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 
vaised 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 11) with Léffler’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, 
