28 ESSENTIALS OF BACTERIOLOGY 



to oxygen the one aerobic, growing in air; the other, anae- 

 robic, living without air. 



Obligate aerobes, those which exist only when oxygen is 

 present. 



Facultative aerobes, those that live best when oxygen is 

 present, but can live without it. 



Obligate or true anaerobes, those which cannot exist where 

 oxygen is ', facultative anaerobes, those which exist better \vhere 

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



Some derive the oxygen which they require out of their 

 nutriment, so that a bacterium may be aerobic and yet not 

 require the presence of free oxygen. 



Aerobes may consume the free oxygen of a region and thus 

 allow the anaerobes to develop. By improved methods of 

 culture many varieties of anaerobes 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; the effect of sunlight is not due to heat-rays. 



Effects of Electricity. Electricity arrests growth. 



Effects of Rb'ntgen Rays. Have little or no effect on arti- 

 ficial cultures, but in the living tissues a pronounced bacteri- 

 cidal effect is produced, perhaps through the stimulation of 

 the body-cells. 



Moisture. Water is necessary for the development of most 

 bacteria; complete drying is usually destructive after a few 

 days. 



Heat. Dry heat is much less destructive than moist heat, 

 steam under pressure most destructive. 



Biologic Activities. Bacteria feeding upon organic com- 

 pounds produce chemic changes in them, not only by the 

 withdrawal of certain elements, but also by the excretion 

 of these elements changed by digestion. Sometimes such 



