THE PROBLEM OF VIRULENCE 15 



producing properties. The dosage necessary to cause infection, 

 therefore, is in inverse proportion to that property of bacteria spoken 

 of as their "virulence." Thus we measure the degree of the so-called 

 virulence of bacteria by determining the smallest quantity, measured 

 by dilution of platinum loops or by fractions of agar slant cultures 

 (both very inexact methods), which will still cause infection and 

 death in susceptible animals of a standard weight. In the case of 

 micro-organisms of extreme virulence, such as the anthrax bacillus 

 or bacilli of the hemorrhagic septicemia group, the inoculation of a 

 very small number of bacteria may suffice to initiate infection. In- 

 deed, it has been claimed for the anthrax bacillus that the injection 

 of a single bacterium will produce fatal disease in a susceptible 

 animal. The inverse relation existing between the degree of viru- 

 lence and the number of bacteria inoculated is well illustrated by the 

 experiments of Webb, Williams, and Barber, 18 carried out upon 

 white mice with anthrax, by the method of inoculation devised by 

 Barber. 19 This technique consists in picking up single organisms with 

 a capillary pipette under microscopic control, from a very thin 

 emulsion of bacteria and injecting directly from the pipette through 

 a needle puncture in the skin. While requiring a considerable de- 

 gree of skill, the method, when successful, permits an actual accurate 

 count of injected bacteria instead of the merely approximate esti- 

 mate which can be made by consecutive dilutions of thicker emul- 

 sions. In their experiments with anthrax in white mice Webb, Wil- 

 liams, and Barber found that the inoculation of a single thread of 

 anthrax bacilli (3 to 6 individuals) taken directly from the blood 

 of a dead animal (that is, in the most virulent condition) would 

 regularly cause death, and it was impossible for this reason to 

 immunize with such bacilli. On the other hand, if taken from 

 12-hour agar cultures of the same strain such small quantities 

 would often fail to kill. The brief period of growth under 

 artificial conditions had sufficiently lessened the virulence of the 

 bacilli so that 2, 3, and more threads could be injected with- 

 out harm. And after several generations of such cultivation as 

 many as 27 and more threads could be inoculated with im- 

 punity. 



Another example of the measurement of relative degrees of- 

 virulence, by a method more commonly employed, may be illustrated 

 as follows : The problem in which this particular measurement was 

 used consisted in the comparison of the virulence of two strains of 

 pneumococcus, one (N 2 ) successively passed through white mice, the 

 other (N\) kept alive for several weeks on serum-agar. To accom- 

 plish this graded quantities of 18-hour broth cultures of the two 



18 Webb, Williams, and Barber. Jour. Med. Res., 1909, Vol. XV. 

 10 Barber. Kansas Univ. Science Bulletin, March, 1907. 



