108 INFECTION 



generally accepted as the ways and means by which bacteria and proto- 

 zoa produce disease, omitting a description of each disease in detail. 



In the great majority of instances disease is produced as the result 

 of chemical substances generated by the metabolic processes of bacteria. 

 Animal parasites and certain bacteria, such as that of anthrax, may do 

 harm mechanically by forming capillary emboli; but, as stated, bac- 

 teria, as a rule, produce their effects chiefly through chemical means. 

 Accordingly, bacteria may give rise to infection and disease through the 

 following agencies : 



1. Soluble or extracellular toxins, which are poisons generated by 

 bacterial cells and discharged into their surrounding media. 



2. Intracellular toxins or endotoxins, which are specific poisonous 

 products of bacterial activity, and are contained within the cells. 



3. Aggressins, or substances secreted by bacteria, that neutralize 

 opsonins and prevent phagocytosis. 



4. Bacterial proteins, which are poisonous protein constituents of the 

 bacterial cells and are responsible for certain general and non-specific 

 lesions. 



5. Ptomains, which are the secondary products of decomposition of 

 the media upon which the bacteria are growing; these may be absorbed 

 and produce symptoms of intoxication. 



6. Mechanical action of bacteria, whereby certain symptoms or 

 lesions may be due to the blocking of small but physiologically important 

 vessels with emboli of bacteria, in addition to the effects of mechanical 

 irritation. 



TOXINS 



Nomenclature. Of all the various means whereby bacteria produce 

 disease, none possesses so much importance as the poisonous substances, 

 known as toxins, elaborated by the metabolic activities of the micro- 

 organisms. A few classes of bacteria secrete this poisonous principle 

 directly into the tissues or artificial culture-media in which they are 

 growing, and hence are known as soluble, exogenous, extracellular, or 

 true toxins. Other bacteria retain most of their toxins within the 

 bacterial cell, and for this reason are called endotoxins, or intracellular 

 toxins; these are liberated upon the disintegration of the bacteria by 

 various mechanical, physical, or chemical means. 



By common consent the term " toxin" is applied to the soluble or 

 true toxins, such as those of diphtheria and tetanus, and hence the term, 

 when used without further qualifications, may be considered to refer to 

 toxins of this class. 



