24 ESSENTIALS OF BACTERIOLOGY 



oxygen is; facultative anaerobins, those which exist better 

 where there is no oxygen, but can live in its presence. 



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. 



Aerobins may consume the free oxygen of a region and thus 

 allow the anaerobins to develop. By improved methods of 

 culture many varieties of anaerobins 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 active only 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 bacteria. 



Effects of Electricity. Electricity arrests growth. 



Effects ofRontgenRays. Have 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 

 compounds produce chemical changes in them, not only by the 

 withdrawal 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. 



Oxidation and reduction are carried on by some bacteria. 

 Ammonia, hydrogen sulphid, and trimethylamin are a few of 

 the chemical products produced by bacteria. Nitrites in the 

 soil are reduced to ammonia. 



Nitrification. Albuminoids changed into indol, skatol, 

 leucin, etc.; then these into ammonia, ammonia into nitrites, 

 nitrites into nitrates. 



Ptomains. Brieger found a number of complex alkaloids 

 closely resembling those found in ordinary plants, and which 

 he named ptomains, from xrwfjLa, corpse, because obtained 

 from putrefying objects. 



