24 ESSENTIALS OF BACTERIOLOGY. 



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

 others again that will not live under a temperature of GO < . 



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 cerofo'c, growing in air ; the other anwrobic, living 

 without air. 



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



Facultative a>robins, those that live best when oxygen is present, 

 but can live without it. 



Obligate or true ancerobins, those which cannot exist where 

 oxygen is. 



Facultative ancerobins, 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. 



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

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

 ture many varieties of anse robins 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 have been killed 

 by a few days' standing in daylight. 



Effects of Electricity. Electricity arrests growth. 



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. 



Oxidation and reduction are carried on by some bacteria. Am- 

 monia, hydrogen sulphide, and trimethylamin are a few of the 

 chemical products produced by bacteria. 



Ptomaines. Brieger found a number of complex alkaloids, 

 closely resembling those found in ordinary plants, and which 



