ABSTRACT
The major mechanisms by which micro-organisms affect the host are invasion and toxin production. Some bacteria, e.g. Bacillus anthracis, must proliferate throughout the entire body in order to kill the host, whereas others remain localized but produce sufficiently large quantities of toxin to affect all the body tissue. An example of the latter is Corynebacterium diphtheriae, which grows in the throat. Two types of bacterial toxin are formed, called exotoxins and endotoxins. C.diphtheriae produces an exotoxin, which is excreted from living cells and is subsequently circulated through the host via the bloodstream. Diseases caused by exotoxins include botulism, cholera, whooping cough, dysentery and gas gangrene. Endotoxins are retained within viable cells and are only liberated on death or lysis of the cell. In contrast to exotoxins, endotoxins are less toxic, more heat stable and cannot be chemically modified to reduce their toxicity. They are frequently responsible for the secondary effects of infection such as fever and shock. The symptoms caused by bacterial invasion and toxin production are normally manifested in full, but in some cases the host
acts only as a carrier of disease, displaying subclinical or no symptoms. The control of a disease such as typhoid is hindered by its long-term persistence in carriers, as the bacterial cells are excreted only sporadically, making detection difficult.
