ISOLATION OF INDICATOR ORGANISMS 2/ 



or so) for convenience of sterilization, but care should be taken 

 to obtain those in which the lower part is of sufficient length to 

 reach well into the diluting bottles. 



While solids are most conveniently recorded as per gramme 

 of material, data as to bacterial content of liquids are returned 

 as per c.c. or litre. 



For B. coli, streptococci, etc. when a series of dilution tubes 

 are used for enumeration purposes the results are not definite 

 amounts, but can only be returned between the limits of the 

 dilutions employed. For example, if a given water sample 

 showed B. coli present in 10 and in 50 c.c. but absent in i 

 and O'l c.c., the deduction would obviously be that there were 

 more than 100 but less than 1000 B. coli per litre of the water 

 and the result would be recorded as B. coli 100 1000 per litre. 

 For routine work and reporting this is a preferable method of 

 recording to stating the amounts examined which were positive 

 and those which were negative. 



CHAPTER III 



WATER 



Water under natural conditions invariably contains bacteria, 

 the actual numbers present varying enormously with the class 

 of water supply, the degree of soil filtration, opportunities of 

 bacterial contamination, etc. 



Natural water supplies as regards their bacterial content are 

 never in a state of equilibrium. On the one hand bacteria- 

 holding substances are constantly being added, thereby raising 

 their bacterial content, while on the other hand certain natural 

 purification agencies are constantly at work tending to reduce 

 the bacteria present. Of these agencies the forces conveniently 

 summed up as sedimentation are the most important, while light, 

 sunlight and deficiency of food material are all factors playing 

 a part. The effects of these different agencies are most readily 

 studied in regard to rivers and streams. An ordinary stream 



