RELATION OF BACTERIA TO DISEASE. 97 



certain extent. The means usually employed are the 

 frequent replanting of cultures and their growth in cap- 

 sules placed in the bodies of susceptible animals. But 

 with all our efforts we are usually only able to restore 

 approximately the degree of toxin-formation which the 

 cultures originally possessed. The adaptation of bac- 

 teria to any nutritive substance, living or dead, so that 

 they will grow more readily, is more easily brought 

 about, provided they will grow at all. The streptococcus 

 from erysipelas and the pneumococcus from pneumonia 

 are typical of this class of bacteria. Inoculate a rabbit 

 with a few streptococci obtained from a case of human 

 sepsis, and, as a rule, no result follows ; inject a few 

 million, and usually a local induration or abscess ap- 

 pears ; but if one hundred million are administered 

 septicaemia develops. From this rabbit now inoculate 

 another, and we find that a dose slightly smaller suffices 

 to produce the same effect; in the next animal inoculated 

 from this still less is required, and so on, until in time, 

 with suitable cultures, a very minute number will surely 

 develop and produce death. The same increase in 

 virulence can be noted when septic infection is carried 

 in surgery or obstetrics from one human case to another. 

 By allowing bacteria to continue to develop under cer- 

 tain fixed conditions they become accustomed to them, 

 and less adapted for all that differ. 



Somewhat distinct, again, from that class of bacteria 

 which multiply rapidly are those which, like the tubercle 

 and leprosy bacilli, develop slowly. Here increase of 

 virulence is shown, as before, by the production of dis- 

 ease through the introduction of very small numbers 

 into the body, but increase in rapidity of development 

 cannot progress except to within certain limits. A sin- 



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