PATHOGENESIS 443 
ceases below 10° C. and above 40° C. The viability of the organism 
in cadavers is considerable; they may remain ali\'e for several weeks. 
In pus and sputum viable cultures may be obtained after one or even 
two weeks. Exposure to sunlight kills the bacilli within a few hours, 
and naked germs (unprotected by mucus or other protein envelope) are 
rapidly killed by drying. An exposure to 58° (\ for an hour, or 100° (\ 
for a few minutes is fatal; 5 per cent carbolic acid and 1 to 1000 bichlor- 
ide of mercury kill the organisms within fifteen minutes. The \iru- 
lence of the bacilli diminishes rather rapidly in artificial media as a 
rule. 
Products of Growth. — Indol is not produced in sugar-free broth. 
Acid, but not gas is produced in glucose, lactose, galactose, mannitol 
and maltose, but not in saccharose, sorbitol, dulcitol, and inulin. 
No enzymes have been demonstrated in cultures of plague bacilli. 
yo.n/?*.— Filtered cultures of plague bacilli possess little or no 
toxicity, although old broth cultures, freed from bacteria by filtration 
through unglazed porcelain, may exhibit slight toxic action. ^ It is 
probable that this toxicity is referable to some endotoxin which has 
been liberated in the medium during the gradual autolysis of the 
organisms. The symptoms of plague are attributed to the action of 
endotoxins which are liberated within the host as the organisms dis- 
integrate. The virulence of plague cultures is variable. Freshly 
isolated strains may occasionally exhibit almost no virulence for 
experimental animals, although as a rule they are very virulent. 
Eberson- has found that a powerful, shock-producing poison may be 
obtained by allowing normal guinea-pig or horse seriun to act upon 
sensitized bacilli. The substances formed are thermostabile, and 
according to Eberson may play some part in determining the virulence 
of the plague bacillus. These proteotoxins have some protective 
power for rabbits against infection with the plague bacillus, when 
administered in proper dosage.^ Prolonged cultivation upon artificial 
media usually results in a decided lowering of virulence, although here 
again exceptions are met with.'* 
Pathogenesis. — .4mi?? a/. — "Plague is primarily a disease of rodents, 
and secondarily and accidentally^ a disease of man."*^ The reservoir 
of plague appears to be certain rodents; the disease exists in chronic 
form in the marmot (Arctomys bobac) of India, and has within recent 
years been discovered in the western United States as a chronic disease 
in the ground squirrel (Citellus beechyi) by \\ herry,^ whose observa- 
tions have been confirmed by McCoy. McCoy ^ and Chapin^ have found 
1 Besredka: Bull. Inst. Pasteur, 1914, 12, 193. 
2 Jour. Inf. Dis., 1917, 20, ISO; 21, Sfi. ' Ibid., 191S, 22, 62. 
< McCoy and Chapin: Public Health Bull., January, 1912, No. 53, p. 1. 
^ This statement may possibly require modification in connection with the pneumonic 
type of plague in man (see human pathogenesis). 
6 Rucker: PubHc Health Reports, .July 19, 1912, p. 1130. 
' Public Health Reports, 1908, 23, 1289; Jour. Inf. Dis., 1908, 5, 485 
» Public Health Bull., April, 1911, No. 43. 
9 Ibid., January, 1912, No. 53, p. 1. 
