THE SEPARATION OF AEROBIC BACTERIA. 57 



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. 16. Before 

 commencing inoculation manipulations, this rack ought to 

 be sterilised by passing it several times through the flame. 



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 produced 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 s , 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. 

 Either simple plates of glass 4 

 inches by 3 inches are used, or, 

 what are more convenient, cir- 

 cular glass cells with similar 

 overlapping covers. The latter 

 are known as Petri's dishes or , FlG - 17- Petri's capsule. 



i /T- \ T>U (Cover shown partially raised. ) 



capsules (Fig. 1 7). 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 



