DIPHTHERIA 45 



narrow rods ; they are either straight or slightly curved in an 

 arc of a large circle or into an /shape (Plate I., Figs, i and 2). 

 Their ends are usually rounded, but it is not uncommon to find 

 forms with one end or both sharply pointed. Lastly, clubbed 

 forms are to be met with in almost all cultures, though they are 

 most frequent in those which have been incubated for several 

 days ; they may be compared to a note of exclamation (!). 



2. Size. — Two well-marked varieties occur. The long form 

 is about as long as a tubercle bacillus (to compare it with 

 an organism with which the practitioner may readily become 

 acquainted), or somewhat longer ; it is decidedly thicker. The 

 short form is only about half as long and thick in proportion. 



We do not know anything as to the difference in pathogenicity 

 (if any) of the long and the short varieties of the diphtheria 

 bacillus. They appear to " breed true " for long periods, and 

 cases of diphtheria caused by the one appear to have as high a 

 mortality as those caused by the other. 



Hoffmann's bacillus is dealt with subsequently. 



3. Staining Reactions. — The diphtheria bacillus stains readily 

 with all the stains in common use for bacteriological purposes. 

 It usually (but not invariably) stains irregularly, deeply-stained 

 portions alternating with others which remain colourless. This 

 gives rise to a beaded appearance, and forms sometimes occur 

 which can hardly be distinguished from short chains of strep- 

 tococci. When a powerful stain is applied for a long time this 

 appearance may be lost. 



The diphtheria bacillus stains deeply with thionin. This often 

 affords a certain amount of help in the diagnosis, as many other 

 bacilli do not stain nearly so deeply in the same time. 



It stains by Gram's method. A beginner should always test 

 his results in this way: If suspicious bacilli do not retain the 

 violet stain they are not those of diphtheria. 



4. Arrangement. — This is a most characteristic feature, but it 

 is one which is difficult to describe. The old comparison to the 

 strokes which form a Chinese letter is a fairly good one; the 

 bacilli lie in little groups, some lying parallel to one another, and 

 some at various angles with these. The characteristic arrange- 

 ment is best seen in a specimen made from a pure culture of the 

 short form. 



Before coming to a conclusion as to the presence or absence of 

 diphtheria bacilH from an examination of a stained film, make a 



