238 BACTERIOLOGY FOR NURSES 



of streptococci. Many varieties in form are seen in smears of 

 material from old lesions. Swollen involution forms with clubbed 

 ends, short, round forms, and long rods may all be found, most of 

 which stain with difficulty. (Fig. 32.) 



Young forms stain well with methylene blue, which brings out 

 clearly the deeper stained bi-polar bodies; they are decolorized 

 by Gram's method. The organism is non-motile and does not 

 form spores. 



Cultivation. The optimum temperature for B. pestis is some- 

 what lower than that of most pathogenic organisms ; growth occurs 

 better at 25 C. to 30 C. than at 37 C. The bacillus is an aerobe 

 and grows well on all ordinary media which have a slightly alkaline 

 reaction. On nutrient agar or gelatin, colonies present a delicate 

 transparent appearance which when seen under the low-power 

 lens show a granular center with a thin, uneven margin. The most 

 characteristic growth takes place in broth, where, if the tubes are 

 not disturbed, a pellicle forms on the surface, and from it long, deli- 

 cate filaments are suspended which hang down into the broth like 

 stalactites from the roof of a cave. Gelatin is not liquefied; a 

 small amount of acid is produced from dextrose, but no other sugars 

 are fermented. 



Resistance. B. pestis exhibits little resistance to heat, dry- 

 ing, and disinfectants. In cultures protected from light and air 

 they have been found alive after ten years. 



Pathogenesis. Plague is primarily a disease of rodents trans- 

 missible to man. In all probability these animals are responsible 

 for the maintenance of foci from which the great epidemics spring. 

 Of the lower animals rats, mice, guinea pigs, and squirrels are 

 particularly susceptible ; dogs, swine, cattle, and horses do not 

 contract the disease naturally, but may be infected by inoculation 

 with large amounts of cultures. Mice and guinea pigs are usually 

 employed for experimental purposes. After inoculation a local 

 inflammatory swelling appears which follows the line of the lym- 

 phatics and terminates in a general infection and death in a few 

 days. On autopsy the internal organs present a congested appear- 

 ance accompanied by extensive hemorrhages. The liver and spleen 



