CULTIVATION OF THE BACILLUS 



403 



by other organisms anti-diphtheritic serum produces no favour- 

 able effect. 



Cultivation. — The diphtheria 

 bacillus grows best in cultures at 

 the temperature of the body; 

 growth still takes place at 22° C., 

 but ceases about 20° C. The best 

 media are the following : Loffler's 

 original, medium (p. 42), solidified 

 blood serum, alkaline blood serum 

 (Lorrain Smith), blood agar, and 

 the ordinary agar media. If inocu- 

 lations be made on the surface of 

 blood serum with a piece of diph- 

 theria membrane, colonies of the 

 bacillus may appear in ' twelve 

 hours, arid are well formed within 

 twenty-four hours, 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 



Fie. 112. — Cultures of the 

 diphtheria bacillus on an 

 agar plate ; twenty-six hours' 

 growth. (Natural size.) 



(a) Two successive strokes ; (b) 

 isolated colonies from the same 

 plate. 



by transmitted light, than 

 the periphery. Their 

 margins are at first regu- 

 lar, but later they become 

 wavy or even crenated. 

 On the second or third 

 day they may reach 3 mm. 

 in size, but when numerous 

 they remain smaller. On 

 the agar media the colonies 

 have much the same ap- 

 pearance (Fig. 112) but 

 grow less quickly, and 

 sometimes they may be 

 comparatively minute, so 

 as rather to resemble 

 those of the streptococcus 

 pyogenes. In stroke cul- 

 tures the growth forms a 

 continuous layer of the 

 same dull whitish colour, the margins of which often show 

 single colonies partly or completely separated. On gelatin 



m 



## 



■9'^^' 



Fig. 113. — Diphtheria colonies, two days 

 old, on agar, x 8. 



