WATER AND SEWAGE 621 



Bacteriological Examination of Water. A bacteriological examina- 

 tion of water requires relentless attention to details, from the collec- 

 tion of the sample to its final analysis and interpretation. 



Collection of Sample. It must be borne in mind that a small volume 

 of water 100 c.c. or less is ordinarily collected as a sample repre- 

 senting thousands or millions of gallons, consequently sampling is an 

 important detail in the bacteriological analysis of water. The col- 

 lecting bottle must be clean and sterile, and the site at which the 

 sample is taken must be representative. 



It is customary to obtain a sample of water from brooks, rivers and 

 lakes at a distance from the shore, and preferably samples from 

 different depths should be taken. The bottle must be immersed below 

 the surface before water is allowed to enter it, to avoid surface scums. 

 If water is taken from faucets or pumps the sample should not be 

 collected until a sufficient flow has been established to make certain 

 that the fluid has come directly from the water mains, or from the 

 well itself. 



As soon as the sample has been collected it should be examined; 

 frequently this is impracticable, and the bottle should be surrounded 

 with ice at once and shipped to the laboratory. Ice restrains bacterial 

 development for some hours and this maintains the sample at approxi- 

 mately its original bacterial content. 



Bacteriological Analysis of Water. A bacteriological examination 

 of water ordinarily includes a determination of the numbers of bacteria 

 w r hich develop in ordinary nutrient media at 20 C. and 37 C., a search 

 for organisms characteristic of the excrement of man or animals, their 

 approximate enumeration, and other tests which vary according to 

 the source of the sample. 



Counting Bacteria. The counting of bacteria ordinarily signifies 

 the numbers of microorganisms which will grow on gelatin incubated 

 at 20 C., and those that develop on agar at 37 C. Unpolluted waters 

 usually contain relatively few bacteria that will grow at body tem- 

 perature, consequently the gelatin plate seeded with the same volume 

 of water as the agar plate will show many more colonies than the 

 latter; polluted waters show a more even distribution of types of 

 bacteria that grow respectively at 20 C. and 37 C. 



The amount of water to be plated in gelatin and in agar depends 

 upon the source of the sample. Water from deep wells and from 

 springs should contain relatively few organisms, and a cubic centi- 

 meter of the sample is usually "planted." Surface waters almost 



