810 PATHOGENIC MICROORGANISMS 



On gcMin, similar colonies appear after two or three days at 20 

 to 22 C. The gelatin is not liquefied. 



In bouillon, the plague bacilli grow slowly. They usually sink to 

 the bottom or adhere to the walls of the tube as a granular deposit 

 and may occasionally form a delicate pellicle. Chain-formation is 

 not uncommon. In broth cultures, moreover, a peculiar stalactite- 

 like growth is often seen, when the culture fluid is covered with 

 a layer of oil and the flasks are incubated in a place where shaking 

 or vibration can be prevented. Delicate threads of growth hang 

 down from the surface of the medium into its depths like stalactites. 



Characteristic involution forms are brought out best when the 

 .bacilli are grown upon agar containing 3 to 5 per cent NaCl. 



Milk is not coagulated. In litmus-milk there is slight acid forma- 

 tion. On potato and on blood serum the growth shows nothing char- 

 acteristic or of differential value. On pepton media no indol is formed. 



Biological Considerations. Bacillus pestis is aerobic. Absence 

 of free oxygen is said to prevent its growth, at least under certain 

 conditions of artificial cultivation. It is non-motile. Outside of the 

 animal body the bacilli may retain viability for months and even 

 years if preserved in the dark and in a moist environment. In 

 cadavers they may live for weeks and months if protected from 

 dryness. In pus or sputum from patients they may live eight to 

 fourteen days. These facts are of great hygienic importance. 



Complete drying in the air kills the bacilli within two or three 

 days. 5 Thoroughly dried by artificial means they die within four 

 or five hours. 



Dry heat at 100 C. kills the bacillus in one hour. 6 Live steam 

 or boiling water is effectual in a few minutes. The bacilli possess 

 great resistance against cold, surviving a temperature of C. for 

 as many as forty days. 



Direct sunlight destroys them within four or five hours. The 

 common disinfectants are effectual in the following strengths: car- 

 bolic acid, one per cent kills them in two hours, five per cent in 

 ten minutes ; bichloride of mercury 1 :1,000 is effectual in ten 

 minutes. 



In a recent communication to the New York Pathological Society, 

 Dr. Wilson reported that plague cultures which he had kept sealed 



Kitasato, Lancet, 1894. 



Abel, Cent. f. Bakt.. xxi. 1897. 



