38 BIOLOGY AND TECHNIQUE 



limits varying but a few degrees from their optimum. Others, on 

 the other hand, like bacilli of the colon group, Bacillus anthracis, 

 Spirillum choleras asiaticae, etc., may develop at temperatures as low 

 as 10 C. and as high as 40 C., or over. The range of temperature 

 at which saprophytic bacteria may develop is usually a far wider 

 one. When temperatures exceed in any considerable degree the 

 maximum growth temperature, the vegetative forms of bacteria 

 perish. Thus, ten minutes' exposure to a temperature of between 

 55 and 60 C. causes death of the vegetative forms of most micro- 

 organisms. Death in such cases is due probably to a coagulation of 

 the protoplasm, and since all such processes of coagulation take place 

 best in the presence of water, the thermal death point of most bac- 

 teria is lower when heat is applied in the form of boiling water or 

 steam, than when employed as dry heat. (See section on Steriliza- 

 tion.) 



When spores are present in cultures, the resistance to heat is enor- 

 mously increased. Exactly what the explanation of this is can not 

 at present be stated. It may be that the high concentration in which 

 the protoplasmic mass is found in the spores renders it less easily 

 coagulable than is the protoplasm of the vegetative body. A more 

 detailed discussion of these relations will be found in the section on 

 Heat sterilization. 



The thermal death points of many bacteria have been carefully 

 studied by Sternberg, 28 by a technique described elsewhere. 



The thermal death points ascertained by him in this way, with an 

 exposure of ten minutes in a fluid medium, for some of the more 

 common non-sporogenic bacteria are as follows : 



Spirillum choleras asiaticse 52 C. 



Diplococcus pneumonise 52 C. 



Streptococcus pyogenes 54 C. 



Bacillus typhosus 56 C. 



Bacillus pyocyaneus 56 C. 



Bacillus mucosus capsulatus 56 C. 



Bacillus prodigiosus 58 C. 



Staphylococcus pyogenes aureus 58 C. 



Gonococcus 60 C. 



Staphylococcus pyogenes albus 62 C. 



The bacillus tuberculosis, though not a spore bearer, seems to be 

 slightly more resistant to heat than other purely vegetative microor- 



28 Sternberg, "Textbook of Bacteriology," New York, 1901. 



