138. PATHOGENIC BACTERIA. 
once to the growth on the serum-tube and make a 
series of strokes on 3 or 4 tubes. When the colonies: 
develop, those in the last tube will be sufficiently 
distinct so that pure cultures may be made from 
them. Another method is to inoculate a bouillon- 
tube or a tube of sterile water or normal salt solu- 
tion, and immediately from this make stroke cult- 
Fic. 73.—Bacterium diphtheriz, colony twenty-four hours 
old upon agar; x 100 (Frankel and Pfeiffer). 
ures on serum-tubes. The growth on these will 
probably be in individual colonies. Either of these 
methods obviates the necessity of making plates. 
g. Study the morphology and biology of the 
diphtheria bacterium obtained above. 
to. The following stains are diagnostic for the 
diphtheria bacterium : 
