CULTURAL CHARACTERISTICS 



769 



the plague bacillus, can be demonstrated in cultures of virulent, but not of aviru- 

 lent, strains of Pasteurella septica. It reaches its maximum development after 

 24 hours at 37° C, and then gradually disappears. Temperatures above and 

 below 37° C. tend to inhibit its formation. Antigenically it is distinct from the 

 somatic substances. 



Bipolar staining is very common and gives to the ovoid bacilli a characteristic 

 appearance. The rod forms often stain irregularly, appearing as granular or barred 

 forms. The reaction to Gram's stain is uniformly negative. 



Cultural Characteristics. — ^Moderate growth occurs on the ordinary media. 

 Past, pseudotuberculosis and Past, aviseptica grow fairly rapidly, giving a confluent 

 growth on agar after 24 hours ; Past, pestis, on the other hand, develops more 

 slowly, and gives a less abundant growth, often barely noticeable after this time. The 

 agar colonies of Past, pestis and Past, pseudotuberculosis resemble each other in 

 many respects, and are characterized by the effuse, clear or slightly granular, 

 peripheral extension that occiirs after 2 to 4 days' growth (Figs. 158, 159) ; the 



Fig. 158. — Pasteurella 



pestis. 



Surface colonies on agar, 

 3 days, 37° C, showing 

 differentiation, and effuse 

 edge ( X 8). 



I 



Fig. 1.59. — Pasteurella pseudo- 

 tuberculosis. 



Surface colonies on agar, 24 

 hours, 37° C, showing irregu- 



• lar granular surface and 

 eff'use edge ( x 8). 



colonies of Past, pseudotuberculosis, however, develop more rapidly, and are larger 

 and more granular. With further incubation the central raised part of the colony 

 may assume a ringed or draughtsman-like appearance. On moist agar the colonies, 

 especially of Past, pestis, are of viscous consistency, and tend to adhere to the 

 medium (Eastwood and Griffith 1914). 



Colonial variants have been described for each member. From Past, pestis 

 Gotschlich (1912) obtained round, slimy, undifferentiated colonies, which were 

 poorly agglutinable and proved avirulent for rats ; they subsequently reverted 

 to the normal virulent type. Variant types, including smooth compact, small- 

 fringed, large irregular, and " sunflower " types, have been described by a number 

 of other workers (see Be-ssouowa, Semikoz, and Kotelnikow 1927, Pirie 1929, 

 Burgess 1930, Bessonowa and Lenskaja 1931, Bhatnagar 1940a), but the exact 

 form produced seems to be so influenced by environmental factors that it is prob- 

 ably better not to refer to these types by the terms rough and smooth. Kakehi 

 (1915-16) described two variants of Past, pseudotuberculosis ; A was almost trans- 

 parent and had a bluish, glimmering appearance ; B was greyish-white and opaque. 

 A rendered broth turbid, B did not. Zlatogoroff and Moghilewskaja (1928a, b) 



P.B. 



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