The bacteriological examination of water supplies.

Date:
1939
Catalogue details

Licence: Public Domain Mark

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

    27/68 (page 19)
    The necessary apparatus and its preparation are described in detail in Appendix B, and recipes and methods for the prepara- tion of media in Appendix C. These prescriptions should be strictly followed if uniform results are to be obtained. For the same reason the use of certain proprietary preparations of estab- lished merit has occasionally been prescribed. The examination should be commenced immediately after receipt of the sample at the laboratory. If this is impracticable, the sample should be placed in the ice-chest until it can be examined. It is desirable that not more than eight hours should elapse between the collection of the sample and its examination ; if this interval is longer, it should be expressly noted on the report. SECTION VII. THE PLATE COUNT. Water contains micro-organisms having different optimum temperatures of development. In general the natural water bacteria, i.e., those bacteria which are found more frequently in water than in any other situation, grow freely at temperatures between 20° and 30° C., while parasitic bacteria and bacteria of soil and sewage origin tend to grow best in the neighbourhood of 37° C. Since these two groups of organisms vary in their sanitary significance, it is desirable to count them separately. For this reason two sets of plates are usually prepared, one of which is incubated at about 22° C. and counted after three days, the other of which is incubated at 37° C. and counted after two days. For routine control purposes counts may be made after a shorter period of incubation, but, since under these con- ditions the number of visible colonies will often be considerably lower, the standard of interpretation of the results will neces- sarily be different. Though in the past gelatin was largely employed for the 22° C. count, it is now being generally replaced by agar. A and costly to prepare, of being unusable in hot countries, and of frequently being liquefied by growing organisms before the plates have had their full period of incubation. The one ad- vantage of gelatin over agar, namely, the differentiation it affords between liquefying and non-liquefying colonies, is of very doubt- ful value, and few bacteriologists nowadays would regard the presence of gelatin-liquefying organisms with greater severity than that of their non-liquefying companions. The use of agar for both the 22° C. and 37° C. counts also avoids a needless multiplication of media. Since both counts are made in the same way, except for the difference of their conditions of incubation, |