98 



A MANUAL OF BACTERIOLOGY 



developed, inoculations can be made from them by means 

 of a platinum needle on to tubes of sterile media. The 

 colonies, having arisen from single organisms, are pure, 

 and the resulting sub-cultures are therefore also pure (it 

 sometimes happens that the colonies are mixed owing to 

 two or more organisms being close together). Different 

 species of organisms usually form colonies having different 

 appearances, so that the colonies are an aid in diagnosis 

 and enable the various species to be picked out from a 

 mixture. The colonies in gelatin are as a 

 rule much more distinctive than those in 

 agar. Whereas the plate cultivation pre- 

 pared from tube No. 1 is generally too 

 crowded, plates 2 or 3, or both, can be made 

 use of, and it is apparent that, to make 

 certain of isolating all the organisms from a 

 mixture, several sets of plates should be 

 prepared. Flat bottles (Fig. 19) may like- 

 wise be used for plate culturing, and are 

 also very useful for growing organisms in 

 bulk for the examination of the constituents 

 and actions of the bacterial cells. 



Golding has devised flat wedge-shaped 

 flasks (having sides at an appropriate angle) 

 for plate culturing, and these are very useful, as the culture 

 medium may be kept in them ready for use. 



Agar plate cultures may be prepared in a similar way. 

 The agar must, however, be brought to a temperature of 

 nearly boiling before it melts ; it is then allowed to cool 

 to nearly 45 C. and the tubes are inoculated in the same 

 manner as for a gelatin plate culture described above. 

 Unless the manipulations be carried out expeditiously the 

 agar will solidify, or the agar film in the Petri dish be 

 lumpy. 



Agar plates should usually be inverted during incuba- 



FIG. 19. 

 " Plate " bottle. 



