92 NUTRITION AND METABOLISM. 



many bacteria belong to this class of organisms, while only a few yeasts 

 have this ability. The absolutely insoluble compounds of protein 

 nature, like the keratin of hair and horn, can be decomposed by only a 

 very few organisms. 



Though very little is known about the nutrition of protozoa, it may 

 be mentioned here that the saprozoic forms are not able to live on any 

 recognized soluble food media. They require solid food (bacteria, algae, 

 diatoms, other protozoa, etc.), and it is recorded that most of them 

 require living organisms for food and reject dead organisms. 



MINERAL FOOD. 



The minerals needed by bacteria are used (with very few exceptions) 

 entirely as structural food. The amount of mineral compounds is very 

 small since they make up not more than about 8 per cent of the solid 

 matter of bacterial cells. It is probable that the cells can exist and 

 multiply even with smaller amounts. The necessary elements are nitro- 

 gen, phosphorus, sulphur, chlorine, potassium, calcium, magnesium, 

 iron and manganese. There is still some doubt about the necessity of 

 certain other elements. Such investigations are extremely difficult 

 because of the mere traces of chemical compounds that enter into con- 

 sideration. These elements are assimilated usually in the form of inor- 

 ganic salts, with the exception of phosphorus and sulphur (and, of course, 

 nitrogen). The organic compounds of these elements may be absorbed 

 occasionally though the mineral sulphates and phosphates are by far 

 more commonly utilized. 



The use of mineral compounds for the production of energy is limited 

 to a very few species of bacteria, while yeasts, molds, and protozoa cannot 

 possibly utilize them. The nitrite and nitrate bacteria and the hypo- 

 sulphite organisms have already been mentioned in connection with this. 

 Another group consisting of a number of bacteria which are morpho- 

 logically as well as physiologically different from the common species 

 and which are known as the sulphur bacteria or thiobacteria (p. 60), 

 oxidize hydrogen sulphide to sulphur, and sulphur to sulphates. 



The sulphur formed by this process is stored in the bacterial cells which 

 are often found nearly filled with sulphur granules. Sulphur takes the 



