50 BACTERIOLOGICAL DIAGNOSIS. 



EXAMINATION OF THE CULTURES. 



I. Naked eye. Each 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 or on ascitic agar 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. 



II. Microscopical. Prepare films by the method de- 

 scribed on page 29, following out all steps in the fullest 

 detail. Stain one of them (step n) with LofBer'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, 



