470 A MANUAL OF BACTERIOLOGY 



On agar the colonies are raised, round and cream- 

 coloured, finely granular, denser at the centre than at 

 the margins, which are regular. Size 0-25 to 0-5 mm. in 

 two days at 37 C. 



On surface agar the B. pestis forms a thick, opaque, 

 moist, smooth, cream-coloured growth, the margins of 

 which are usually markedly crenated ; the growth is 

 very sticky and tenacious. Haffkine states that when 

 grown on dry agar (agar which has been kept in the warm 

 incubator for two to three weeks) and viewed from behind 

 the growth has an appearance like that given by the back 

 of a mirror i.e. a dull, silvery appearance. 



On a salt agar (2-5-3-5 per cent, of sodium chloride) 

 Hankin describes the development of remarkable spherical 

 or pear-shaped involution forms. 



On gelatin the colonies are whitish, filmy, finely granular 

 with regular margins. Size, 0-1 to 0-25 mm. in five days 

 at 22 C. 



On surface gelatin the organism forms a thin, white, 

 granular growth, with slightly irregular surface and 

 margins, and nearly confined to the inoculation track 

 (Fig. 45). The growth does not penetrate into the medium, 

 nor does it render it cloudy. The growth is very adherent. 



In a stab gelatin culture a delicate whitish, finely 

 granular growth develops to the end of the stab, with little 

 tendency to spread from the needle track. The gelatin 

 is not liquefied. Both in agar and gelatin cultures fresh 

 punctate growths sometimes develop in the original 

 growth, simulating a contamination. No growth occurs 

 on ordinary potato, and milk is not coagulated. 



In broth the growth is somewhat characteristic. For 

 two or three days the broth remains perfectly clear, but 

 a flocculent growth forms and gradually increases in 

 amount on the bottom and sometimes upon the sides of 

 the tube. After some days the broth may become a little 



