450 CORYNEBACTERIUM 



place during the growth of the Kving cell. This character, among others, decided 

 the American Committee to separate this genus from the Euhacteriales, and include 

 it with some others in a family of the order Aclinomycetales. It has, however, 

 been noted in Chapter 2 that rudimentary branching has occasionally been observed 

 in such typically bacillary forms as those of the genus Bacterium. 



The morphology of other species of corynebacteria departs, to a greater or less 

 extent, from that of C. diphtherice. Sometimes, as with C. ovis and C. murium, 

 the resemblance may be so close that an experienced observer would be unable 

 to distinguish either of these species from the true diphtheria bacillus on morpho- 

 logical appearances alone. Sometimes, as with C. Jwfmanni, the differences 

 are so clear-cut that little difficulty arises. 



It is characteristic of many species of diphtheroids that their morphology, as 

 displayed in films from young cultures, is far less variable than that of C. dij)h- 

 therice. A film of C. xerosis, for example, may show a marked resemblance to some 

 of the average long forms of C. dipJitJierice, but while pleomorphism will usually 

 be minimal in the former it will be marked in the latter. Similarly the so- 

 called C. coryzce segmentosum may be very similar to the barred form which 

 is sometimes assumed by C. diphtherice ; but again pleomorphism is slight or 

 absent. 



With regard to the staining reactions of this group the bare statement that 

 the corynebacteria are Gram-positive bacilli needs some qualification. The type 

 species retains the stain to a sufficient degree to differentiate it quite clearly from 

 the frankly Gram-negative bacteria ; but it is decolorized by alcohol more easily 

 than are many Gram-positive species. The metachromatic granules, on the other 

 hand, retain the stain tenaciously ; so that moderate overdecolorization, followed 

 by the use of a red or brown counterstain, may give a picture very similar to that 

 afforded by the use of Neisser's stain. Of the other species within this genus, 

 C hofmanni is very resistant to decolorization ; so that the application of Gram's 

 stain, followed by a prolonged exposure to alcohol (15 mins. or so), affords a useful 

 differential criterion between these two species. Of the rest, some species behave 

 as C. hofmanni, others as C. diphtherice. Absence of flagella and lack of motility 

 are characteristic of the genus, as also is the absence of capsulation. 



Cultural Characters. 



With the exception of C. acnes, the members of this group grow on ordinary 

 nutrient agar, though to a variable degree. Many of the diphtheroids, including 

 C. hofmanni, develop freely, but the growth of all members is improved by the 

 presence of natural animal protein. For several decades Loeffler's serum con- 

 stituted the medium of choice. In stroke cultures on this medium there is a 

 fairly abundant growth within 24 hours, having a moist, slightly creamy and 

 sometimes faintly pigmented appearance. The degree of development, however, 

 varies, and all transitions are seen between a slightly raised colourless film and 

 a profuse succulent pigmented growth. 



Since the introduction of blood tellurite medium attention has been concen- 

 trated mainly on the appearance of individual colonies, particularly in relation 

 to the diphtheria bacillus. In 1931 McLeod and his colleagues at Leeds (Anderson, 

 Happold, McLeod and Thomson 1931) recorded the differentiation of two types 

 of this organism. One, which was prevalent in severe cases, they called gravis ; 

 the other, which was isolated from milder cases, they called mitis. In a subsequent 

 report (Anderson, Cooper, Happold and McLeod 1933) they extended these observa- 



