6 Dairy Bacteriology. 



bibe their food from material in solution. They are capable 

 of living on solid substances, but in such cases, the food 

 elements must be rendered soluble, before they can be ap- 

 propriated. If nutritive liquids are too highly concen- 

 trated, as in the case of syrups and condensed milk, bacteria 

 cannot grow therein, although all the necessary ingredients 

 may be present. Generally, bacteria prefer a neutral or 

 slightly alkaline medium, rather than one of acid reaction; 

 but there are numerous exceptions to this general rule, 

 especially among the bacteria found in milk. 



Temperature. Growth of bacteria can only occur with- 

 in certain temperature limits, the extremes of which are 

 designated as the minimum and maximum. Below and 

 above these respective b'mits, life may be retained in the 

 cell for a time, but actual cell-multiplication is stopped. 

 Somewhere between these two cardinal temperature points, 

 and generally nearer the maximum limit is the most favor- 

 able temperature for growth, known as the optimum. The 

 temperature zone of most dairy bacteria in which growth 

 occurs ranges from 40-45 F. to somewhat above blood- 

 heat, 105-110 F., the optimum being from 80-95 F. 

 Many parasitic species, because of their adaptation to the 

 bodies of warm-blooded animals, generally have a narrower 

 range, and a higher optimum, usually approxima'ing the 

 blood heat (9S-99 F). The broader growth limits of bac- 

 teria in comparison with other kinds of life explain why 

 these organisms are so widely distributed in nature. 



Air supply. No living organism, can thrive without 

 oxygen and most species require atmospheric oxygen; but 

 there exist certain bacteria (and yeasts as well), which 

 possess the peculiar property of not being able to utilize 

 elemental oxygen. These secure the necessary element for 



