BACTERIOLOGY. 41 



bacteria were first grown upon an artificial pabulum in 

 the laboratory. The adaptability of bacteria to various 

 kinds of foods explains the spread of many diseases by 

 foods and water, and also the tenacity that certain 

 diseases display in clinging to a house or to clothing. 

 Almost everywhere there are small amounts of organic 

 matter and moisture, and these microscopic forms 

 of life which we are considering require such infini- 

 tesimal amounts of either, that it is possible for them 

 to live where nothing else can. 



Harmful bacteria, that is, those that are 

 ATHOGENic (.g^p^^g ^f producinff disease, are called 

 AND NON- ^, . , , . . 



PATHOGENIC pc^^hogemc; harmless varieties, or sapro- 



Bacteria. phytes, are called non- pathogenic. A 

 pathogenic bacterium, however, cannot 

 always produce disease when it finds lodgment within 

 the body. This special characteristic is extraordinarily 

 variable, appearing and disappearing under a variety 

 of circumstances. Pathogenicity in a bacterium depends 

 upon its power to produce toxins (poisons), a function 

 which is extremely susceptible of loss by exposure 

 to many natural agencies. A pathogenic bacterium 

 that can produce its specific toxins, and therefore bring 

 about disease, is spoken of as virulent^ one that cannot, 

 as* non-virulent. For the foregoing reasons we recognize 

 in all pathogenic bacteria both varieties. However, 

 virulence and non-virulence are evanescent qualities, 

 requiring only alterations in environment to be made 



