MODES OF BACTERIAL ACTION 153 



specific organism may produce lesions through which the other 

 organisms gain entrance, e.g. in typhoid, diphtheria, etc. A 

 notable example of diminished resistance to bacterial infection is 

 seen in the case of diabetes 7 ; tuberculosis and infection with 

 pyogenic organisms are prone to occur in this disease and are of 

 a severe character. It is not uncommon to find in the bodies of 

 those who have died from chronic wasting disease, collections of 

 micrococci or bacilli in the capillaries of various organs, which 

 have entered in the later hours of life ; that is to say, the 

 bacterium-free condition of the blood has been lost in the period 

 of prostration preceding death. 



The methods by which the natural resistance may be specific- 

 ally increased belong to the subject of immunity, and are 

 decribed in the chapter on that subject. 



Modes of Bacterial Action. In the production of disease 

 by micro-organisms there are two main factors involved, namely, 

 (a) the multiplication of the living organisms after they have 

 entered the body, and (b) the production by them of poisons 

 which may act both upon the tissues around and upon the body 

 generally. The former corresponds to infection, the latter is of 

 the nature of intoxication or poisoning. In different diseases one 

 of these is usually the more prominent feature, but both are 

 always more or less concerned. 



1 . Infection and Distribution of the Bacteria in the Body. 

 After pathogenic bacteria have invaded the tissues, or in other 

 words after infection by bacteria has taken place, their further 

 behaviour varies greatly in different cases. In certain cases 

 they may reach and multiply in the blood stream, producing a 

 fatal septicaemia. In the lower animals this multiplication of the 

 organisms in the blood throughout the body may be very exten- 

 sive (for example, the septicaemia produced by the pneumococcus 

 in rabbits) ; but in septicaemia in man, it very seldom, if ever, 

 occurs to so great a degree, the organisms rarely remain in large 

 numbers in the circulating blood, and their detection in it during 

 life by microscopic examination is rare, and even culture methods 

 may give negative results unless a large amount of blood is 

 used. In such cases, however, the organisms may be found 

 post mortem lying in large numbers within the capillaries of 

 various organs, e.g. in cases of septicaemia produced by strepto- 

 cocci. In the human subject more frequently one of two things 

 happens. In the first place, the organisms may remain local, 

 producing little reaction around them, as in tetanus, or a well- 

 marked lesion, as in diphtheria, pneumonia, etc. Or in the 

 second place, they may pass by the lymph or blood stream to 



