FOOD 33 



organic compounds are derived, two classes of bacteria 

 may be recognized, viz., saprophytes, which obtain them 

 from the dead and decaying bodies of plants and 

 animals, or the lifeless organic matter in soil, milk, water, 

 and other common materials ; the parasites , which feed 

 upon materials present in the tissues of living organisms, 

 their so-called hosts, and produce abnormal physiological 

 disturbances or diseases in the latter. Bacteria which 

 produce disease are spoken of as pathogenic organisms ; 

 examples are the bacilli which cause tuberculosis, anthrax, 

 diphtheria, and enteric fever in man and the lower 

 animals. 



The elements composing the food of bacteria are 

 practically the same as those utilised by plants, viz., 

 carbon, hydrogen, oxygen, sulphur, phosphorus, and 

 certain metallic elements, the chief of which are sodium, 

 calcium, magnesium, and iron. The first five enter into 

 the composition of the proteins, which form much of 

 the body substance of bacteria. The metallic elements 

 appear to be necessary for the growth and physiological 

 functions of these organisms, but how they are utilized 

 is not clearly known. It is said that pure cultures of 

 certain species of bacteria have been grown without 

 calcium, sodium, or potassium, but the experiments on 

 which these statements are made are not altogether 

 conclusive ; exceedingly minute traces a*e difficult to 

 eliminate from culture media, and these small amounts 

 are often quite sufficient for the needs of bacteria. 



The food elements are obtained from many sources. 

 Some of them, e.g. oxygen and nitrogen, may be absorbed 

 in a free state ; usually, however, the latter can only 

 be used when presented in a combined form. Potassium, 

 calcium, and magnesium are taken from sulphates, 

 C 





