304 MICRO-ORGANISMS AND DISEASE [chap. 



branes two species of bacilli, similar in morphological 

 respects and in the mode of growth on and in Agar plates, 

 on serum, and on potato ; but one species is not constant, 

 and is probably the pseudo-diphtheria bacillus of Hoffmann 

 and Loffler, while the other is present in all cases, and in 

 some almost in pure culture ; it is pathogenic on guinea- 

 pigs. It grows abundantly on broth at 37° C, making this 

 uniformly turbid already in 24 hours ; this increases during 

 the next day, while a whitish, powdery precipitate appears, 

 and on the surface a filmy membranous-like pellicle. 



On gelatine the colonies are at first rounded, white, 

 prominent dots, which enlarging in breadth thicken in the 

 middle and become here slightly yellowish, dark brown in 

 transmitted light, the peripheral part being thin, plate- 

 like, and angular (Fig. 120). In the streak cultivation on 

 gelatine the streak becomes marked as a white band, at first 

 made up of droplets, but soon becoming confluent into a 

 uniform band ; at the margin the droplets and knob-like 

 expansions can still be recognised ; the middle is thick and 

 prominent ; in stab culture in gelatine the stab becomes 

 indicated by a line of droplets, white in reflected, brownish 

 in transmitted light ; the upper point of the stab is occupied 

 by a crenate, convex, white plate. Of course on gelatine, at 

 19-21° C, the growth is much slower than on Agar-Agar at 

 35-37° C. In milk kept at 20° C. our bacillus grows 

 luxuriantly and produces already after three days, or even 

 less, slight curdling of the milk, minute flakes of coagulated 

 casein; at 37° C. the growth is curiously less abundant in 

 the same space of time, and the curdling far less. The 

 diphtheria bacilli are killed by heating to 60° C. for five 

 minutes ; they do not form spores. The diphtheria bacilli 

 when transmitted through several subcultures acquire the 

 power to grow more and more rapidly on gelatine at 20-2 i°C., 



