HELATION TO ENVIRONMENT— CLASSIFICATION 33 



from the bacillus of human tuberculosis in that its optimum growth 

 temperature lies at 41°-42° C, a temperature which exceeds the op- 

 timum temperature for the human type by as much as the normal tem- 

 perature of birds exceeds that of man. The same principle is illustrated 

 by the facts that the bacteria which have a very low optimum tem- 

 perature are usually those isolated from water, and the so-called ther- 

 mophile or high-temperature bacteria are obtained from hot springs and 

 from the upper layers of the soil, where, according to Globig,' occasion- 

 ally temperatures ranging from about 55° C. occur. 



As stated before, one and the same species may develop within a 

 wide temperature range, and it may be possible, by persistent cultiva- 

 tion at special temperatures, to adapt certain bacteria to grow luxu- 

 riantly at temperatures removed by several degrees from their normal 

 optimum. In such cases it may often occur that special characteristics 

 of the given species may be lost. An example of this is the loss of viru- 

 lence and of spore-formation which takes place when anthrax bacilli 

 are cultivated at 42° C, or the loss of the power to produce pigment 

 when bacillus prodigiosus is grown at temperatures above 30° C. 



The vegetative forms of most of the pathogenic bacteria may grow 

 at temperatures ranging between 20° C. and 40° C. This can, however, 

 by no means be regarded as applicable to all of the pathogenic bacteria, 

 as some of these, like the gonococcus, the pneumococcus, the tubercle 

 bacillus, and others, are delicately susceptible to temperature changes 

 and have the power of growing only within limits varying but a few 

 degrees from their optimum. Others, on the other hand, like bacilli of 

 the colon group. Bacillus anthracis, Spirillum choleras asiaticse, 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 

 microorganisms. 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 bacteria 

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

 than when employed as dry heat. (See section on Sterilization.) 



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



« Gldbig, Zeit. f. Hyg., iii. 



