32 ESSENTIALS OF BACTERIOLOGY 



produce toxins in artificial media may do so in the human 

 body. These toxic substances are formed by the bacteria to 

 combat the body defenses, and have been called by Bail 

 aggressins. They have a paralytic action on phagocytes. 

 A sublethal dose of bacteria, if injected along with aggressin, 

 will cause death. 



Toxins are not stable, though tetanus toxin has been kept 

 in powdered form for a number of years. They are soluble 

 in water, destroyed by heat (thermolabile), and precipitated 

 by ammonium sulphate. 



The Cardinal Conditions for Infection. (i) The 

 microorganism must be sufficiently virulent; (2) it must enter 

 in sufficient numbers and by appropriate channels; and (3) the 

 host must be susceptible. 



Virulence is a very variable quality, and depends on the 

 ability of the micro-organism to invade or produce toxin or 

 both. The virulence may be decreased by repeated trans- 

 planting on artificial culture-media or by. the action of heat. 

 It may be increased by adding animal juices to the culture- 

 medium, by inclosing the micro-organism in a collodion sac, 

 and placing the sac in the abdominal cavity of an animal, and 

 by repeatedly passing it through animals. 



Infection Depends on Quantity of Bacteria. Unless 

 a sufficient number of bacteria enter the tissues no infection 

 follows, because the body defenses immediately destroy the 

 bacteria. The number necessary to cause infection depends 

 on their virulence and the susceptibility of the host. Strep- 

 tococci may become so virulent that a single coccus will cause 

 death in a rabbit. It has been found that 820 tubercle bacilli 

 are necessary to kill a guinea-pig, and 1,000,000 staphylo- 

 cocci to kill a rabbit. The period of incubation can be ex- 

 plained on the supposition that the organism requires a 

 definite time to generate the amount of toxin necessary to 

 produce symptoms. 



Avenues of Infection. The organism must gain en- 

 trance into the tissue or find lodgment on some part of the body 

 that has been injured. Even when several avenues of infec- 



