56 BACTERIOLOGY. 



bacteria. An amount of acid or alkali insufficient to 

 prevent the development of bacteria may still suffice to 

 rob them of some of their most important functions, 

 such as the production of poison. The different effect 

 upon closely allied varieties of bacteria of a slight 

 excess of acid or alkali is sometimes made use of in 

 separating those which may be closely allied in many 

 other respects. 



The influence of one species upon the growth of 

 another, either when the bacteria grow together or 

 follow one another, is very marked. The development 

 of one variety of bacteria in a medium causes that sub- 

 stance, in the majority of instances, to become less suit- 

 able for the growth of other bacteria. This isxdue 

 partly to the impoverishment of the food-stuff, but 

 more to the production of chemical substances or 

 enzymes, which are antagonistic not only to the growth 

 of the bacteria producing them, but to many other 

 varieties also; less frequently the changes produced by 

 one variety of bacteria in the food-stuff are favorable 

 for some other form. 



For the growth of bacteria a suitable temperature is 

 absolutely requisite. For different varieties the most 

 favorable temperature varies, but for all a range of 

 about 2J C. above or below this most favorable point 

 covers the limits for their most vigorous growth. Few 

 bacteria grow well under 10 C. and few over 40 C. ; 

 2 C. is about the lowest temperature that any bacteria 

 have been found to grow and 70 C. the highest. 



In many instances the temperature of the soil in 

 which the bacteria are deposited is the controlling 

 factor in deciding whether growth will or will not take 

 place. Thus nearly all parasitic hacleria require a 



