20 INFECTION AND RESISTANCE 



bacteria, in which true capsule formation has not been observed, may 

 show swelling or enlargement under conditions in which their of- 

 fensive activities in the infected animal body are called into play. 32 

 Radziewsky 33 has noticed such swelling of B. coli in fatal guinea- 

 pig infections, and spoken of it as "one of the characteristic signs 

 of infectiousness." Kisskalt 34 has described the same thing in the 

 case of streptococci, and Eisenberg interprets this as signifying an 

 ectoplasmic hypertrophy comparable in principle to capsule forma- 

 tion. He looks upon the ectoplasmic zone as a protective layer, and 

 calls attention to the observation of Liesenberg and Zopf) 35 who 

 showed that capsulated strains of leukonostoc mesenteroides will 

 withstand 85 C., a temperature at which uncapsulated forms are 

 rapidly killed. 



There is a considerable amount of evidence, then, which seems 

 to indicate that the development of a capsule is at least one im- 

 portant method by which the bacteria can protect themselves against 

 the onslaught of the defences of the invaded animal body and in so 

 doing become more virulent. 



It is not likely, however, that this merely passive increase of the 

 resistance to injury on the part of the bacteria accounts for the 

 entire train of phenomena included in an enhancement of virulence. 

 It has been suggested by a number of observers that definite active 

 offensive characteristics distinguish the virulent from the avirulent 

 bacteria, in that the former may secrete, within the living body, 

 substances by which the destructive powers of serum and leukocytes 

 are neutralized or held at bay. A very definite suggestion of such a 

 possibility we find expressed in the now classical paper of Salmon 

 and Smith 36 on hog cholera immunity, published in 1886. They 

 say : ". . . the germs of such maladies are only able to multiply in 

 the body of the individual attacked, because of a poisonous principle 

 or substance which is produced during the multiplication of these 

 germs." 37 Bouchard formulated such a theory in 1893 by speaking 

 of the "produits secretes par les microbes pathogeniques," substances 

 which he found in cultures of virulent bacteria, and which seemed 

 to reenf orce the invasive powers of the germs. Kruse 38 also within 

 the same year developed a similar idea. He assumed that bacteria 

 may secrete enzyme-like substances which paralyze the destructive 

 properties of animal serum, and in this way gain the power to 



i2 These forms Bail has spoken of as. "thierische Bazillen." 



33 Radziewsky. Zeitschr. f. Hyg., Vol. 34. 



34 Kisskalt. Cited after Eisenberg, loc. cit. 



35 Liesenberg and Zopf. CentralbL f. Bakt., Vol. XII, 1892. 



36 Salmon and Smith. Proc. Biol Soc., Washington, D. C., Ill, 1884, 

 6, p. 29. 



37 A typewritten copy of this paper was kindly put at my disposal by 

 Prof. Theobald Smith. 



38 Kruse. Ziegler>s Beitrage, Vol. XII, 1893. 



