156 BACTERIA IN WATER 



soil, even when it is the only organism present. In experiments 

 made by sowing cultures of cholera and diphtheria in plots in a 

 field, it was found that after, at the longest, forty days they were 

 no longer recognisable. Further, it is a question whether 

 ordinary disease organisms, even if they remain alive, can 

 multiply to any great extent in soil under natural conditions. 

 If we are dealing with a sporing organism such as the b. 

 anthracis, the capacity for remaining in a quiescent condition of 

 potential pathogenicity is, of course, much greater. The most 

 important principle to be deduced from these experiments is that 

 the ordinary conditions of soil rather tend to be unfavourable 

 to the continued existence of pathogenic bacteria, so that by 

 natural processes soil tends to purify itself. It must, however, 

 be noted that such an organism as the typhoid bacillus can exist 

 long enough in soil to be a serious source of danger. 



WATER. 



In the bacteriological examination of water three lines of 

 inquiry may have to be followed. First, the number of bacteria 

 per cubic centimetre may be estimated. Second, the kinds of 

 bacteria present may be investigated. Third, it may be necessary 

 to ask if a particular organism is present, and, if so, in what 

 number per c.c. it occurs. 



Methods. Collection of Samples. In all water examinations it is pre- 

 ferable that the primary culture media (i.e. those to which the water is 

 actually to be added) should be inoculated at the spot at which the sample 

 is collected. When this is not possible, the samples should be packed in 

 sawdust and ice and the primary inoculations made as soon as possible. 

 Otherwise the bacteria will multiply, and an erroneous idea of the number 

 present will be obtained. Immediately after collection a slight diminution 

 in numbers may be observed, but at any rate after six hours an increase 

 over the initial numbers is manifest. 



When samples have to be taken for transport to the laboratory, these 

 are best collected in 8-ounce, wide-mouthed, stoppered bottles, which 

 are to be sterilised by dry heat (the stopper must be sterilised separately 

 from the bottle and not inserted in the latter till both are cold, otherwise 

 it will be so tightly held as to make removal very difficult). 



In the case of water taken from a house tap, the water should be allowed 

 to run for some time before the sample is taken, as water standing in 

 pipes in a house is under very favourable conditions for multiplication of 

 bacteria taking place, and if this precaution be not adopted an altogether 

 erroneous idea of the number present may be obtained. 



With river waters it is best to immerse the sampling bottle and then 

 remove the stopper with forceps. Care must be taken not to touch the 

 river bed, as the vegetable matter covering it contains many organisms. 

 When water has to be taken from below the surface of a well or lako, a 

 weighted sample bottle must be used. Several special bottles have been 



