MORPHOLOGY 449 



long, rather slender bacillus, often slightly curved, with rounded, somewhat swollen 

 ends and sometimes with localized swellings elsewhere, and staining unevenly with 

 such dyes as methylene blue : but in the same, or in other cultures, there will also 

 be found much shorter forms, cells which stain solidly and evenly, cells in which 

 the irregular staining takes the form of a series of transverse bars, and cells in 

 which the combination of uneven staining and localized swellings gives to a single 

 bacillus the appearance of a short chain of streptococci. This diversity of structure 

 has led to attempts to classify the various forms of C. diphtherice into definite types, 

 indicated by names, numbers or letters, and to a description of individual strains in 

 accordance with the predominant morphological form. It is quite true that different 

 strains of this organism differ very markedly in morphology and that a notable 

 frequency of one particular type of cell may characterize a particular strain through- 

 out repeated subcultures. It seems, however, very doubtful whether it is wise 

 to assign labels to different strains of this organism on the basis of morphological 

 characters alone. Moreover, as Goldsworthy and Wilson (1942) have shown, the 

 morphology of diphtheria bacilli varies greatly on different batches of Loeffler's 

 serum, depending on its mode of preparation. A strain which, on one batch, 

 develops into typical long, curved, granular forms may, on another batch, appear 

 as short rods devoid of granules, closely resembling an ordinary diphtheroid bacillus. 



The interest shown by the older workers in the morphology of the diphtheria 

 bacillus as a criterion of differentiation from other members of the group was due 

 mainly to the extensive use of stroke cultures on LoeflSer's serum. Since the 

 introduction of blood tellurite medium reliance has been placed to an increasing 

 extent on the appearance of individual colonies, and morphology has been degraded 

 to a secondary position. Furthermore, each of the three types of diphtheria 

 bacillus now recognized, gravis, intennedius, and mitis, has been found to possess 

 its own more or less characteristic morphology, with the result that microscopical 

 examination is used more to distinguish between these three types than between 

 the diphtheria bacillus and diphtheroid bacilli. 



Another feature that characterizes C. diphthericB as a species, and serves to 

 distinguish it from some, but by no means all, of the related " diphtheroids," is 

 the presence of the metachromatic granules described by Babes (1886) and by Ernst 

 (1888, 1889). These granules are coloured a reddish purple when a film prepara- 

 tion is stained with a suitable sample of methylene blue. They may be demon- 

 strated more clearly by the differential stain devised by Neisser, or by one of its 

 many modifications. A single cell may contain one or more of these granules, 

 seldom more than half a dozen, usually two or three. When only one or two 

 are present they show a definite tendency to be situated at one or both poles. 



The arrangement of the bacilli in film preparations is at least as characteristic 

 as the form of the individual cells. Adjacent cells tend to lie at any angle to one 

 another, forming a V or an L according to the degree of angular displacement ; and 

 groups of such pairs form characteristic clusters, resembling Chinese letters, or 

 cuneiform writing. It would appear, from the observations of Hill (1898-1902), 

 that this particular arrangement results from incomplete separation at the moment 

 of division, the daughter cells remaining attached at one point, and bending on 

 this attachment as on a hinge as growth proceeds. 



Finally, it may be noted that C. diphtherice provided the first instance in which 

 true branching was demonstrated in a bacillary species. The observations of Hill 

 showed that this appearance was not an artefact, but could be observed to take 

 PB. Q 



