MORPHOLOGY AND PHYSIOLOGY OF BACTERIA. II 5 



Moisture is indispensable to the growth of bacteria, and dry- 

 ing causes the death of certain kinds, as, for instance, the spiril- 

 lum of cholera, while others remain alive, but do not grow. 



Food. — There are a few species of bacteria that contain 

 chlorophyll, but it is wanting in most forms. On account of 

 the absence of chlorophyll, bacteria require, as part of their food, 

 organic compounds, such as sugar, as a source of carbon. They 

 are unable, with very few exceptions, such as the nitrifying bac- 

 teria, to derive their carbon from the carbon dioxide of the atmos- 

 phere, or from inorganic 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 become manifest 

 in connection with their important work in decomposition, 

 putrefaction and fermentation. A culture-medium having a 

 sHghtly 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 quaUty 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 metaboHsm. Many 

 bacteria refuse to grow on culture-media at all; at least the 

 suitable artificial medium has not yet been found for them. 

 Some species are extremely fastidious, and can only be propa- 

 gated on particular nutrient substances. But bacteria show 

 great adaptability, and, once they have been made to start, they 

 can be further cultivated with less and less diflSculty as a rule. 



Relation to Oxygen. — Oxygen is indispensable to the 



