BACILLUS PESTIS 145 



Growth. Bacillus pestis is an aerobe and grows best at temperatures 

 between 3oC. and 38C. 



Bouillon growth appears as a whitish sediment, which sticks to the sides or 

 falls to the bottom of the container. The medium usually remains clear, 

 occasionally it becomes turbid; slight pellicle formation may occur. 



Bouillon to which sufficient sterile oil or butter has been added to form 

 globules on the surface, if kept at rest during incubation, has a stalactite forma- 

 tion if bacteria develop. Around each globule these stalactites are whitish 

 and when the medium is agitated they fall to the bottom. 



Agar, after incubation at 37C. for 24 hours, shows small, round, irregular- 

 edged, transparent whitish colonies. Subcultures show more abundant growth 

 than those made from tissue. 



Salt-agar is especially valuable to demonstrate involution forms. 



Gelatin surface cultures show round yellowish colonies. 



Gelatin stab cultures show a fine thread-like whitish growth along the stab 

 with a yellowish growth at the surface. Gelatin is not liquefied. 



Blood serum growth appears as on agar. 



Milk is not coagulated; there may be slight acidulation. 



Potato shows slight, if any growth without any distinctive feature. 



Bacillus pestis forms acid, but no gas in glucose; there is neither acid nor 

 gas production in lactose and saccharose. Spores are not formed. Indol is 

 not formed. 



Resistance. Exposure to direct sunlight will kill the bacillus pestis in 

 several hours. In dried pus it remains alive for weeks. It has been found alive 

 in soil, protected from light, after several months and in water after i month. 

 It remains alive and virulent in the bodies of those dead of plague for weeks; 

 when putrefaction occurs bacillus pestis is said to disappear in 15 to 30 days. 

 Five per cent, carbolic acid solution kills it in 10 minutes, so does i : 1000 

 bichloride. Hot-air sterilization requires an exposure of J^ hour at i5oC.; 

 in a moist state 6oC. for i hour. Boiling kills it instantly. 



Toxin. Bacillus pestis produces an intracellular toxin. 



Agglutinins. The occurrence of agglutinins and precipitins in the blood of 

 infected and immunized persons and animals is irregular and slight. 



Pathogenesis. Plague is a disease said to occur in many animals other than 

 man and rats; among them may be mentioned squirrels and guinea-pigs, rabbits, 

 cats, chickens and monkeys. 



Much the most common and important is the occurrence of the disease in 

 rats and man. The disease usually prevails in rats of a district shortly before 

 the occurrence of an epidemic among the people; this association is almost a 

 rule and the agent of transmission is the flea. Fleas which bite stricken rats 

 imbibe the bacilli and implant them upon the skin of people whom they bite. 

 Some believe the flies and mosquitoes may play a minor part in the dissemination 

 of plague. 



Two forms of the disease occur in man: the bubonic, distinguished by the 

 enlargement of many or all of the superficial lymph glands, with or without, 



