20 BACTERIA 



THE INFLUENCE OF EXTERNAL CONDITIONS ON 

 GROWTH OF BACTERIA 



Nutritive Medium. In the very earliest days of the study 

 of micro-organisms it was observed that they mostly con- 

 gregate where there is pabulum for their nourishment. 

 The reason why fluids such as milk, and dead animal matter 

 such as a carcass, and living tissues such as a man's body 

 contain so many microbes is because each of these three 

 media is favourable to their growth. Milk affords almost 

 an ideal food and environment for microbes. Its tempera- 

 ture and constitution frequently meet their requirements. 

 Dead animal matter, too, yields a rich diet for some species 

 (saprophytes). In the living tissues bacteria obtain not only 

 nutriment, but a favourable temperature and moisture. Out- 

 side the human body it has been the endeavour of bacterio- 

 logists to provide media as like the above as possible, 

 and containing many of the same elements of food. Thus 

 the life-history may be carried on outside the body and 

 under observation. By means of cover-glass preparations 

 for the microscope we are able to study the form, size, 

 motility, flagella, spore formation, and peculiarities of stain- 

 ing, all of which characters aid us in determining to what 

 species the organism under examination belongs. By means 

 of artificial nutrient media we may further learn the char- 

 acters of the organism in " pure culture," ' its favourable 

 temperature, its power or otherwise of liquefaction, the curd- 

 ling milk, or of gas production, its behaviour towards 

 oxygen, its power of producing indol, pigment, and chemical 

 bodies, as well as its thermal death point and resistance to 

 light and disinfectants. It is well known that under artificial 

 cultivation an organism may be greatly modified in its 

 morphology and physiology, and yet its conformity to type 



1 A " pure culture " is a growth in an artificial medium outside the body of 

 one species of micro-organism only. 



