MORPHOLOGY 465 



The bacillus does not form spores, and is generally said to be non-motile, 

 [but Gordon states that it is furnished with flagella and is motile]. In pre- 

 parations made from blood the organism is rather longer than in the buboes 

 and often appears as though surrounded by an hyaline capsule. 



In broth cultures the organism grows in chains. On agar more or less 

 elongated forms are seen among the ordinary short cocco-bacillary forms. 



In old cultures and on agar containing salt [2-5 per cent.] the plague bacillus gives 

 rise to involution forms consisting of large ball- like swellings and under these 

 conditions the organisms stain feebly (fig. 232). 



FIG. 231. Plague bacillus. From a broth FIG. 232. Plague bacillus. Involution forms, 

 culture. (After Yersin.) Agar cultures (6 days). ^Vn. 



Staining methods. The plague bacillus stains readily with the ordinary 

 basic aniline dyes ; carbol- violet or carbol-thionin can be recommended. It 

 is gram-negative. 



When stained with weak dyes the ends of the organism stain more deeply 

 than the centre, so that the bacillus often presents the appearance of a 

 shuttle. 



[To obtain good polar-stained bacilli fix the film by heat, float the cover- 

 glass on to the staining bath (frichsin 1 per cent., carbolic acid 3 per cent., 

 glycerin 40 per cent.), wash almost immediately in 60 per cent, alcohol, pass 

 rapidly through water (Jordansky and Kladnitsky). Giemsa's and Roma- 

 nowsky's stains also give very good results.] 



2. Cultural characteristics. 



1. Conditions of growth. The plague bacillus is an aerobic organism. It 

 grows easily on the ordinary, slightly alkaline, media. Growth begins about 

 + 5 C., is rapid at 20 C. but best at 30-38 C. 



2. Characters of growth on culture media, (a) Broth. The growth of the 

 plague bacillus in broth is similar to that of some streptococci : the medium 

 is clear while minute flakes adhere to the walls and subsequently fall to the 

 bottom of the tube. Occasionally a pellicle is formed on the surface. Some- 

 times and especially when the broth is sown from a previous broth culture 

 a more or less marked turbidity occurs. 



The best medium, according to Yersin, consists of an alkaline solution of peptone 

 (2 per cent.) containing 1 to 2 per cent, of gelatin. 



In broth culture the bacillus will, under suitable conditions, give rise to 

 stalactites (p. 473). 



[To obtain good stalactites in a plague culture the Advisory Committee 

 point out that the first essential is an absolute lack of vibration of the shelf 

 on which the flask stands. The addition of oil is an advantage but not an 

 essential. A neutral broth was used and in a typical stalactite growth the 



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