86 BACTERIOLOGY FOR NURSES 



are compared it is generally found that the microscopic method 

 gives a much higher count than the plate method. If, however, 

 the clumps of bacteria seen in the former are given only the value 

 of one the two counts closely agree. Microscopic examination 

 of pasteurized milk is not practical, since it offers no means of dis- 

 tinguishing between living and dead bacteria. 



Milk Standards. Several cities have endeavored to obtain 

 a purer milk supply by admitting for sale only the milk that reaches 

 the standard they have set. The requirements are based on farm 

 conditions and chemical and bacteriological analyses. The num- 

 ber of bacteria permissible in the milk sold in New York City is 

 as follows : 



Grade A l must not contain more than 60,000 bacteria per c.c. 

 It may be raw or pasteurized ; the raw is obtained 

 from cows that have successfully passed the tuber- 

 culin test. 



Grade B is all pasteurized. Before pasteurization it may 

 contain 1,500,000 and after pasteurization 50,000 

 per c.c. 



Grade C is all pasteurized. It may contain any number within 

 reason before pasteurization and not more than 

 100,000 per c.c. afterwards. It is to be used for 

 cooking purposes only. 



Sour Milk. Under ordinary circumstances milk, after a short 

 period, becomes sour. The milk sugar, lactose, is fermented and 

 lactic acid is produced ; curdling results as a precipitation of the 

 casein from solution by the acid. 



When it was first discovered that the souring of milk was due 

 to bacteria it was thought that only one species was responsible 

 for the change, and an organism isolated was named Bacillus 

 acidi lacti because it had the ability to decompose lactose into 

 lactic acid. Later it was discovered that more than a hundred 

 other species have the same power in a varying degree. The 

 amount of acid produced depends largely upon the bacteria pro- 



1 Park and Williams, "Pathogenic Microorganisms," 1918, p. 642. 



