24 BACTERIOLOGY. 



Temperature. The influence of temperature on bacteria will be 

 found to vary according to the species, but still for the majority we 

 may distinguish a maximum, optimum, and minimum temperature. 



Many grow best at the temperature of the blood, and hence the 

 value of nutrient agar-agar, which is not liquefied at 37 C. The 

 tubercle bacillus will only grow satisfactorily at a temperature 

 varying between 30 C. and 41 C. On the other hand, many forms 

 grow between the limits of 5 C. and 45 C. At these temperatures 

 their functional activity is paralysed, but they are not destroyed, 

 for by removal to favourable conditions they spring again into life. 

 Bacteria seem to have -a special power of resisting the effects of cold. 

 It has been stated that comma-bacilli exposed to a temperature 

 of 10 C. for an hour, and bacilli of anthrax after exposure to a 

 temperature of 110 C., still retained their vitality. Temperatures 

 over 50 to 60 C. destroy most bacteria, but not their spores ; spores 

 ,of anthrax retain their vitality after immersion in boiling water, but 

 are destroyed by prolonged boiling. Roughly speaking, all patho- 

 genic bacteria grow best at the temperature of the blood, and 

 non-pathogenic bacteria at the ordinary temperature of the room. 



Movement. Bacteria probably grow best when left undisturbed. 

 Violent agitation of a vessel in which they are growing certainly 

 retards their growth, but a steady movement is stated not to affect 

 it ; at any rate, anthrax bacilli grow with enormous rapidity in the 

 blood-vessels, in spite of the circulation. 



Compressed Air. Paul Bert maintained that a pressure of 

 twenty-three to twenty-four atmospheres stopped all development 

 of putrefactive bacteria. Oxygen, under a pressaire of five or six 

 atmospheres, is stated to stop their growth. Other observers have, 

 however, obtained different results. 



Gases. Hydrogen and carbonic acid are stated to stop the 

 movements of the motile bacteria. Chloroform is believed to arrest 

 the changes brought about by the zymogenic species. 



Electricity. Cohn and Mendelssohn found that a constant 

 galvanic current produced a deleterious effect owing to electrolysis. 

 At the positive pole the liquid became distinctly acid, and at the 

 negative pole distinctly alkaline. With a weak current there 

 appeared to be no effect, two powerful cells at the very least being 

 necessary. 



Light. Downes has shown that sunlight is fatal to putrefactive 

 bacteria. This is believed to be due to a process of induced hyper- 

 oxidation, from which living organisms ordinarily are shielded by 

 protective developments of the cell-wall, or of colouring-matter, 



