BACTERIOLOGY OF WATER 229 



In the purest upland streams and lakes the number of 

 bacteria in 1 c.e. is frequently under 100, while in town 

 sewage there are many millions in the same volume. In 

 ordinary rivers the number is generally between 1,000 

 and 100,000 per c.c. In water from deep-seated springs 

 the presence of more than 100 organisms per c.c. is con- 

 clusive evidence of some contamination with surface water. 



In waters of poorer quality the numbers may approach 

 500 per c.c. Anything over this casts suspicion on the 

 water, and 1,000 per c.c. or more should probably condemn 

 the sample, always supposing, of course, that multiplica- 

 tion in vitro can be excluded by the proper storage of the 

 sample bottle in ice. In Victoria aerated waters may not 

 contain more than 40 bacteria per c.c. 



Before taking a portion of the sample for any purpose 

 the water should be thoroughly shaken, the American 

 committee interpolating the procedure, ' Shake at least 

 twenty-five times the bottle which contains the sample.' 

 They also stipulate that the ' cool ' incubator should be 

 well ventilated, and that its atmosphere should be practi- 

 cally saturated with moisture. To avoid fictitious 

 accuracy they stipulate that numerical results should be 

 returned to the nearest 5, 10, 25, etc., number of organisms 

 depending on the number found. Counting a fractional 

 part of the organisms is sometimes done when plates are 

 crowded, by means of a Pakes's disc, but it is far prefer- 

 able to count all the organisms. As a thousand organisms 

 can be counted on a 10-centimetre dish, there is generally 

 little need to dilute, and if the organisms exceed this on 

 a plate made from 0*2 c.c., they can be truly returned as 

 " uncountable.' 



Crowded plates are to be avoided, as, quite apart from 

 the labour of counting, the results tend to be low: some 

 colonies may be hidden under others, two colonies may 

 coalesce and only count as one, and the metabolic 

 products excreted by quick-growing species may diffuse 

 into the medium and inhibit growth of neighbouring 

 colonies. Therefore it is desirable that 2CO colonies 

 should be the maximum allowed on a dish, and when 

 dealing with a water known to contain a large number 

 of organisms, plates should also be made from dilutions 

 of the sample. It is very desirable that duplicate sets 

 of plates should be made for all counts. 



