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 is due 
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 23° 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 hacteria require a 
