268 BIOLOGY OF SPECIALIZED GROUPS 



the abdominal cavity or under the skin of a susceptible animal. 

 The virulence may be decreased by growing the bacteria on 

 culture media, and by growing the cultures at a comparatively 

 high heat. Virulence depends upon the ability of the bacteria 

 or microorganisms to grow in the body and to subsequently 

 form toxic substances. 



(2) Number. The number of bacteria or other micro- 

 organisms necessary to produce an infection in the body and 

 overcome its resistance varies with the species. For example, 

 it has been found that the introduction of one single Bact. 

 anthracis is all that is necessary to produce anthrax. This 

 one bacterium, of course, soon multiplies very rapidly in the 

 body. Observations have been made on tuberculosis, and 

 it has been demonstrated that at least eight hundred and 

 twenty of Bact. tuberculosis are necessary to produce the 

 disease experimentally in a guinea pig. In all probability, 

 in man and animals it requires only a few bacteria to start a 

 tubercular infection naturally. 



Watson-Cheyne found on experimentation with B. vulgaris 

 that six million bacilli injected under the skin produced no 

 lesion, eight million produced an abscess, fifty-six million a 

 phlegmon, and that two hundred and twenty-five million 

 were necessary to produce a fatal result in two hours. 



It has been also demonstrated that two hundred and fifty 

 million of M. pyogenes var. aureus were required to produce 

 an abscess, and one billion were necessary to cause death in 

 an experimental animal, such as a rabbit. 



