174 



GENERAL BIOLOGY OE MICRO-ORGANISMS 



perature for growth of each species. Ordinarily the optimum 

 temperature is, only a few degrees below the maximum at which 

 growth will take place. The following table from Marshall's 

 Microbiology illustrates the relation of these temperatures. 



[Species 



Temperatures 



Minimum 



Optimum 



Maximum 



PenicUlium glaucum 



Aspergillus niger 



Saccharomyces cereidsice I. . . . 

 Saccharomyces pasteurianus I 



Bacterium phospkoreum 



Bacillus subtilis 



Bacterium anthracis 



Bacterium Pudwigii. ..■ 



7°-io° 



i°-3° 



°.S° 



below o° 



6° 



io° 



25 -2 7 



33°-37° 

 28°-3o° 



2S°-30° 



i6°-i8'' 



30° 



30°-37° 



S5°-S7° 



3i°-36° 



40°-43° 

 40° 



34° 

 28° 

 SO- 

 43° 

 80° 



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 

 Uquid 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 



