MORPHOLOGY AND PHYSIOLOGY OF BACTERIA. 23 



gelatinisation, a part is poured into the first tube, so that when it 

 sets the piece of tissue is completely embedded. A piece of tissue 

 from an animal suffering from maKgnant oedema is treated in the 

 same way, and the tubes are placed in the incubator. If we 

 examine them after two or three days, we shall find no change in 

 the anthrax tube ; the bacillus being eminently aerobic, no growth 

 whatever has occurred. In the tube containing the bacilli of 

 malignant oedema there will be a more or less characteristic 

 cultivation. 



The nitrogen which is essential for building up their protoplasm 

 can be obtained either from albumins, or from ammonia and its 

 derivatives. That the albumins can be dispensed with was shown 

 by Pasteur, who employed an artificial nourishing solution consti- 

 tuted upon a formula representing the essential food constituents. 



Carbon is derived from such substances as cane sugar, milk 

 sugar, and glycerine, and, in some cases, by the splitting up of 

 complex proteid bodies. 



Water is essential for their growth, but deprivation of water 

 does not kill all bacteria. Desiccation on potato is employed for 

 preserving some micro-organisms, as a new growth can be started, 

 when required, by transferring some of the dried potato to fresh 

 nourishing ground. Comma-bacilli, on the other hand, are 

 destroyed by drying. Sugar is used in making preserves, because 

 by abstracting water it prevents the development of micro- 

 organisms. 



Mineral or inorganic substances, such as compounds of sodium 

 and potassium, and different phosphates and sulphates, are necessary 

 in small propoi'tions. 



Circumstances affecting the Growth of Bacteria. 



Mature of tlie Soil. — Though we know the elements necessary, 

 we are, nevertheless, as yet unable to provide a pabulum suitable for 

 all kinds of bacteria. Thus we are quite unable to cultivate some 

 species artificially. Others will only grow upon special media. 

 Many grow vipon nutrient gelatine ; but some species only if it be 

 acid or alkaline respectively. Whether in the latter case this is due 

 purely to the reaction or to the presence of the particular ingredients 

 is an unsettled point. Though the comma-bacillus of Koch, like the 

 majority of organisms, grows best on an alkaline medium, yet it 

 is well known to flourish at the temperature of the blood on the 

 surface of potato, which is acid. 



