74 EXPERIMENTAL DAIRY BACTERIOLOGY 



indications as to whether the organism produces spores or 

 not. A broth culture forty-eight hours old, grown at 37 C., 

 is used. In a water bath heated to 80 C. place four tubes of 

 nutrient broth in immediate contact with the thermometer. 

 After fifteen minutes' exposure to this temperature inoculate 

 each tube, without removing from the water, with three loop- 

 fuls of the broth culture, using care not to contaminate the 

 glass above the surface of the liquid. Continue the exposure 

 for fifteen minutes, remove the tubes, and cool at once. Incu- 

 bate for at least seven days. Organisms producing true endo- 

 spores are able to withstand this temperature. These are the 

 types that are so difficult to destroy in the sterilization of 

 milk. Organisms resisting this temperature are classed as 

 spore forming. 



For the more detailed determination of the thermal death 

 point of the organism the student is referred to standard 

 text-books on general technique. 



Exercise. Determine the optimum temperature for growth of the 

 organisms furnished, and also whether or not they produce spores. 



Chromogenesis. The formation of pigment is usually most 

 apparent on agar cultures. Cultures should be incubated at 

 both 20 and 37 C. Some organisms produce pigment only 

 under special conditions, as B. mesentericus ruler when grow- 

 ing on potato. Free oxygen is an essential with practically all 

 types. Therefore the cultures must be under obligate aerobic 

 conditions. 



Action on milk. The action of the various organisms on 

 milk is of the very greatest importance to the dairy bacteri- 

 ologist, and especial attention should be paid to the determi- 

 nation of the points which are enumerated on the following 

 page. 



