the quantitative estimation of faecal coli is likely in the future to play a dominant part in the routine control of water supplies. This does not mean to say that other types of coliform organism can be neglected, because there is evidence to show that some of these types survive longer in water than the faecal coli type, and hence may reveal by their presence more distant excretal contamination which, though not dangerous at the time of sampling, may nevertheless indicate that the water is subject to potentially dangerous pollution. The whole question will be discussed more fully at a later stage (pp. II-14). SECTION IV. FREQUENCY AND [TYPE OF EXAMINATION. In the past the practice among many water undertakers has been to have a complete chemical and bacteriological examina- tion of their water supplies made at infrequent intervals, such as three months or so, and to trust that the water supplied in between these examinations would not undergo any substantial deterioration. As has been pointed out in the Introduction, the objection to this practice is that it affords no effective control over the quality of the water supplied during the greater part of the year. There are extremely few supplies of any size so circumstanced that regular bacteriological control is unneces- sary. In the great majority, contamination, as is well known, may occur at any moment, and may come from numerous different sources. Unless frequent examinations are made, disease may break out among the human population before any suspicion is cast on the water. To prevent this effectively, constant bacteriological supervision must be maintained, and