VIRULENCE OF ORGANISMS 3 



may be altered by favorable or unfavorable conditions. Virulence, 

 however, does not depend entirely upon characters inherent in the 

 infectious agent, because the production of disease is an exhibition 

 of reaction between invading organism and host. We may, there- 

 fore, say that virulence depends upon two groups of factors, those 

 inherent in the invading organism and those dependent upon the 

 resistance exhibited by the attacked individual. This resistance on 

 the part of the host is represented by the condition of immunity and 

 will be discussed subsequently. The capacity of the infecting organ- 

 ism in the production of disease depends upon certain inherent ele- 

 ments of virulence which are not well understood, upon the capacity 

 of the organism to protect itself against the defensive mechanism of 

 the host, upon the capacity to produce certain substances which aid 

 invasion, and upon the development of poisonous bacterial products. 



Demonstration of Virulence. Inherent virulence of organisms 

 may be demonstrated by the administration of accurately measured 

 doses of the organism and observance of the effects upon susceptible 

 animals. Ordinarily the dose is measured in the form of certain 

 quantities of fluid culture. Growths on solid media may be meas- 

 ured by the use of a platinum loop so standardized as to take up 

 approximately 2 mg. of the organisms. Such growths may also be 

 measured by suspension in a suitable menstruum. If 5.0 c.c. of salt 

 solution are added to a slant agar culture, fractions of the resulting 

 5.0 c.c. suspension contain equivalent fractions of the total surface 

 growth. The most accurate method is that of Barber, who has developed 

 a technic in which the use of a capillary tube permits picking a single 

 organism out of a suspension. Of importance in considering viru- 

 lence from this point of view is not only the quantity of organisms 

 injected, but also the length of time they have lived upon artificial 

 media, inasmuch as prolonged cultivation leads to deterioration of 

 virulence. If a culture is maintained for a period of time without 

 transplantation considerable numbers of the organisms die, and 

 therefore may constitute a part of the bulk injected, at the expense 

 of living organisms. This is not a true decrease of virulence and 

 constitutes a factor of error. The route of injection is also of im- 

 portance, because certain organisms may be virulent by one route 

 of injection and not so by others. For example, the cholera vibrio 

 may produce disease by introduction into the intestinal tract and is 

 entirely without pathogenic effect when introduced subcutaneously. 



The Basis of Virulence. The studies of pathogenic bacteria have 

 shown that they may acquire or in certain instances may lose viru- 

 lence by passage through animals, and that they may lose virulence 

 by cultivation upon artificial media. The method whereby they 

 acquire virulence has been extensively studied. It is well known 

 that the pneumococcus possesses a capsule when growing in ani- 

 mal tissue, but that it loses its capsule after artificial cultivation. 

 This is true of certain other organisms, and it has been demon- 

 strated that the protection afforded by the capsule makes these 



