BACTERIOLOGICAL EXAMINATIONS 233 



tin to one liter of milk, boil, filter, neutralize, boil and 

 filter again. Or, one can use agar-agar in place of gela- 

 tin in making this culture medium. In this case, it is 

 necessary to first remove the casein by the use of ren- 

 net. By this method, one may obtain agar or gelatin 

 that is fairly transparent. The culture media should be 

 placed in test tubes in the usual way. The examination 

 can be made as follows: By the use of an ordinary 

 cover glass preparation, examined under the microscope, 

 one ascertains whether the milk is especially rich in bac- 

 teria. If there are only a few bacteria, usually three gel- 

 atin tubes will suffice, otherwise four or five must be used. 

 One c.c. of milk is measured out and is mixed with 9 c.c. 

 of fluid gelatin in a test tube. After mixing, one c.c. 

 of the mixture is placed in another tube containing 9 c.c. 

 of fluid gelatin. From this mixture a third tube is inoc- 

 ulated, and so on. In this way different dilutions of milk 

 are obtained. Tube no. 1 contains 0.9 c.c. of milk; 

 no. 2, 0.09; no. 3, 0.009; no. 4, 0.0009; no. 5, 0.0001. If 

 nothing is taken out of tubes 2, 3 and 4 for the purpose 

 of making further dilutions, they contain 0.1, 0.01 and 

 0.001 c.c. of milk. Of course one can make mixtures in 

 any other desired proportion. 



After making the proper dilutions the fluid is poured 

 in a Petri dish which, of course, must have been steril- 

 ized. After the mass of gelatin becomes firm, the covered 

 dish is put away and is kept at an appropriate tempera- 

 ture. In the course of time more or less colonies form 

 on the gelatin, these are counted on the whole plate or 

 on a definite sub-division, and by this means the approx- 

 imate number of bacteria in one c.c. of milk may be 

 estimated. The result of the count is by no means exact 

 for a number of reasons, but this method gives an 

 approximation that is sufficient for practical purposes. 

 [The milk may be diluted with water instead of being 



