Gelatin colonies: Slow development. Very small, grayish, lobulate. Gelatin stab: Slight growth on surface and scant growth in stab. No liquefaction. Agar slant: Scant, grayish, granular, translucent growth, with irregular margin. Blood-tellurite media: Produces gray to black colonies. Colony forms: Smooth (S) colony form: Round and umbonate or convex, with even margin and smooth surface. Opaque when viewed by transmitted light, glistening and somewhat moist in appearance when viewed by reflected light. Colonies about 1 to 3 mm in diameter. Growth frequently slowed or inhibited by the presence of potassium tel- lurite in the medium. Rough (R) colony form: Flat, margin is very irregular. Surface is pitted and very uneven. Very little light reflected from sur- face. Translucent when viewed by trans- mitted light. Colonies about 1 to 5 mm in diameter. Intermediate colony forms: Several col- ony forms are found in this group since the term includes all forms between the pure S form and the pure R form. Sr forms very nearlj' approach the S colonies and the sR forms nearl}' approach the pure R forms. The SR form shows properties distinct from either the S or R forms. The colonies are 3 to 5 mm in diameter. The margin usually shows indentations. The surface is raised but not convex; it may be nearly level or show a central elevation surrounded by a concentric depression and elevation. Dwarf (D) colony form: Colonies very small, about 0.2 mm or less in diameter. Margin round and even. Surface convex. All of the above colony forms have been isolated from cases of diphtheria (Morton, Jour. Bact., ^0, 1940, 768 ff.). Broth: Uniform turbidity produced by S form, pellicle produced by SR form, sedi- ment produced by the R form. Litmus milk: Unchanged. Potato: No visible growth. Blood serum: Growth grayish to cream- colored, moist, smooth, slightly raised, margin entire. May be bright yellow or oc- asionally reddish (Hill, Sci., 17, 1903, 375). Indole not produced. All strains produce acid from glucose and fructose; some strains also ferment galac- tose, maltose, sucrose, dextrin and glycerol. Nitrites produced from nitrates. Does not hydrolyze urea (Merkel, Zent. f. Bakt., I Abt., Orig., U7 , 1941, 398). A highly poisonous exotoxin is produced in fluid media. This toxin represents the principal disease-producing agency of the organism. Toxin production may fail in otherwise tj-pical strains. A highly potent antitoxin can be pro- duced by repeated injection of toxin into experimental animals. The antitoxin pos- sesses both curative and protective proper- ties. Serological types : In a study of 250 strains of Corynebacterium diphtheriae, Murray (Jour. Path, and Bact., 41, 1935, 439-45) was able to classify 228 strains into 11 sero- logical types, and 22 strains remained un- classified (Morton, Bact. Rev., 4, 1940, 196). McLeod et al. (Jour. Path, and Bact., 34, 1931, 667; ibid., 36, 1933, 169; Lancet, 1, 1933, 293) describe three tj-pes which have been confirmed by other workers; these are distinguishable by colony form on McLeod's blood-tellurite medium, they are antigeni- cally different with subtypes, there is some difference between their toxins (Etris, Jour. Inf. Dis., 50, 1934, 220) and the severity of disease is associated with the type. Corynebacterium diphtheriae type gravis grows with dark gray, daisy-head colonies; ferments dextrin, starch and glycogen; is not hemolytic; has very few small metachro- matic granules; forms a pellicle and a gran- ular deposit, and there is an early reversal of pH in broth. Corynebacterium diphtheriae type mitis grows in convex, black, shiny, entire colo- nies; no fermentation of starch and glycogen, and it is variable with dextrin; hemolytic; metachromatic granules are prominent; diffuse turbidity, infrequent pellicle, and there is a late reversal of pH in broth. Corynebacterium diphtheriae type intenne- dius produces a small, flat, umbonate colony with a black center and slightly crenated periphery; not hemolytic; barring of bacilli is accentuated; there is no fermentation of starch and glycogen, and it is variable with dextrin ; forms no pellicle but a fine granular
