THE SEPARATION OF AEROBIC BACTERIA. 61 



Fig. 1 6. Before commencing inoculation manipulations, 

 this rack ought to be sterilised by passing it several 

 times through the flame. 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. 



The general principle underlying the methods of separa- 

 tion is the dilution of the bacterial mixture, till each 

 microbe is sufficiently separated from its neighbours to 

 allow it to multiply into a growth (called a " colony "), 

 without the latter coming in contact with the colonies pro- 

 duced by other microbes present. In order to render the 

 colonies easily accessible, the medium is made to solidify 

 in as thin a layer as possible, by being poured out on glass 

 plates. 



As the optimum temperatures of organisms vary, it is 

 necessary to adapt to the process a low melting-point 

 medium, such as gelatine, and a high melting-point medium, 

 such as agar. Many pathogenic organisms, e.g., pneumo- 

 coccus, B. diphtheriae, etc., grow too slowly on gelatine 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 Gelatine Media. With both the gelatine 

 and agar media the fluid medium containing bacilli is 

 poured out on plates of glass, and, therefore, when growth 

 takes place, " plate cultures " are said to be obtained. As 

 the naked-eye and microscopic 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 



