The bacteriological examination of water supplies.

Date:
1939
Catalogue details

Licence: Public Domain Mark

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

    12/68 (page 4)
    spite of the large amount of work carried out it was found that these tests were not sufficiently delicate or specific for the detection of minor degrees of sewage contamination. With the rise of bacteriology, efforts were made to supple- ment the chemical analysis. These consisted at first of approxi- mate estimations of the numbers of bacteria capable of growing at atmospheric and at body temperatures. The greater the number of colonies developing at atmospheric temperature, the greater presumably was the amount of organic matter in the water; and the greater the number of colonies developing at body temperature, the more likely were organisms of sewage and parasitic origin to be present. These tests, however, had little advantage over the methods evolved by the chemists. Attention was therefore directed to the demonstration of bacterial species of known excretal origin, particularly organisms of the coliform group, faecal streptococci, and Clostridium welch. Since these organisms are constantly present in the human intestine, usually in numbers greatly exceeding those of pathogenic intestinal bacilli, and since their death rate in water is rather slower than that of organisms of the enteric group, it follows that whenever typhoid or para- typhoid bacilli, for example, gain access to a water supply through excretal pollution they are always accompanied by the natural organisms inhabiting the intestine. The finding of coli- form bacilli in water shows, therefore, that recent excretal pollution has probably occurred, and, though not constituting of itself conclusive evidence of danger, is nevertheless sufficient to indicate that the water is potentially dangerous. Attention became more and more directed to the coliform group rather greater ease with which coliform bacilli could be demonstrated and partly because C/. welchu was found to survive considerably longer in water than members of the coliform and typhoid- paratyphoid groups. More recent work has rendered it evident that the coliform bacilli constitute a group of organisms of diverse origin. Some of them, such as Bact. coli, have their normal habitat in the human and animal intestine, whereas others, such as Bact. certain irregular types, appear to be distributed mainly: in vegetable matter and soil. It is true that these non-parasitic types may gain access to the intestine by means of food and drink and may survive in it for a time, but the available evidence suggests that they are seldom present in numbers equal to those of the typical Bact. coli. So far as we know Bact. coli does not lead a saprophytic existence, except in certain special conditions, and the presence of this organism in water can be regarded as almost undoubted evidence nk ee ihe i iis cet 2