104 DISEASES DUE TO BACTERIA 



Anli-bacterial action is brought about by two agencies : — 



(i) Phagocytosis. The leucocytes digest the bacteria. 



(2) Action of anti-bacterial substances in the blood serum which kill 

 and dissolve the bacteria outside the leucocytes. 



Bacteriolysins. Exemplified in anti-streptococcus, anti-cholera, 

 anti-plague, anti-pneumococcus, and anti-typhoid sera. 



Phagocytes. Is increased by the action of a substance called 

 an opsonin produced by the action of the bacteria and the toxins in the 

 blood. 



Opsonic Index is the proportion of bacteria devoured by leucocytes 

 m an infected blood to those devoured by leucocytes in normal blood. 



Directly after an injection of a vaccine (bacteria of toxins) the 

 opsonic index falls = negative opsonic phase. During this time a 

 further dose of vaccine does harm. 



After a few days the opsonic index rises -- positive opsonic phase. 

 During this time a further dose of vaccine increases the phagocytosis. 



DISTRIBUTION. 



In air and water. Plentiful in proportion to warmth and proximity 

 to organic life. 



In earth. Always abundant near the surface. 



In animals (apart from disease). 



On and in the skin, hair follicles, and sweat glands. 



Mouth, especially round the teeth. 



Whole alimentary canal. Most numerous at the end of the small 

 and beginning of the large intestine. 



Lower parts of the urethra, vagina and nose. 



INFECTION. 



The invasion of the living tissues by bacteria in such a manner as 

 to produce local or general disease. 



CONDITIONS NECESSARY FOR INFECTION, 



(i) Dose of bacteria. Bacteria must be introduced in a certain 



number, or they will be destroyed before they produce any 



effect. 

 (2) Virulence. — This varies enormously naturally. 



May be increased artificially : By passage through successive 



animals. 



May be decreased artificially : By cultivation outside the body, 



especiall}^ if done under unfavourable conditions. 

 f,3) Pathogenicity. — Only certain kinds of bacteria are capable of 



producing disease in certain kinds of animals. 



