THE SEPARATION OF AEROBIC BACTERIA. 



S3 



Fig. i8. — Rack for platinum needles. 



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. i8). 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 replac- 

 ing. 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 separa- 

 tion 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 as thin a layer as 

 possible, by being poured out on glass plates — hence the term 

 " plate-cultures," — or in Petri's dishes. 



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. diphtherias, 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 separa- 

 tion, 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 colo- 

 nies are the same, then the cultures may be held to be pure. 



Fig. 19. — Petri's capsule or 

 dish. (Cover shown partially 

 raised.) 



