BACTERIA IN WATER 151 



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 au 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 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 lake, a 

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

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

 two 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. Houston recommends slight modifications in the composi- 

 tion of these media when they are to be used for enumerations of writer 

 bacteria. After considerable experience we can endorse his opinion as to 

 their efficiency. The gelatin medium consists of beef broth (p. 36)250c.c, 

 gelatin 120 grms., Lemco 5 grms., peptone 10 grms., water to 1000 c.e. ; 

 and the agar, of agar 20 grms., beef broth 1000 c.c, peptone 10 grms., 

 sodium chloride 5 grms. The medium in each case is made distinctly 

 alkaline to litmus or slightly alkaline to turmeric with 5 per cent, solution 

 of potassium hydrate. The gelatin plates, incubated at 20° C, give an idea 

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



