BACTERIA IN WATER 157 



devised for such a purpose. Quite good results are obtained by tying 

 two short lengths of string to the neck and stopper of an ordinary bottle 

 respectively, winding them round the neck and enveloping in cotton 

 wool ; any required length of string can afterwards be knotted on these. 

 A piece of lead can be attached to the bottom of the bottle by wires 

 passing round the neck. The whole is then wrapped in paper and 

 sterilised. For use the bottle is carefully lowered to the required depth 

 by the string attached to the neck, the stopper is jerked out, and the 

 bottle filled. If the bottle and stopper be rapidly jerked through the 

 topmost layers, contamination with surface bacteria does not appear as a 

 serious factor. 



Counting of Bacteria in Water. This is done by adding a given quantity 

 of water to 10 c.c. of liquefied gelatin or agar, plating, and counting the 

 colonies which develop. The amount of water added depends on its 

 source, and varies from '1 c.c. of a water likely to have a high bacterial 

 content to 5 c.c. of a purer water. It is usual to inoculate both gelatin 

 and agar tubes. The former, incubated at 20 C., gives an idea of the 

 numbers of bacteria present which grow at summer heat ; the latter, 

 incubated at 37 Q 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. With regard to the summer-heat bacteria it is important 

 to note that gelatine of a slightly greater alkalinity than that ordinarily 

 prepared such an increased degree as is caused by the addition of 

 01 grm. of Na 2 C0 3 to 10 c.c. peptone gelatin will give a greater yield 

 of colonies. In the case of both gelatin and agar plates usually forty- 

 ight 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 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 in the bacteriological examination of waters to be used 

 for industrial purj>oses. 



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 with regard to a water is directed to the investigation 

 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 applying 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 prefer- 

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

 present. In this medium the members of the b. coli group cause changes 

 n suiting in the formation of acid and gas. It is thus convenient to put 

 the nu-dium into Durham's fermentation tubes. In practice we employ 



