10 INFECTION AND RESISTANCE 



imating so closely to that of the host that the latter reacts but slightly 

 and then only after a long period of stimulation." Into this class 

 he places the syphilis spirochseta and, in a somewhat modified sense, 

 the tubercle bacillus. 



We have seen, then, that a micro-organism may be pathogenic 

 and still be saprophytic in its mode of life. In order that this can 

 occur, however, it is necessary that it should possess the power of 

 producing at the place of lodgment a poison or toxin which can be 

 absorbed and cause disease. The condition w r hich ensues is not, prop- 

 erly speaking, an infection, but rather a "toxemia" differing from 

 the toxemias resulting from the ingestion of drugs or other poisons 

 only in so far as the toxins are manufactured at some point of bac- 

 terial lodgment within the body of the victim. Typical tetanus and 

 diphtheria, for instance, can be produced as readily by ingestion of 

 the bacteria-free culture filtrates as by inoculation with the bacteria 

 themselves. And although these bacteria may, on occasion, become 

 invasive and thereby satisfy the criteria of true infection, this is not 

 necessary for their pathogenicity. 



In the large majority of bacterial diseases, however, it is neces- 

 sary that the germs shall be capable of producing a true infection 

 before they can become pathogenic, and it is our task therefore to 

 attempt to analyze those bacterial attributes upon which the invasive 

 power or virulence may be said to depend. 



In the realm of infectious micro-organisms a wide range of cul- 

 tural variations is encountered which indicates that some of these 

 germs have adapted themselves very closely to the specific environ- 

 mental conditions found in the living animal body, while others can 

 take up with ease and under the simplest cultural conditions a purely 

 saprophytic existence. 



Many pathogenic micro-organisms have so far defied all attempts 

 at cultivation in artificial media. These we cannot use for examples 

 since it may well be that the failure of attempts in many of them 

 may hinge upon such simple alterations of method as the exclusion 

 of oxygen, the addition of fresh tissue, or the supplying of amino- 

 acids, which have made possible the cultivation of the spirochaeta 

 pallida and the leprosy bacillus. But among those which we can 

 cultivate there are many which require for successful cultivation 

 the production of artificial conditions simulating closely those ob- 

 taining in the living body. Thus malarial plasmodia can be made to 

 multiply only if furnished with uninjured human red blood cells, 

 within which they can develop. The gonococcus requires, in its 

 first cultures outside the body, a medium containing human protein ; 

 and the hemophile bacteria, among them the influenza bacillus, re- 

 quire hemoglobin. Other organisms like pneumococci, many strep- 

 tococci, diphtheria bacilli, and many others, though easily grown on 

 artificial media, are still fastidious in their requirements and develop 



