76 Infection 



THE CARDINAL CONDITIONS OF INFECTION. 



Infection can take place only when the micro-organisms 

 are sufficiently virulent, when they enter in sufficient num- 

 ber, when they enter by appropriate avenues, and when the 

 host is susceptible to their action. 



Virulence. Virulence may be denned as the disease- 

 producing power of micro-organisms. It is a variable 

 quality, and depends upon the invasiveness of the bacteria, 

 or the toxicity of their products, or both. 



A few bacteria are almost constant in virulence and can 

 be kept under artificial conditions for years with very little 

 change. Other bacteria begin to diminish in virulence so 

 soon as they are introduced to the artificial conditions of 

 life in the test-tube. Still others, and perhaps the greater 

 number, can be modified, and their virulence increased or 

 diminished according to the experimental manipulations 

 to which they are subjected. 



Variation in virulence is not always a peculiarity of the 

 species, for the greatest difference may be observed among 

 individuals of the same kind. Thus, the streptococcus 

 usually attenuates rapidly when kept in artificial media, 

 so that special precautions have to be taken to maintain it, 

 but Hoist observed a culture whose virulence was unaltered 

 after eight years' continuous cultivation in the laboratory 

 without any particular attention having been devoted to it. 

 What is true of different cultures of the same organisms, is 

 equally true of the individuals in the same culture. To 

 determine such individual differences is quite easy among 

 chromogenic bacteria. If these are plated in the ordinary 

 way it will be found that some colonies are paler and some 

 darker than others. Conn found that by repeating the 

 plating a number of times and always selecting the palest 

 and darkest colonies he was eventually able to produce two 

 cultures, one brilliant yellow, the other colorless, from the 

 same original stock. By applying the same idea to virulence 

 we can understand how the artificial selection is brought 

 about inadvertently in transplanting cultures. 



Decrease of virulence under artificial conditions prob- 

 ably depends upon artificial selection of the organisms in 

 transplantation from culture to culture. When planted 

 upon artificial media, the vegetative members of the bacte- 

 rial family proceed to grow actively and soon exceed in 



