18 TEXT-BOOK OF BACTERIOLOGY. 



which they recover as soon as they are again placed under favor- 

 able circumstances. If the temperature departs still further from 

 the favorable mean it acts directly prejudicially, or even destruc- 

 tively, on the majoi"ity of the micro-organisms known to us. Heat 

 of 60° C. kills quickly and to a certainty many very important 

 bacteria, such as the typhus, cholera, glanders (malleus), and tuber- 

 cle bacilli. Others have a greater power of resistance, and particu- 

 larly in a dry state some, such as the pus cocci, keep their vital 

 power even at 80° C. 



In like manner the influence of cold is manifested. If fluids con- 

 taining bacteria be allowed to freeze, we observe that the greater 

 part of the micro-organisms perish. The bacteria are specially 

 sensitive, as shown by the investigations of Prudden, to repeated 

 freezings and thawings. Yet here also we see differences in the 

 different species, some of which display a high power of resistance, 

 while others are soon destroyed. 



Still more important is the fact that even within the same species 

 the separate individuals are by no means similar in their powers of 

 resistance : some yield more quickly, others very much more slowlj', 

 to the influences of a prejudicial temperature. As the same differ- 

 ence has been remarked with respect to other external influences 

 also, it may be regarded as certain that among the bacteria, as 

 also among higher organisms, there are strong and weak, young 

 and old, healthy and diseased, individuals, all of which must not be 

 considered subject alike to the same influence. 



It is well to note here some further striking observations which 

 in part go to contradict the general rules above given, the correct- 

 ness of which, however, is beyond all doubt. 



On one side, Fischer and Forster have made us acquainted with 

 certain bacteria occurring in sea-water and in the earth which 

 even at the freezing point (0° C.) cannot only exist, but are able to 

 grow without interruption. 



On the other hand, bacteria have been discovered by Globig and 

 Miquel which continue to develop and multiply at 60° C, and even 

 at 70° C. Nay, there are even some which require this high tem- 

 perature, and are unable to thrive at a lower temperature than 

 about 60° C. It is hard to conceive how and where in a state of 

 nature such organisms can find the necessary conditions for their 

 growth and reproduction. 



After heat, oxygen is a very important factor in the life of bac- 

 teria. By far the greater part of the micro-organisms yet known 

 cannot thrive in the absence of oxygen. Some, indeed, are so sensi- 

 tive on this point that even a slight diminution of oxygen in their 



