152 BACTERIA IN WATER 



(which should be incubated in the inverted position), incubated at 37° C, 

 those which grow at blood-heat. As the pathogenic and intestinal bacteria 

 ■ grow at this temperature, the determination of the numbers of blood-heat 

 bacteria is important. The counts on the two media usually differ as 

 each is favourable to the growth of its own group of organisms. In the 

 ease of both gelatin and agar plates usually forty-eight hours' incubation 

 is allowed before the colonies are counted, but, with the former, difficulties 

 may arise hi consequence of the presence of rapidly liquefying colonies, 

 and it may thus be necessary to count after twenty-four hours. 



Probably no one medium will support the growth of all the organisms 

 present in a given sample of water, and under certain circumstances special 

 media must therefore be used. Thus Hansen found that in testing 

 waters to be used in brewing it was advisable to have in the medium 

 employed some sterile wort or beer, so that the organisms in the test 

 experiments should be provided with the food materials which would be 

 present in the commercial use of the water. Manifestly this principle 

 applies generally jin the bacteriological examination of waters to be used 

 for industrial purposes. 



Detection of the Pzcsence of Special Organisms. — (a) The B. coli Group. — 

 In ordinary public health work, it may be taken that the most frequent 

 and important inquiry with regard -to a water is directed to the investiga- 

 tion of the presence or absence of the b. coli and its congeners. Here 

 the method adopted is to determine the smallest quantity of a water 

 which gives evidence of containing organisms of this typp. In apply- 

 ing any method with this object in view it is, we consider, absolutely 

 necessary that it shall be carried out at the spot at which samples are 

 collected. 



The usual method is to use as the primary culture medium one of the 

 bile-salt preparations, of which the best is MacConkey's bile-salt lactose 

 bouillon to which litmus has been added. In this medium the members 

 of the b. coli group cause changes resulting in the formation of acid and 

 gas. It is thus convenient to put the medium into Durham's fermentation 

 tubes. In practice we employ 2-ounce cylindrical medicine bottles 4| in. 

 high by 1J in. in diameter. The' medium,' along with the inverted test- 

 tube, is placed in these ; rubber stoppers are inserted in the mouths, and 

 they are sterilised. It is customary to test for the presence of the 

 organisms in any sam pie by adding to a series of such tubes the following 

 quantities of the water : 50c.c. (two samples), 20 c.c, lOc.c, 5c.c, 1 c.c, 

 and, it maybe, in specially suspicious waters, '5 c.c, - 1 c.c, and even '01 

 c.c. The result is estimated in terms of the smallest amount of water with 

 which the occurrence of acid and gas formation is observed. By starting 

 with a concentrated MacConkey's mixture, it is arranged that, when the 

 sample is added, the resulting fluid shall be of the concentration of 

 MacConkey's medium as ordinarily prepared. Thus, in the bottle to 

 which the 50 c.c. sample is to be added, there are placed 10 c.c. of a six- 

 fold concentration of MacConkey's medium. In the 20 c.c. tube, there 

 are present 20 c.c. of a medium of double strength ; in the 10 c.c tube, 

 10 c.c. of a mixture of double strength ; and in the 5 c.c. tube, 5 c.c. of 

 a mixture of double strength. With smaller samples, we simply use the 

 ordinary MacConkey's medium. 



For the taking of the samples, sterile 8-ounce stoppered bottles are 

 convenient, and 'for each sample it is necessary to have sterile 25 c.c, 

 10 c.c. (graduated to tenths), and 1 c.c. (graduated . to hundredths) 

 pipettes. The armamentarium being thus simple, there is no difficulty 



