NON-SPORING BACILLI 255 



with a limited outbreak of pneumonic plague in man, was 

 (September, 1910) the occasion of considerable anxiety and 

 of increased vigilance and action. In California the last 

 case of plague was noted in 1909, but an epizootic of plague 

 among squirrels, causing thousands of deaths among these 

 animals, was only reported as quiescing in 191 1. 



In 1894, Kitasato and Yersin independently discovered 

 the bacillus in large numbers in the buboes, cultivated it 

 in pure growth, and reproduced the disease in susceptible 

 animals by inoculation, and from them recovered the 

 bacillus. The proof in the human subject was given 

 later, when by an accidental infection a physician and a 

 nurse died of plague. 



Description. B. pestis is a short thick bacillus with 

 well-rounded ends, thus appearing as a small oval rod, 

 two to three times in length what it is in breadth (1-5 

 to 175 X 0-5 to 07). The bacilli appear singly, though 

 at times in pairs, and in fluid cultures in chains. In 

 young cultures they show marked variations in size, and 

 less polar staining. In old cultures, involution forms 

 appear, as swollen coccoid forms, or as longer club-shaped 

 diphtheroid forms. In the tissues they are sometimes found 

 to possess a capsule. They stain readily with all the 

 usual aniline dyes, dilute aqueous fuchsin and methylene- 

 blue having been mostly used, and these show the polar 

 staining well. Special polar stains have been devised. 

 Involution forms are developed more rapidly when NaCl 

 is added to the medium, and " salt agar " containing 2 to 5 

 per cent of NaCl is used for diagnostic purposes, the 

 bacilli showing the usual shape on plain agar, on salt 

 agar exhibiting coccoid, root-shaped, large, globular, and 

 sausage-shaped forms, when the higher percentage is used ; 

 with the lower percentage, the most striking feature is a 

 general enlargement of all the bacilli. 



Dr. R. M. Buchanan, city bacteriologist of Glasgow, 

 in a Report to the Local Government Board for 

 Scotland ("Thirteenth Annual Report of the L.G.B., 

 Scotland, 1907," page 81), describes a new culture 

 medium for B. pestis as follows : " Rat agar as a 

 culture medium for B. pestis. The susceptibility 

 of the rat to plague suggested the use of rat tissues 



