BACILLUS PESTIS 557 



On gelatin, 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 un- 

 common. In broth cultures, moreover, a peculiar stalactite-like growth 

 is often seen, when the culture fluid is covered with a layer of oil. 

 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 

 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 condi- 

 tions 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. 1 

 Thoroughly dried by artificial means they die within four or five hours. 



Dry heat at 100 C. kills the bacillus in one hour. 2 Live steam or boil- 

 ing water is effectual in a few minutes. The bacilli possess great resist- 

 ance 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: carbolic 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 for as 

 long as ten years in the ice chest were found living and virulent at the 

 end of this time. This ability to go into a quasi latent stage under 

 suitable conditions is of the greatest importance in connection with 

 the problem of prevention. 



1 Kitasato, Lancet, 1894. 2 Abel, Cent. f. Bakt., xxi, 1897. 



