42 PRINCIPLES OF BACTERIOLOGY 



Influence of Reaction of Media upon Growth. The reaction of the 

 nutritive media is of very great importance. Most bacteria grow best in 

 those that are slightly alkaline or neutral to litmus. Only a few varieties 

 require an acid medium, and none of these belong to the parasitic bac- 

 teria. An amount of acid or alkali insufficient to prevent the develop- 

 ment 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. 



Influence of One Species upon the Growth of Others. 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 substance, in the majority 

 of instances, to become less suitable for the growth of other bacteria. 

 This is due partly to the impoverishment of the foodstuff, 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 foodstuff are favorable for some other form. The 

 pneumococcus which usually develops very tiny delicate colonies upon 

 nutrient agar, grows as luxuriant succulent colonies when grown with 

 certain bacilli. 



Temperature Suitable for Growth. For the growth of bacteria a suit- 

 able temperature is absolutely requisite. For different varieties the 

 most favorable temperature varies, but for all a range of about 2.5 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 bacteria require a tem- 

 perature near that of the body for their development, while many sapro- 

 phytic bacteria can grow only at much lower temperatures. Bacteria 

 when exposed to lower temperature than suffices for their growth, while 

 having their activities decreased, are not otherwise injured unless actually 

 frozen; while exposure to higher temperatures than allows of growth 

 destroys the life of the bacteria. The relations of the temperature 

 to bacterial life and death will be dealt with more fully in the next 

 chapter. 



