338 BACTERIOLOGY. 
Growth on Blood-serum. Blood-serum, especially in 
the form of Loffler’s mixture, is the most favorable 
medium for the growth of the diphtheria bacillus, 
and is used particularly for diagnostic purposes in 
examining cultures from the throats of persons sus- 
pected of having diphtheria. For its preparation, 
see p. 377. If we examine the growth of the diph- 
theria bacillus in pure culture on blood-serum we shall 
find at the end of from eight to twelve hours small colo. 
nies of bacilli, which appear as pearl-gray, whitish-gray, 
or, more rarely, yellowish-gray, slightly raised points. 
The colonies when separated from each other may in- 
crease in forty-eight hours so that the diameter may 
be one-eighth of an inch. The borders are usually 
somewhat uneven. The colonies lying together be- 
come confluent and fuse into one mass, when the 
serum is moist. During the first twelve hours the 
colonies of the diphtheria bacilli are about equal in size 
to those of the other pathogenic bacteria which are often 
present in the throat; but after this time the diphtheria 
colonies become larger than those of the streptococci and 
smaller than those of the staphylococci. The diph- 
theria bacilli in their growth never liquefy the blood- 
serum. 
Growth on Agar. On 1 per cent. slightly alkaline, 
plain nutrient or glycerin-agar the growth of the 
diphtheria bacillus is less certain and luxuriant than 
upon blood-serum; but the appearance of the colonies 
when examined under a low-power lens, though very 
variable, is often far more characteristic. (See Fig. 30, 
page 229, and Fig. 47, page 339.) The diphtheria 
bacillus obtained from cultures, which have developed 
for some time on culture media grows weil, as a rule, 
