130 THE ENUMERATION OF BACTERIA IN MILK 



the area of the field is of such a value as will give a value to 

 y having as many ciphers as possible. The following are the 

 most satisfactory. 



When R = 0.080 m.m., y = 500,000 

 When 22 = 0.089 m.m., 2/=400,000 

 When jR = 0.101 m.m., 2/ = 300,000 



When the desired result is obtained the position of the draw 

 tube is noted and always set at this point in future examinations. 

 In order to get results comparable with the plate method, only 

 the groups or clumps, together with isolated bacilli are counted; 

 individual cocci, diplococcus or streptococcus chains, and rod 

 forms where the plane of division shows clearly, are counted as 

 individuals. The number of fields to be examined must be 

 determined by the frequency of the organisms. It is obvious 

 that with a factor of 300,000 to 500,000, this method is of the 

 greatest advantage when the count averages one clump or more 

 per field; with high-grade milks under 10,000 bacteria per 

 c.cm. the number of fields to be examined would be so large, 

 if reasonable precision is to be obtained, as to consume as much 

 time as the plate method. Dead bacteria are counted with the 

 hving, so that this process is not applicable to pasteurised 

 products; it would, however, be of advantage in determining 

 the quaUty before pasteurisation. A collateral advantage of 

 this method is that in addition to the quantitative estimation of 

 the bacteria, a cell count can be made at the same time and 

 information obtained regarding the bacterial flora. 



As an indirect method for estimating the number of bacteria, 

 Barthol,^^ in 1908, suggested the employment of methylene 

 blue. It was found by Barthol and confirmed later by Jensen 

 and Muller, that the time required to decolourise methylene 

 blue bears a relationship to the number of bacteria present. 

 Fred ^^ showed that 21 of 23 species of milk bacteria were capa- 

 ble of reducing methylene blue and that each species has a 



