1 8 BACTERIA 



acids and wax have been found. In others, xanthin bases, cellu- 

 lose, starch, chitin, iron salts, and sulphur grains have been dis- 

 covered. The essential protein of the cell body is highly nitrog- 

 enous and is usually combined with some carbohydrate as a 

 glyconucleo-protein. The salts in the ash are mostly composed 

 of various phosphates. Intracellular toxins in combination 

 with the cytoplasm are found in certain groups of bacteria, 

 e.g., B. typhosus. 



BIOLOGICAL CONDITIONS 



Bacteria are arbitrarily classes as parasites, or saprophytes. 

 They may be so dependent upon the tissues of the infected 

 organism as to be a strict parasite and incapable of growth under 

 any other condition (Mycobact. leprce), or they may be capable 

 of life on artifical culture media (tubercle bacillus), or of life in 

 the body, on culture media containing organic matter (influenza 

 bacillus), or in the soil (B. tetani). 



Saprophytes are bacteria capable of living upon dead organic 

 matter, in soil, in water, in air; they are not parasitic and do not 

 resist the defenses of the living body. 



Certain biological conditions are essential for the growth of 

 bacteria: water, oxygen, carbon ; nitrogen, and salts are neces- 

 sary. For certain parasitic bacteria, highly complex substances 

 are indispensable: meat albumins, peptones, milk, egg albumin, 

 blood serum, and sugars are the ingredients of various culture 

 media. 



The chemical reaction of such media is important: it should 

 either 'be faintly acid or faintly alkaline. The greatest number 

 of water bacteria grow in media that are slightly acid, while 

 diphtheria produces its strongest toxins and grows best in 

 alkaline media. Salt-free media is required for a number of 

 pathogenic bacteria, e.g., the gonococcus, B. leprae. 



All bacteria require for their growth either free oxygen, as in 

 air, or combined oxygen, as in albumin, water, etc. Those that 



