42 BACTERIOLOGY 



Other methods of isolation may sometimes be employed, for 

 example, less resistent organisms in a mixture may be killed 

 off by graduated heat (see p. 27) or by chemicals ; or animals 

 susceptible to the special bacterium may be inoculated, and 

 the organism " recovered" from their tissues. 



CHAPTER V 

 HEALTH AND DISEASE 



The Relations of Bacteria to the Living Body and to 

 the Production of Disease Susceptibility and Im- 

 munity 



As we have already seen, bacteria of various kinds are present 

 in enormous numbers upon the surface of the body and in the 

 mouth and other parts of the alimentary canal. How are they 

 prevented from obtaining access to and invading the blood and 

 the tissues of the body, which are normally free from them ? 

 Microbes, like the poor, are always with us. We all have 

 pneumococci and numberless other organisms in our mouths 

 and throats. The tubercla bacillus in the present stage of our 

 civilisation is present in the sputum-contaminated dust of our 

 towns and in our milk-supplies. Tetanus bacilli lurk in our 

 gardens and fields and on our roads. Staphylococci in crowds 

 lie on our skins, and our bath-water contains them by the 

 million after every ablution and yet some of us at all events, 

 manage to resist their attacks, whilst others are caught un- 

 prepared and suffer accordingly, and some are " never well." 



By Immunity against Infective Disease is meant the 

 power of resistance against such attack by pathogenic micro- 

 organisms of all kinds. It is an extraordinarily complex pro- 

 cess, of which science has only just begun to unravel the 

 elements. The perfectly healthy body, by means of its specially 

 evolved covering and " linings " the skin and mucous mem- 

 branes can prevent the entrance of organisms ; but few or 

 none can boast of " perfect health." Some damage to these 

 surfaces may allow of the entrance of organisms at any moment, 

 and at any part of the bodily surface (and for our purpose here, 

 and scientifically speaking, the mucous membrane, say of the 

 stomach or intestine, is just as much a covering or surface as 

 the skin, the food being really "outside," i.e. not within the 

 tissues of) the body. Such damage may be produced by any 

 number of causes. Mechanical injuries such as cuts or scratches, 

 abrasions and even bruises, may allow the microscopic invader 

 to enter or it may be that a mucous membrane is damaged 



