474 A MANUAL OF BACTERIOLOGY 



fluid in the buboes, either alone or mixed with streptococci 

 or micrococci, and in the sputum in the pneumonic form. 

 They are not usually found in any number in the blood 

 except in the septicsemic variety, or shortly before death, 

 and in stained preparations appear as short plump bacilli, 

 often in pairs, with polar staining and unstained centres 

 (Plates XIV, c, and XV, a). If the organisms are found to 

 be free and numerous in the buboes the prognosis tends 

 to be grave, but if they are largely present within the 

 phagocytic polymorphonuclear leucocytes the prognosis 

 is better and the disease will probably remain localised. 



Toxins. The plague bacillus forms but little toxin, the 

 minimal fatal dose of the most active filtered broth culture 

 for a mouse being about 0-02 c.c. 



Macfadyen obtained an endotoxin by triturating the 

 bacilli frozen with liquid air. 



Vaccines and immunity. Of the plague vaccines, that 

 of Haffkine, the Haffkine prophylactic, is the best known, 

 and has been extensively employed. It consists essen- 

 tially of a four to six weeks old butter-fat broth culture 

 of the plague bacillus, killed by heating to 65 C. for an 

 hour, with a small addition of antiseptic. As to the 

 value of Haffkine' s prophylactic a mass of figures is 

 available. By its use both the incidence of, and mortality 

 from, plague are markedly diminished. Wilkinson col- 

 lected the following data of the efficiency of the vaccine : 

 Among the inoculated the case incidence was 1-8 and the 

 case mortality 23-9 per cent. ; among the uninoculated 

 the figures were 7-7 and 60-1 respectively. The immunis- 

 ing products seem to be mainly intracellular, but the broth 

 itself is not without action. 



Other vaccines have also been devised. Lustig and Galeotti 

 prepared one by digesting the growth from agar cultures with 

 1 per cent, caustic soda solution, filtering through paper, and 

 precipitating with very dilute acetic or hydrochloric acid, or by 



