44 PRINCIPLES OF BACTERIOLOGY 



injured. Indeed, it is the usual custom in laboratories to preserve 

 bacteria which die readily (such as streptococci) by keeping them in the 

 refrigerator at about 4 to 6 C., after cultivation for two days at 30 C., 

 as a means for retaining their vitality without repeated transplantation. 

 Temperatures even far under C. are only slowly injurious to bacteria, 

 different species being affected with varying rapidity. This has been 

 demonstrated by numerous experiments in which they have been exposed 

 for hours in a refrigerating mixture at 18 C. They have even been 

 subjected to a temperature of 175 C. by immersing them in liquid 

 air kept in an open tube for two hours, arid 15 to 80 per cent, were found 

 to still grow when placed in favorable conditions. We found about 

 10 per cent, of typhoid bacilli alive after thirty minutes' exposure to 

 this low temperature. Staphylococci were more resistant. Spores were 

 scarcely killed at all. 



Effect of High Temperatures. Temperatures from 5 to 10 C. over 

 the optimum affect bacteria injuriously in several respects. Varieties 

 are produced of diminished activity of growth, the virulence and the 

 property of causing fermentation are decreased, and the power of spore 

 formation is gradually lost. These effects may predominate either in 

 one or the other direction. 



If the maximum temperature is exceeded the organism dies; the 

 thermal death point for the psychrophilic species being about 37 C., 

 for the mesophilic species about 45 to 55 C., and for the thermophilic 

 species about 75 C. There are no non-spore bearing bacteria which 

 when moist are able to withstand a temperature of 100 C. even for a 

 few minutes. A long exposure to temperatures between 60 and 80 C. 

 has the same result as a shorter one at the higher temperatures. Ten 

 minutes' exposure to moist heat will at 60 C. kill the cholera spirillum, 

 the streptococcus, the typhoid bacillus, and the gonococcus, and at 

 70 C. the staphylococcus, the latter being among the most resistant 

 of the pathogenic organisms which have no spores. 



Effect of Dry Heat. When micro-organisms in a desiccated condition 

 are exposed to the action of heated dry air the temperature required for 

 their destruction is much above that required when they are in a moist 

 condition or when they are exposed to the action of hot water or steam. 

 A large number of pathogenic and non-pathogenic species are able to 

 occasionally resist a temperature of over 100 C. dry heat for an hour. In 

 any large number of bacteria a few are always more resistant than the 

 majority. A temperature of 120 to 130 C. dry heat maintained for one 

 and a half hours will destroy all bacteria, in the absence of spores. 



Resistance of Spores to Heat. Spores are far more resistant to all 

 injurious influences than vegetative forms. They retain their power 

 of germination for years without either nourishment or water, and are 

 much more indifferent to the action of gases than bacilli, the spores of 

 the anaerobic species being especially resistant to the action of oxygen. 

 Spores possess a great power of resistance to both moist and dry heat. 

 Dry heat is comparatively well borne, many spores resisting a tempera- 

 ture of over 130 C. for as long as three hours. Exposed to 150 C. for 



