SEPARATION OF AEROBIC ORGANISMS 57 



that no strands of cotton fibre are adhering to the inside of the 

 necks. As these might be touched with the charged needle and 

 the plug thus be contaminated, they must be removed by heating 

 the inoculating needle red-hot and scorching them off with it. 

 When the platinum wires are not in use they may be laid in a 

 rack made by bending up the ends of a piece of tin, as in 

 Fig. 15. To prevent contamination of cultures by bacteria 

 falling on the plugs while these are exposed to the air during 

 inoculation manipulations, some bacteriologists singe the plugs 

 in the flame before replacing. This is, however, in most cases a 

 needless precaution. If the top of a plug be dusty it is best to 

 singe it before extraction. 



THE METHODS OF THE SEPARATION OF AEROBIC ORGANISMS. 

 PLATE CULTURES. 



The general principle underlying the methods of separation 

 is the distribution of the bacteria in one of the solid media 

 liquefied by heat and the dilution of the mixture so that the 

 growths produced by the individual bacteria called colonies 

 shall be suitably apart. In order to render the colonies easily 

 accessible, the medium is made to solidify in a thin layer by 

 being poured out on glass plates hence the term "plate 

 culture." 



As the optimum temperature varies with different bacteria, 

 it is necessary to use both gelatin and agar media. Many 

 pathogenic organisms, e.g., pneumococcus, b. diphtherise, etc., 

 grow too slowly on gelatin to allow its ready use. On the other 

 hand, many organisms, e.g., some occurring in water, do not 

 develop on agar incubated at 37 C. 



Separation by Gelatin Media, As the naked-eye and micro- 

 scopic appearances of colonies are often very characteristic, 

 plate cultures, besides use in separation, are often taken 

 advantage of in the description of individual organisms. The 

 plate-culture method can also be used to test whether a tube 

 culture is or is not pure. The suspected culture is plated (three 

 plates being prepared, as will be described). If all the colonies 

 are the same, then the culture may be held to be pure. 



Either simple plates of glass 4 inches by 3 inches are used, 

 or, what are more convenient, circular glass cells with similar 

 overlapping covers. The latter are known as Petri's dishes or 

 capsules (Fig. 16). They are usually 3 inches in diameter and 

 half an inch deep. The advantage of these is that they do not 

 require to be kept level by a special apparatus while the medium 



