THE GRAVIS, MIT IS AND INTERMEDIUS TYPES OF G. DIPHTHERIA 463 



TABLE 30 

 Characters of the Types of Diphtheria Bacillus. 



Gravis. 



Morphology on 

 tellurite blood 



Colonial appear- 

 ance on tel- 

 lurite blood 

 agar. 



Usually short rods, 

 resembling irregular 

 Hofmann. Staining 

 fairly uniform, few or 

 no granules, and often 

 a narrow unstained bar 

 dividing the rod un- 

 equally. Some degree 

 of pleomorphism with 

 irregularly barred, 

 snow-shoe and tear- 

 di'op forms. May be 

 coccoid on first isola- 

 tion. Occasional strains 

 resemble intermedius 

 or mitis type. 



x4n 18-hr. colony is 1-2 

 mm. q., circular, low 

 convex, pearly grey 

 or with greyish -black 

 centre and paler semi- 

 translucent periphery, 

 with a smooth matt or 

 rarely liquorice type 

 of surface, and a com- 

 mencing crenation of 

 the edge. The colony 

 is coherent, tending, 

 when touched, to move 

 as a whole on the sur- 

 face of the medium ; 

 has a consistency of 

 cold margarine, and 

 breaks up radially into 

 small masses that are 

 not easily emulsified. 

 Slight hemolysis a- 

 round colonies of some 

 strains. In 2-3 days 

 colony reaches 3-5 mm. 

 in diameter, is flat- 

 tened with a slightly 

 raised centre, is slaty- 

 grey or greyish-black 

 in colour, often darker 

 at the centre than at 

 the periphery, has a 1 

 frosted surface and i 

 a crenated edge, and 

 shows radial striation, 

 especially towards the 

 margin. When stria- 

 tion and differentiation 

 are well developed, the 

 term " daisy head " 

 colony is applicable. 



Intermedius. 



Usually long, irregu- 

 larly barred rods, often 

 with termmal club- 

 bing. Granulation 

 generally poor. Pleo- 

 morphism always pre- 

 sent. Distinguished 

 from diphtheroids, 

 which may closely 

 resemble them, by 

 irregularity of size, 

 shape, barring and 

 arrangement. Some 

 strains indistinguish- 

 able from mitis. 



Colonies are uniform, 

 small, discrete, deli- 

 cate, almost misty in 

 appearance, and under- 

 go little increase in 

 size between 24 and 

 48 hours. At 18 hours 

 the colony is less than 

 I mm. q., is slightly 

 raised, with or with- 

 out umbonation, or is 

 of sugar-loaf appear- 

 ance ; centre is greyish- 

 black and generally 

 darker than periphery; 

 sui'face smooth or very 

 finely granular, and 

 edge entire or slightly 

 spiky. Consistency is 

 intermediate between 

 the brittle gravis and 

 the butyrous mitis 

 type. Hsemolysis never! 

 seen. At 48 hours 

 colony is not much 

 larger, has a dull 

 granular centre and a 

 smoother more glisten- 

 ing periphery, and is 

 dark in colour except 

 for a lighter ring near 

 the edge — frog's-egg 

 appearance. Edge may 

 be entii'e or finely 

 crenated. On further 

 incubation, colony may 

 enlarge and come to 

 resemble a daisy-head 

 colony of the gravis 

 type. 



Mitis. 



Usually long, curved, 

 pleomorphic rods with 

 prominent metachro- 

 matic granules. Ex- 

 cept for some shadow 

 areas, protoplasm 

 stains evenly. Some 

 strains show barring, 

 with or without 

 granules. Occasional 

 strains are coccoid and 

 others yeast-like. 



Very variable in size, 

 usually ranging be- 

 tween intermedius and 

 gravis. At 18 hours 

 colonies may be less 

 than 1 mm. q. up to 

 1-5 mm. q. They are 

 circular, convex, us- 

 ually of a mushroom- 

 grey colour, darker 

 than that of gravis, 

 though varying con- 

 siderably . with a 

 smooth, glistening sur- 

 face and entire edge. 

 Consistency is of soft 

 butter, and emulsi- 

 fiability is easy. Small 

 ring of haemolysis is 

 usual. At 48 hours 

 colony is 2-4 mm. q., 

 undifferentiated, and 

 dark greyish - black 

 with sometimes a nar- 

 row paler margin. On 

 further incubation 

 colony may become 

 flatter with a central 

 elevation — poached- 

 egg appearance — and 

 the surface may be- 

 come granular and 

 contoured, or develop 

 concentric rings or 

 papular excrescences. 



