66 OUTLINES OF BACTERIOLOGY 



constituents. In green plants, for example, a very small quantity 

 of iron is necessary ; because, without it no chlorophyll can be 

 formed, and without chlorophyll there can be no building up of 

 carbon dioxide and water into formaldehyde, which is the first step 

 in the construction of organic substance. To find the nature of 

 the substances that enter into the composition of any organic body 

 a chemical analysis must be made. An analysis of this kind for 

 bacteria shows that they do not essentially differ from other 

 organisms. The elements present are carbon, hydrogen, oxygen, 

 nitrogen, sulphur, phosphorus, potassium, calcium, iron, and mag- 

 nesium. In addition, for many, especially pathogenic bacteria, 

 sodium, and chlorine, though not absolutely necessary, yet make 

 a great difference to the well-being of the organism. In the list 

 just given the first six are necessary, because protoplasm consists 

 of these elements. The others mentioned are found to be neces- 

 sary because, just as in other plants, they are connected with the 

 machinery, as it were, by which protoplasm is constructed. Thus 

 calcium is necessary probably to neutralise the poisonous effect on 

 protoplasm of acids secreted as waste products. With regard to iron, 

 all green plants and almost all the lower animals require this sub- 

 stance, though in such small quantities that no special provision need 

 be made for its presence. The same is true for bacteria, but the exact 

 role of these elements is not yet ascertained. To supply the necessary 

 mineral constituents the following mixture may be used in making up 

 nutrient solutions : 



(CaCl 2 ) Calcium chloride, - (H gram. 



(MgS0 4 ) Magnesium sulphate, - 0*2 ,, 



(K 2 HP0 4 ) Potassium hydrogen phosphate, 0-1 ,, 

 Distilled water, 1000 c.c. 



In ordinary cultures, however, where the exact analysis of the 

 nutrient solution is not required, ordinary tap-water may be used 

 instead of this mixture, especially if flesh extracts are used in the 

 preparation of the nutrient media. It will be noticed that the pro- 

 portion of water is very great, but it must be remembered that the 

 proportion of water in living protoplasm is very high, that no food 

 can be taken except in solution in water, and also that the process 

 of digestion, that is the changing of insoluble into soluble substances 

 fit for absorption, cannot take place unless water be present, for 

 ferments act only in the presence of water. Another reason for 

 this large percentage lies in the fact that if the nutrient substances 



