Cultivation and Identification of Bacteria 121 



in the solidified agar. Each separate cell develops in the medium 

 producing a visible growth, called a colony, and these usually 

 represent pure cultures which arise from either a single cell or 

 from a group of like cells. If the dilutions are carried out in 

 series, the developing colonies will be far enough apart to facilitate 

 their being picked from the agar with the aid of a sterile wire or 

 loop. By subculturing these isolations to tubes of sterile broth or 

 to solid media, many different isolations are possible from different 

 colonies on a single culture plate, and they will represent those 

 organisms found in low numbers as well as the predominating 

 species in a given mixed culture. 



This solid medium technic is a decided improvement over the 

 serial broth procedure, but it still has certain undesirable features 

 which can readily be overcome. 



One of the principal disadvantages of the method is the dif- 

 ference in the appearance of colonies of the same species when the 

 organisms develop at different oxygen tensions. Colonies growing 

 on the surface of agar plates and having full access to atmospheric 

 concentrations of oxygen are usually larger than subsurface, im- 

 bedded colonies. In the discussion of chromogenesis in the pre- 

 vious chapter it was emphasized that only in the presence of an 

 abundant oxygen supply can pigmentation by chromogenic organ- 

 isms be assured. Upon examination of an agar dilution plate made 

 of such organisms, it would appear on the basis of differences in 

 pigmentation that more than one species of organism was present 

 in the plate. By inhibiting chromogenesis and reducing colony 

 size of sub-surface colonies, one is faced with serious diagnostic 

 disadvantages. Plates over-crowded with colonies is another un- 

 desirable feature of this culturing technic. 



STREAK PLATES 

 It is possible to overcome the above criticisms by a simple ex- 

 pedient. Instead of mixing the bacteria with the liquefiable-solid 

 medium before transferring it into a petri dish, the agar can be 

 poured into the dish first, allowed to harden, and then the culture 

 can be smeared or streaked on the hardened surface of the medium. 



