336 



DIPHTHERIA. 



serum, alkaline blood serum (Lorrain Smith), blood agar, 

 and the ordinary agar media, though glycerine agar is less 

 suitable than the others. If inoculations be made on the 

 surface of blood serum with a piece of diphtheria membrane, 

 colonies of the bacillus appear within twenty-four hours, 

 and often before any other growths are 

 visible. The colonies are small circular 

 discs of opaque whitish colour, their 

 centre being thicker and of darker greyish 

 appearance when viewed by transmitted 

 light than the periphery. On the second 

 or third day they may reach 4 mm. in 

 size, but when numerous they remain 

 smaller. On the agar media the colonies 



FIG. 83. Colonies 

 of the diphtheria 

 bacillus on sloped 

 agar, showing darker 

 centres (with trans- 

 mitted light). Two 

 days' growth at 37 C. 

 Natural size. 



FIG. 84. Diphtheria bacilli from a twenty 

 four hours' culture on agar. 



Stained with methvlene-blue. x 1000. 



have much the same appearance (Fig. 83) but grow less 

 quickly, and sometimes they may be comparatively minute, 

 so as rather to resemble those of the streptococcus 

 pyogenes. In stroke cultures the growth forms a continuous 

 layer of the same dull whitish colour, the margins of which 

 often show single colonies partly or completely separated. 



