CHAPTER XXXIX 

 BACILLUS PESTIS 



Bacillus pestis is present in the lymphatic glands and in the blood of in- 

 fected animals and man; in the sputum of those suffering from the pneumonic 

 form of the disease, in the carcasses of those dead of plague and in fleas which 

 have fed upon infected rats. It has been found several times in the soil of 

 districts where plague is endemic. 



Morphology. Bacillus pestis is subject to marked variations in size and 



FIG. 28. BACILLUS PESTIS. 



Upper half of field shows polar staining. Lower half of field shows solid staining. 

 (4X eyepiece and Mz oil immersion objective.) 



shape. Bacilli removed from the lymphatic glands of those suffering with 

 plague are about i /* to 2 /* have decidedly rounded ends, stain deeply at each 

 end and show a clear band in the middle. They are arranged singly. Young 

 (24 to 48 hours) cultures in bouillon, agar and salt-agar have a different appear- 

 ance; bacilli from bouillon stain solidly, are about i n by 2 /i and are arranged 

 in pairs, end to end, and in filaments, as well as singly. Those from agar and 

 salt-agar have the same appearance, but do not show filaments as in bouillon. 



Old agar and salt-agar cultures (5 to 6 days old) show marked involution 

 forms, bacilli 3 or 4 n long and i /* wide showing polar staining, and larger bacilli 

 which stain solidly and are curved or club-shaped. Similar involution forms 

 may be found in tissue. Bacillus pestis is not motile. 



Staining. Bacillus pestis stains with all the basic anilin dyes and is Gram 

 negative. 



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