MICROBIAL DISEASES OF MAN AND DOMESTIC ANIMALS 831 



acteristic in artificial media. There are no spores. It is non-motile. Some 

 observers have demonstrated a gelatinous capsule. Occasionally very distinct 

 branching occurs (Hill). It stains easily with aniline dyes, particularly at the 

 poles which may show round or oval granules. It is Gram-negative. Its minimum 

 temperature for growth is about 12, the optimum 30, the maximum 40. It is 



FIG. 173Bact. pestis. (After Yersinfrom Williams.) 



aerobic. Agar after twenty-four hours shows small granular grayish colonies with 

 a thickened center and indented margin. Broth shows a granular deposit and 

 sometimes a pellicle with dependent outgrowths, the medium remaining otherwise 

 clear. Gelatin growths are as on agar, and the medium is not liquefied. Litmus 

 milk may show slight acid formation and no coagulation. Potato shows nothing 

 characteristic. The toxins appear to be largely endotoxins, though soluble poi- 

 sons have been found in old cultures. No indol is formed. Resistance toward 

 heat is not great, boiling kills in a few minutes. Light kills in a few hours. They 

 do not resist drying well, but in a moist condition remain viable for over a year. 

 The usual strengths of ordinary disinfectants kill in about ten minutes. 



Rats, mice, guinea-pigs, rabbits, and monkeys are particularly 

 susceptible to inoculation and even insects die from infection. 



The bacterium enters the body through the (usually abraded) skin 

 or respiratory tract. After involvement of the nearest lymphatic 

 glands the bacteria are distributed through the blood. 



Single attacks immunize. The antibodies developed are agglutinins. 

 probably bacteriolysins, and possibly antitoxins. The agglutination 

 reaction is of value in diagnosis. 



