214 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 



Special mention of the preparation of tissue media is made in the 

 sections on Specific Anaerobic Organisms. 



The Incubation of Bacterial Cultures. The growth of bacteria in 

 artificial media is markedly influenced by the temperature to which 

 they are exposed. A majority of those organisms parasitic upon or 

 pathogenic for man develop most luxuriantly at the temperature of 

 the human body, 37 C. Exposure to temperatures but slightly above 

 37 C. leads to rapid death of these organisms, consequently incuba- 

 tors must be available within which cultures may be safely exposed 

 to a uniform and constant degree of heat equal to that of the human 

 body. Gelatin cultures must be maintained at temperatures not 

 exceeding 22 C. 



Incubators are single- or double-walled chambers of various sizes, 

 heated directly by gas or electricity, or indirectly through a water 

 jacket. The latter run more uniformly, because water receives and 

 imparts heat more slowly than air. On the other hand, large incuba- 

 tors cannot be surrounded with water jackets because of mechanical 

 difficulties. The regulation of temperature within incubators is con- 

 trolled by bimetallic regulators which actuate valves or electromagnets 

 controlling the supply of gas or electricity which heats the chamber, 

 or by mercurial thermoregulators working upon the principle of the 

 mercury thermometer. Bimetallic regulators, in which the movement 

 imparted to the regulator of the source of supply of heat is due to the 

 differential expansion or contraction of two dissimilar metals, are more 

 sensitive to slight variations in heat and they possess the additional 

 advantage of being less fragile than mercurial regulators. Various 

 patterns of thermoregulators of tried efficiency are on the market and 

 a selection between them is largely a matter of mechanical adaptability 

 to local needs. 



VI. The Study of Bacterial Cultures. I. Growth in Solid Media. 

 (a) Colonies. The macroscopic appearance of bacterial colonies upon 

 solid media is of considerable value for the differentiation and recog- 

 nition of the various types; in a similar manner their microscopic 

 appearance, stained or unstained, permits of some differentiation. 



The aspect of a colony is influenced. 



1. By the kind of organism Streptococcus colonies, for example, 

 are habitually small and nearly transparent; anthrax colonies are 

 habitually larger and opaque. 



2. By the consistency of the medium in firm, dense media the 

 growth of bacteria is limited and relatively dry; in moist, semisolid 



