EXAMINATION OF WATER. 437 



The number of colonies in 30 fields should be counted in order to 

 obtain a good average number. With 1,500 or more colonies on a 

 plate it is preferable to count with a microscope. This is espe- 

 cially true in water examinations and the like where different species 

 are present, some of which grow rapidly and form large colonies 

 whereas others grow slowly and give rise to very small ones. . A 

 cross-wire ocular micrometer should be used when the number of 

 colonies in a field is large. The number of colonies, as calculated for 

 the whole plate, is approximate but, if the work is properly done, the 

 error need not exceed 12 or 15 per cent. 



In case the number of bacteria is exceedingly great, the Thoma- 

 Zeiss apparatus for counting blood corpuscles can be used to deter- 

 mine the number of bacteria present. The result thus obtained will 

 be higher than that obtained by plate cultivation. 



To obtain accurate results, however, it would be necessary to 

 dilute a given volume of the water with a known volume of sterile 

 water and then plate portions of this mixture. Thus, 1 c.c. of the 

 water might be added to 99 c.c. of sterile water, and portions of 

 1, 0.5, 0.25 and 0.1 c.c. of this diluted material could then be plated in 

 the manner described. 



The number of colonies counted on a plate are taken to 

 represent the number of bacteria present in the water. In 

 reality, this represents the minimum and not the actual 

 number of bacteria present. A colony may be derived from 

 several cells. Thus, a bacillus growing- in pairs or in short 

 threads, or a diplococcus or streptococcus may be the start- 

 ing- point of the colony. Again, the conditions of cultiva- 

 tion may not be such as to cause the development of all the 

 bacteria that may be present. As pointed out, heretofore, 

 the reaction and composition of the gelatin, as well as the 

 prevailing temperature will influence the number of colon- 

 ies that develop. Some bacteria may be present that will 

 develop only at the temperature of the body. Moreover, 

 the method takes into account only the aerobic bacteria. 

 The anaerobic bacteria present will not develop. Conse- 

 quently, the number of bacteria present in a water, as as- 

 certained by the method given, is merely an approximate 

 number and serves to roughly indicate the amount of or- 

 ganic matter present. 



