XIV. METHODS OF CULTIVATION. 



CULTIVATIONS of micro-organisms are usually pre- 

 pared in the laboratory in one of three ways : 



Tube cultures. 

 Plate cultures. 

 Hanging=drop cultures. 



These may be incubated either aerobically (i. e., 

 in the presence of oxygen) or anaerobically (i. e., in 

 the absence of oxygen, or in the presence of an in- 

 different gas, such as hydrogen, nitrogen, or carbon 

 dioxide) . 



With regard to the temperature at which the culti- 

 vations are grown, it may be stated as a general rule 

 that all media rendered solid by the addition of gela- 

 tine are incubated at 20 C., or at any rate at a tem- 

 perature not exceeding 22 C. (that is, in the "cold" 

 incubator) ; whilst fluid media and all other solid media 

 are incubated at 3 7 C. (that is, in the ' ' hot " incubator) . 

 Exceptions to this rule are numerous. For instance, 

 in studying the growth of the psychrophylic bacteria, 

 the' yeasts and the moulds, the cold incubator is em- 

 ployed for all media. 



Tube cultivations are usually packed in the incubator 

 in small tin cylinders, such as those in which American 

 cigarettes are sold, or in square tin boxes. Beakers or 

 tumblers may be used for the same purpose, but being 

 fragile are not so convenient. Metal test-tube racks, 

 long enough to just fit into the interior of the incuba- 

 tor and each accommodating two rows of tubes, are 

 also exceedingly useful. 



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