394 A MANUAL OF BACTERIOLOGY 



slightly raised above the surrounding medium. The 

 blood-serum is not liquefied. 



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. 44). 

 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, simu- 

 lating 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 



