768 PA8TEV BELLA 



ment ; they are generally disposed singly, in pairs, in short chains, or in small 

 groups. The most striking feature is their pleomorphism, which is most noticeable 

 with Past, pestis, least with the haemorrhagic septicaemia bacilli. Though well 

 marked on ordinary media, it is best brought out by cultivation on nutrient agar 

 containing 3 per cent, sodium chloride. The growth on this medium is poor ; 

 microscopically, as well as the usual ovoid or short baciilary forms, there will be 

 found shadow forms, filamentous snake-like forms, club forms, large yeast-like 

 globules, and other irregular forms. Past, pestis and Past, pseudotuberculosis are 

 larger and more ovoid than Past, aviseptica, but their shape varies considerably with 

 the medium on which they are grown. Generally speaking, the bacilli of all three 

 species tend to be ovoid and to show bipolar staining when taken from smooth 

 colonies, and to be more baciilary, filamentous, or pleomorphic, without bipolar 

 staining, when taken from rough colonies, but many exceptions occur. 



Past, pestis and Past, aviseptica 

 are non-motile. Past, pseudo- 

 tuberculosis, on the other hand, 

 though usually non-motile in cul- 

 tures incubated at 37° C, is often 

 motile in broth cultures grown for 

 18 hours at 20-22° C. The posses- 

 sion of motility by this organism 

 is of considerable value in differ- 

 entiating it from Past, pestis, with 

 which it may easily be confused 

 (Arkwright 19 27). The value of 

 this test, however, is limited. 

 Weitzenberg (1935), for example, 

 made observations on 25 strains 

 of pseudotuberculosis, and found 

 that, though all were motile at 

 Fig. 151.— Pasteurella pestis. room temperature, some were very 



From an agar culture, 3 days, 37° C. ( X 1000). poorly so, and had to be examined 



repeatedly before their motility 

 could be definitely established. A negative result, therefore, on a single examina- 

 tion cannot be regarded as conclusive. Both Levinthal (1930) and Weitzenberg 

 (1935) have demonstrated the presence of flagella on Past, pseudotuberculosis. 

 The usual number appears to be 1-2, arranged at one or both poles. 



In the animal body Past, pestis may form a true capsule with a definite edge 

 (Kitasato 1894). More often it is surrounded by a gelatinous envelope of ill-defined 

 capsular material, which is soluble in weak alkalies (Rowland 1914a). The same 

 envelope may be formed in artificial media, especially in 10 per cent, inactivated 

 horse serum broth incubated at 36° C. According to Schiitze (1932a), it develops 

 best at 37° C, poorly at 26° C, and not at all at 20° C. By serological methods 

 it can be shown to contain a special antigen distinct from that present in the 

 body of the organism. Past, aviseptica may show an indefinite capsule in the 

 animal body. From this capsule Hoffenreich (1928) has extracted a polysaccharide, 

 which, though unable to give rise to precipitins on injection into rabbits, yet 

 reacts to a high titre with a specific precipitating serum. Priestley (1936a, c) 

 has shown that an envelope substance, apparently similar to that formed by 



