358 



DIPHTHERIA 



remain smaller. 



n 



Fig. 120. Diphtheria bacilli of larger size 

 than in previous figure, showing also ir- 

 regular staining of protoplasm. From a 

 three days' agar -culture. 



Stained with weak carbol-fuchsin. x 1000. 



On the agar media the colonies have much 

 the same appearance (Fig. 

 117) 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 at 22 C. a 

 puncture culture shows a 

 line of dots along the 

 needle track, whilst at the 

 surface a small disc forms, 

 rather thicker in the 

 middle. In none of the 

 media does any liquefac- 

 the organism produces a turbidity 



tion occur. In bouillon 

 which soon settles to the 

 bottom and forms a pow- 

 dery layer on the wall of 

 the vessel. By starting 

 the growth on the surface 

 and keeping the flasks at 

 rest a distinct scum forms, 

 and this is especially suit- 

 able for the development 

 of toxin. Ordinary bouillon 

 becomes acid during the 

 first two or three days, 

 and several days later 

 again acquires an alkaline 

 reaction. If, however, the 

 bouillon is glucose -free 

 (p. 75) the acid reaction Fig. 121. Involution forms of the diphtheria 



does not occur. 



In these media the bacilli 

 show the same characters 

 as in the membrane, but the irregularity in staining is more marked 



bacillus ; from an agar culture of seven 

 days' growth. 

 Stained with carbol-thionin-blue. x 1000. 



