24 ESSENTIALS OF BACTERIOLOGY. 



A few varieties exist only at freezing point of water; and 

 others again will not live under a temperature of 60 C. 



For the majority of Bacteria a temperature of 60 C. is de- 

 structive ; and several times freezing and thawing very fatal. 



Influence of Oxygen. Two varieties of bacteria in relation to 

 oxygen. The one aerobic, growing in air; the other, anserobic, 

 living without air. 



Obligate asrobins, those which exist only when oxygen is present. 



Facultative aerobins, those that live best when oxygen is present, 

 but can live without it. 



Obligate or true anasrobins, those which cannot exist where 

 oxygen is. Facultative anserobins, those which exist better where 

 there is ^ oxygen, but can live in ito prcQGncc.'* > ' v> ^*^'*^' 



Some derive the oxygen which they require out of their nutri- 

 ment, so that a bacterium may be aerobic and yet not require 

 the presence of free oxygen. 



^robins may consume the free oxygen of a region and thus 

 allow the anserobins to develop. By improved methods of cul- 

 ture many varieties of anserobins have been discovered. 



Influence of Light. Sunlight is very destructive to bacteria. 

 A few hours' exposure to the sun has been fatal to anthrax 

 bacilli, and the cultures of bacillus tuberculosis. The sun's 

 rays, however, must come in direct contact with the germs, and 

 are usually only active on the surface-cultures. The rays at 

 the violet end of the spectrum are the most active. The 

 electric arc light has much the same effect as sunlight on bac- 

 teria. 



Effects of Electricity. Electricity arrests growth. 



Effects of Rontgen Rays. Haye little or no effect on artificial 

 cultures, but in the living tissues a pronounced bactericidal 

 effect is produced, perhaps through the stimulation of the 

 body-cells. 



Vital Actions of Microbes. Bacteria feeding upon organic com- 

 pounds produce chemical changes in them, not only by the with- 

 drawal of certain elements, but also by the excretion of these 

 elements changed by digestion. Sometimes such changes are 

 destructive to themselves, as when lactic and butyric acids are 

 formed in the media. 



