PHYSIOLOGY OF MICRO-ORGANISMS 



I6 7 



Heating above the maximum temperature for growth injures 

 the microbe and exposure for a short time kills it. A temperature 

 of 60 C. for 20 to 30 minutes destroys most vegetative forms of 

 bacteria. Cooling, on the other hand, merely checks and inhibits 

 growth. Freezing destroys some of the germs contained in a 

 liquid but many of them remain alive. Still lower temperatures 

 seem to be entirely without further effect. Bacteria gradually 

 die in frozen material. 



Germicides. Unfavorable environmental factors, germicides 

 and antiseptics have been considered in an earlier Chapter 

 (Chapter II). 



Microbic Variation. A microbic species is very stable in its 

 characters when maintained under fairly constant conditions in 

 its normal habitat. Change in environment brings about rather 

 quickly change in some of the characters of a bacterial species. 

 The alterations in virulence or ability to produce disease, which 

 may be produced by methods of artificial culture, are perhaps 

 best known. It would seem that the descendants of a single 

 cell are not all identical, but they vary among themselves within 

 fairly narrow limits in respect to a great many characters, fluc- 

 tuating about a mean type which is that best adapted to the 

 environment. With a change in surrounding conditions, this 

 mean or normal type may no longer be best adapted, but a 



