338 BACTERIOLOGY. 



Growth on Blood-serum. Blood-serum, especially in 

 the form of L6 frier'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 well, as a rule, 



