158 BACTERIA IN WATER 



hydrate. The gelatin plates, incubated at 20 C., gives an idea of the 

 numbers of bacteria present which grow at summer heat ; the agar (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 

 case 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 in 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 Han sen 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 in the bacteriological examination of waters to be used 

 for industrial purposes. 



Detection of the Presence of Special Organisms. (a] The B. coli Group. 

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

 important inquiry W'ith 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 type. 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 glucose 

 bouillon to which litmus has been added glucose being used in preference 

 to lactose in order to bring out the b. enteritidis of Gaertner if this be 

 present. 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 1^ 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 sample by 

 adding to a series of such tubes the following quantities of the water : 

 50 c.c. (two samples), 20 c.c., 10 c.c., 5 c.c., 1 c.c., and, it may be, 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 con- 

 centrated 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, Ave simply use the ordinary MacConkey's 

 medium. 



