^ 



BIOLOGY OF BACTERIA. 49 



Most bacteria are killed by temperatures above 6o°-75° 

 C. The spores can resist boiling water for some minutes, 

 but are killed by dry heat if exposed to 150 C. for an 

 hour or to 175 C. for five to ten minutes. 



The resistance of low forms of life to low temperatures 

 is most astonishing. Cold inhibits the growth of all bac- 

 teria, and immersion in freezing-mixtures will destroy a 

 few. The spores seem capable of resisting almost any 

 degree of cold. Ravenel 1 performed some interesting 

 experiments with liquid air. Anthrax spores were ex- 

 posed to its action for three hours ; diphtheria bacilli for 

 thirty minutes, typhoid bacilli for sixty minutes, and 

 Bacillus prodigiosus for sixty minutes. The temperature 

 to which the cultures were reduced was about — 31 2° 

 F., yet in no case was the vegetative capability of the 

 bacteria destroyed, and when planted in culture bouil- 

 lon they all grew normally. His researches corroborate 

 those of Pielet and Young and others. 



Bacteria usually grow best at the ordinary temperature 

 of a comfortably heated room, and are not affected by its 

 occasional slight changes. Some, chiefly the pathogenic 

 forms, are not cultivable except at the temperature of the 

 animal body (3J C); others, like the tubercle bacillus, 

 grow best at a temperature a little above that of the 

 body — 40 C. 



(/) x-Rays. — The action of the ^r-rays upon bacteria 

 has been investigated by Bonome and Gros, 2 Pott, 3 and 

 others. When the cultures are exposed to their action 

 for prolonged periods their vitality and virulence seem 

 to be slightly diminished. They are not killed by the 

 .r-rays. 



(k) Antiseptics, etc. — The presence of certain substances 

 — especially some of the mineral salts — in an otherwise 

 perfectly suitable medium will prevent the development 

 of bacteria, and when added to grown cultures of bac- 



1 The Medical News, June 10, 1899. 



* Giornal. med. del Regis Esercito, an. 45, u. 6. 



s Lancet, vol. ii. No. 21, 1897. 



