126 MANUAL OF BACTERIOLOGY. 



tions, like the nitrifying bacteria, to derive their carbon 

 from the carbon dioxide of the atmosphere, or from inor- 

 ganic carbon compounds. Although some species are 

 able to obtain nitrogen from inorganic salts, most bacteria 

 flourish best if organic substances containing nitrogen, like 

 peptone and albumen, are furnished them as part of their 

 food. The complicated, unstable, organic molecules with 

 high potential energy are converted by them into simple 

 and more stable compounds like carbon dioxide, ammonia 

 and water, with the liberation of energy. These facts be- 

 come manifest in connection with their important work in 

 decomposition, putrefaction and fermentation. A culture- 

 medium having a slightly alkaline or neutral reaction is 

 favorable to most bacteria. 



The prolonged artificial cultivation of bacteria may or 

 may not modify their properties. The pathogenic bacteria 

 are likely to undergo considerable modification both in the 

 quality and luxuriance of their growth and the intensity of 

 their pathogenic characters. 



The growth of bacteria may eventually be hindered by 

 the accumulation of the products of their own metabolism. 

 Many bacteria refuse to grow on culture-media at all. Some 

 species are extremely fastidious, and can only be propagated 

 on particular sorts of nutrient substances. 



Relation to Oxygen. Oxygen is indispensable to the 

 growth of some bacteria, aerobes. Its absence is equally 

 indispensable to certain others, anaerobes. Others still are 

 able to flourish either in the presence or absence of oxygen, 

 facultative aerobes or anaerobes. The first-named varieties 

 are sometimes called strict, or obligate aerobes or anaerobes. 



Effects of Sunlight. Direct sunlight kills the- vegeta- 

 tive forms of bacteria more or less rapidly, and constitutes 

 one of the most efficient among the natural methods of dis- 

 infection. Diffuse daylight acts much more slowly. Elec- 



