MODES OF BACTERIAL ACTION 179 



organism apparently spreading widely from individual to indi- 

 vidual in the community. Further facts will be given in con- 

 nection with the special diseases. 



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 

 extensive (for example, the septicaemia produced by the pneumo- 

 coccus in rabbits) ; but in septicaemia in man it is seen in less 

 degree, the organisms rarely remain in large numbers in the 

 circulating blood, and their detection in it during life by micro- 

 scopic 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 streptococci. In the human 

 subject more frequently one of two things happens. In the 

 first place, the organisms may remain local, producing little 

 reaction around thenv as in tetanus, or a well-marked lesion, 

 as in diphtheria, etc. Or in the second place, they may 

 pass by the lymph or blood stream to other parts or organs 

 in which they settle, multiply, and produce lesions, as in 

 tubercle. 



2. Production of Chemical Poisons. — In all these cases the 

 growth of the organisms is accompanied by the formation of 

 chemical products, which act generally or locally in varying 

 degree as toxic substances. The toxic substances become 

 diffused throughout the system, and their effects are manifested 

 chiefly by symptoms such as the occurrence of fever, disturbances 

 of the circulatory, respiratory, and nervous systems, etc. In 

 some cases corresponding changes in the tissues are found, for 

 example, the changes in the nervous system in diphtheria, to be 



