42 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



Examination of the Cultures. 



1. Naked-eye.— Esich living diphtheria bacillus which has been 

 deposited upon the surface of the culture medium and kept at a 

 suitable temperature will develop into a colony of bacilli; and 

 these colonies are fairly distinctive, being different from those 

 which are formed by most other organisms. The expert 

 bacteriologist can often give an accurate guess as to the presence 

 or absence of diphtheria bacilli by mere inspection of the 

 cultures. The colonies formed by diphtheria bacilli on solidified 

 blood-serum are small round raised spots; they are variable in 

 size, but rarely exceed that of the head of a medium-sized pin. 

 They are white or grey in colour, and opaque. They do not 

 tend to run together so as to form a uniform film over the 

 surface of the medium, but remain discrete even when closely 

 packed. Some cocci form colonies which closely resemble those 

 of diphtheria, but they rarely become elevated so high above the 

 surface in the same space of time. 



2. Microscopical. — Prepare films by the method described on 

 p. 22 following out all steps in the fullest detail. Stain one of 

 them (step ii) with Loffler's blue or carbol thionin, allowing the 

 stain to act for two minutes, and the other by Gram's method. 



In removing some of the growth to make the film, remember 

 the facts just stated as to the characters of the colonies of the 

 bacillus, and select a colony presenting those characters (especially 

 that of elevation), if one is present. If there is no apparent 

 growth in the tube take " sweeps " of the whole surface. This is 

 conveniently done by means of a platinum loop shaped like a 

 stirrup, the flat bar being drawn along the surface of the medium 

 from bottom to top, just as a rake is drawn along a flower-bed. 



Now examine your specimens in the way described on p. 27. 



Characters of the Diphtheria Bacillus. 



The following are the chief points which are considered in 

 deciding whether a given stained slide does or does not show the 

 diphtheria bacillus. 



I. The shape of the bacillus is very variable, and this is a 

 feature which often affords us great assistance ; a specimen in 

 which all the bacilli present resemble each other exactly in shape 

 and size is not from a case of diphtheria. Diphtheria bacilli are 



