254 



THE DIPHTHERIA BACILLUS 



(a) Isolated colonies (grown on the surface). After 18 hours (at 37 C.) 

 greyish white points are seen which rapidly grow to the size of a pin's head : 

 by transmitted light the colonies are more opaque in their centres than at 



their margins : as growth proceeds they 

 attain a diameter of 5 mm., remain regular 

 and are sometimes pale yellow in colour. 



(ft) Stroke cultures. Colonies similar to 

 those described above appear along the line 

 of sowing ; these soon become confluent and 

 form a fairly broad greyish band with ir- 

 regularly serrated margins. 



The long, short and medium sized varieties of 

 the diphtheria bacillus cannot be distinguished 

 by the characters of the growth on serum ; 

 colonies of the short variety are, however, some- 

 times whiter and moister than usual ; these 

 whiter colonies will be found to grow at room 

 temperature, and are in fact colonies of the 

 pseudo-diphtheria or Hofmann's bacillus (vide 

 infra). 



3. Agar. Colonies on agar are very similar 

 to those on serum though sometimes larger 

 and whiter. Growth is somewhat slower. 



4. Gelatin. Stab cultures in gelatin (15 

 per cent.) at 22-24 C. give rise to a very 

 poor growth consisting of small white punc- 

 tiform colonies along the line of the stab. 



us 169 - Cultures of the di P htheria The diphtheria bacillus does not liquefy 



1. Surface culture on agar (3 days at gelatin. 



Ill hours) 2 ' IS lated COl nieS n Serum 5 - Potato. The diphtheria bacillus does 



not appear to grow on potato ; some ob- 

 servers have however described a delicate growth consisting of an almost 

 invisible yellowish glaze. 



6. Egg albumin. SakarofE recommends white of egg coagulated by heat 

 as a medium in place of blood-serum (p. .53, B). When sown on the surface 

 of this medium, and incubated for 24 hours, the diphtheria bacillus grows in 

 the form of small, dull, slightly transparent, hemispherical colonies somewhat 

 darker in colour than the medium : sometimes towards the twelfth day the 

 growth may be of a brown or flesh colour. 



7. Milk. The diphtheria bacillus grows luxuriantly in fresh milk. The 

 medium is not coagulated. 



SECTION III. BIOLOGICAL PROPERTIES. 

 1. Vitality and virulence. 



Vitality. 



The diphtheria bacillus remains alive in culture for a considerable length 

 of time : sub-cultures can be grown from a culture 5 or 6 months old. 



In moist cultures the bacillus is readily destroyed : exposure to a tem- 

 perature of 58 C. for a few minutes is sufficient to sterilize a broth culture. 



When dried, the bacilli are more resistant to the action of heat and can 

 be subjected to a temperature of 95 C. for several minutes without being 

 killed. 



The diphtheria bacillus shows even greater powers of resistance to heat 



