16 BACTERIOLOGY FOR NURSES 



will not take place and its minimum temperature below which it 

 is inactive. Death does not necessarily occur at these limits 

 but reproduction does not take place. The maximum and mini- 

 mum for each species has a range of from twenty to thirty degrees 

 and its optimum does not extend ordinarily more than five degrees. 

 The maximum for some may be below the minimum for others, 

 for example, for B. phosphorescens the minimum temperature at 

 which growth occurs is C., the optimum 20 C., and the maxi- 

 mum 37 C., while for B. thermophilus, an organism found in fer- 

 menting manure, the minimum temperature is 40 C., the opti- 

 mum about 66 C., and the maximum 72 C. Generally speaking, 

 the optimum temperature for bacteria is the ordinary temperature 

 of their natural habitat. The most favorable temperature then 

 for pathogenic organisms is that of the human body. If grown on 

 culture media at a higher temperature they may lose their viru- 

 lence. 



The vegetative forms of most bacteria are killed by half an hour's 

 exposure in the presence of moisture to a temperature of from 

 55 C. to 58 C. or by 10 minutes' exposure to a temperature of 

 60 C. to 80 C. There are no non-spore-bearing forms, except 

 a few cocci, that can live in boiling water even for a few minutes. 

 Most of the pathogenic bacteria, including the cholera spirillum, 

 the typhoid bacillus, and the tubercle bacillus, are destroyed in 

 ten minutes when exposed to moist heat at 60 C. Thus milk 

 properly pasteurized or water brought to the boiling point are 

 rendered harmless so far as these germs are concerned. 



Dry heat is much less effective than moist ; many pathogenic 

 organisms can withstand in the absence of moisture a temperature 

 of 100 C. for half an hour. Spores are especially resistant to both 

 dry and moist heat. Practically all forms, however, are killed by 

 exposure to dry heat for one hour at 150 C. or to steam under 

 pressure in an autoclave for 15 minutes at 125 C. 



Bacteria are affected by low temperatures much less than by 

 high. Many have been subjected to a temperature of liquid air 

 (about -190 C.) without being destroyed. 1 In a culture of 

 1 Park and Williams, " Pathogenic Microorganisms," p. 56. 



