152 RELATIONS OF BACTERIA TO DISEASE 



In increasing the susceptibility of a given individual, condi- 

 tions of local or general diminished vitality play the most 

 important part. It has been experimentally proved that 

 conditions such as exposure to cold, fatigue, starvation, etc., 

 all diminish the natural resistance to bacterial infection. Rats 

 naturally immune can be rendered susceptible to glanders by 

 being fed with phloridzin, which produces a sort of diabetes, a 

 large amount of sugar being excreted in the urine (Leo). 

 Guinea-pigs may resist subcutaneous injection of a certain dose 

 of the typhoid bacillus, but if at the same time a sterilised 

 culture of the bacillus coli be injected into the peritoneum, they 

 quickly die of a general infection. Also a local susceptibility 

 may be produced by injuring or diminishing the vitality of a 

 part. If, for example, previous to an intravenous injection of 

 staphylococci, the aortic cusps of a rabbit be injured, the 

 organisms may settle there and set up an ulcerative endocarditis ; 

 or if a bone be injured, they may produce suppuration at the 

 part, whereas in ordinary circumstances these lesions would not 

 take place. The action of one species of bacterium is also often 

 aided by the simultaneous presence of other species. In this 

 case the latter may act simply as additional irritants which 

 lessen the vitality of the tissues, but in some cases their presence 

 also appears to favour the development of a higher degree of 

 virulence of the former. 



These facts, established by experiment (and many others 

 might be given), illustrate the important part which local or 

 general conditions of diminished vitality may play in the pro- 

 duction of disease in the human subject. This has long been 

 known by clinical observation. In normal conditions the blood 

 and tissues of the body, with the exception of the skin and 

 certain of the mucous surfaces, are bacterium-free, and if a few 

 organisms gain entrance, they are destroyed. But if the vitality 

 becomes lowered their entrance becomes easier and the possibility 

 of their multiplying and producing disease greatly increased. In 

 this way the favouring part played by fatigue, cold, etc., in the 

 production of diseases of which the direct cause is a bacterium, 

 may be understood. It is important to keep in view in this con- 

 nection that many of the inflammation-producing and pyogenic 

 organisms are normally present on the skin and various mucous 

 surfaces. The action of a certain organism may devitalise the 

 tissues to such an extent as to pave the way for the entrance of 

 other bacteria ; we may mention the liability of the occurrence 

 of pneumonia, erysipelas, and various suppurative conditions in 

 the course of or following infective fevers. In some cases the 



