2 MANUAL OF BACTERIOLOGY. 



the oxidation of the carbohydrates, which may appear indirectly 

 in the form of nervous or muscular activity or warmth. 



Those plants that are devoid of chlorophyll are compelled 

 to some extent to use the same kinds of food as animals. They 

 are unable to decompose carbon dioxide (in most cases), and 

 procure their nourishment from substances derived from the 

 dead or living bodies of other plants or animals. Since they 

 have no chlorophyll, light is of no advantage to them, and is 

 often a positive detriment. Bacteria contain no chlorophyll, 

 and consequently are unable to decompose carbon dioxide and 

 to use it as food.* 



There is another well-known property, possessed by yeasts 

 especially, which may be useful in explaining the work done by 

 bacteria. It is a fact of every-day observation that, when 

 ordinary yeast grows in fruit juice or other fluids containing 

 sugar, alcohol and gas are formed. It not only appears that 

 bacteria sometimes form alcohol and gas from sugar, but that 

 with different kinds of bacteria and different kinds of food 

 material a great number of substances are formed, some of which 

 are powerful poisons. In most, if not in all, of the diseases 

 caused by bacteria such poisons are produced within the hving 

 body of the affected individual, and the symptoms of the dis- 

 ease and the changes produced in the body are certainly due to 

 these poisons, as a rule, rather than to the direct action of 

 bacteria. 



The extreme smallness of the bacteria prevents us from seeing 

 them as individuals without the aid of the microscope, although 

 great numbers of them taken together may form a plainly visible 

 mass of growth. When they are examined with the microscope 

 they appear as little, round, rod-shaped or curved bodies, which 

 may be likened to so many periods, dashes and commas. It is 

 at once perceived that each bacterium is an individual by itself, 



* See Part II., Chapter I. 



