THE BIOLOGY OF BACTERIA 



23 



the low temperatures gelatine is chosen ; as a medium for the 

 higher temperatures agar. 



Moisture ha.s been shown to have a favourable effect upon 

 the growth of microbes. Drying will of itself kill many 

 species (^.^., the spirillum of cholera), and, other things being 

 equal, the moister a medium is, the better will be the growth 

 upon it. Thus it is that the growth in broth is always more 

 luxuriant than that on solid media. Yet the growth of 



Culture Media Ready for Inoculation 



Bacillus subtilis and other species is an exception to this 

 rule, for they prefer a dry medium. 



Temperature. Most bacteria grow well at room temper- 

 ature, but they will grow more luxuriantly and speedily at 

 blood-heat. The optimum temperature is generally that of 

 the natural habitat of the organism. In exceptional cases 

 growth will occur as low as 5" C. or as high as 70° C. 

 Indeed, some have been cooled to -20° C. and -30° C, and 

 yet retained their vitality,' whereas some few can grow at 



1 Some pathogenic germs (suppuration and typhoid) can withstand freezing 

 for weeks. 



