CULTURAL CHARACTERS 451 



tions, and concluded that certain strains which they had found previously to 

 corresjjond to neither the gravis nor the ryiitis type constituted a third type — now 

 generally known as mterniedius. Robinson and Marshall (1934) at Manchester, 

 and numerous subsequent workers, have confirmed the accuracy of these observa- 

 tions. The three colonial types with their associated morphological, biochemical 

 and pathogenic characters are now generally recognized as stable varieties of the 

 diphtheria bacillus (see Siemens 1938). 



A general description of the colonial appearance of the gravis, intermedins and 

 mitis types is rendered peculiarly difficult because of the absence of a standard 

 medium. McLeod and his colleagues originally used a heated rabbit-blood tellurite 

 agar, on which the differentiation of the three variants was excellent. Later, 

 however, it was found that some mitis and occasional gravis strains failed to develop 

 on this medium. Numerous modifications have therefore been introduced. Rabbit 

 and guinea-pig blood in 5-10 per cent, concentration give the best type differentia- 

 tion, but are difficult to obtain in quantity. Sheep, horse and ox blood are less 

 satisfactory, but they are improved either by heating the medium in which they 

 are contained or by lysis of the red cells (Neill 1937, Hoyle 1941). Against the 

 first method is the objection, noted above, that some strains, particularly of the 

 mitis type, are inhibited by heated blood (Glass 1937, 1939a) ; against lysis there 

 is the objection that the ability of the different variants to cause haemolysis on 

 blood agar plates is obscured. On the whole, therefore, we favour a medium 

 made with unheated and unlysed blood containing sufficient potassium tellurite 

 to prevent or restrain the growth of some of the organisms that are likely to be 

 present in the nose or throat. A 10 per cent, sheep blood 0-5 per cent, glucose 

 agar medium containing a final concentration of 0-03-0-04 per cent, potassium 

 tellurite is satisfactory in practice, though the type differentiation is not so good 

 as on McLeod's medium. 



The colonial appearances of the three types will be summarized more con- 

 veniently, along with their morphological, biochemical and other characters, in 

 a later section of this chapter (p. 462). Suffice it to say here that diphtheroid 

 bacilli also develop on a tellurite medium, forming colonies which are usually low 

 convex, undifferentiated, about 1-2 mm. in diameter, varying in colour from pearl 

 grey to jet black, with an entire edge, and a smooth, finely granular, or liquorice 

 type of surface. Most of them can be easily recognized, but some strains form 

 colonies so closely resembling the gravis, intermedins or mitis types of the diphtheria 

 bacillus that they can be distinguished from them only after careful study of their 

 other characters. 



Growth occurs readily in broth, but is seldom abimdant . The degree of turbidity, 

 pellicle formation, and the amount and nature of the deposit vary with different 

 members of the group ; with some species, as for instance the three types of 

 diphtheria bacillus, they are of value in identification. 



Growth in gelatin at room temperature is generally poor to moderate. A few 

 members of the group, like C. ovis, C. pyogenes, and certain diphtheroids, liquefy 

 the medium, but most members, including the diphtheria bacillus, do not. Wright 

 (Report 1942) has drawn attention to gravis-\ike strains of a toxigenic diphtheroid 

 bacillus that liquefy gelatin in slope but not in stab culture. 



Resistance. — C. diphtherice is readily killed by heat, suspensions of the bacilli 

 failing to survive 10 minutes' heating at 58° C. It is also easily destroyed by most 

 of the usual antiseptics. It would appear to be relatively resistant to drying, 



