56 METHODS OF CULTIVATION OF BACTERIA 



used, and a little of the culture is smeared in a line along the 

 surface of the medium from below upwards. In inoculating 

 tubes, it is always well, on removing the plugs, to make sure 



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 scorch- 

 FIG. 16. Rack for platinum needles. ing 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. 16. 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 as thin a layer as 

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

 "plate cultures." 



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. diphtheriae, 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 



