CHAPTER II 

 PRODUCTS OF METABOLISM 



General CoNSibERATiONS. — ^The great difference in the meta- 

 bolism of animals and of bacteria, even though they feed essentially 

 on the same foods, is the incomplete metabolism of most bacteria, 

 contrasting sharply against the very complete oxidation of food in the 

 animal body. The food of the animal is decomposed by the body cells 

 to carbon dioxide, water and urea. It is the most complete decom- 

 position possible, excepting urea which, however, is very near the final 

 decomposition product, ammonium carbonate. Microorganisms, on 

 the contrary, are charEcterized by incomplete nietabolism. They do 

 not commonly oxidize their food to the end products but many of them 

 produce organic compounds which are not farther decomposed by 

 them. It is this partial decomposition of organic matter which makes 

 bacteria play such an important r61e in life and industries. Our 

 modern bacteriology is dated from the time when Pasteur showed that 

 the alcohol in the beer fermentation, the lattic acid in the souring of 

 milk, the acetic acid in the vinegar fermentation are products of 

 microbial activity. The existence of bacteria had been known for 

 nearly 200 years, but they were considered largely as a curiosity; 

 as soon as they were recognized as the cause of fermentations,' and 

 of toxins, they received at once the greatest attention. Not all 

 bacteria cause incomplete decompositions; some oxidize as com- 

 pletely as animals do. Others, again, form first intermediary products, 

 which they later decompose completely; among these, are found many 

 molds, the sulphur bacteria, and some species of the vinegar bacteria. 



The Chemical Equations of Fermentations 



The metabolism of all Organisms is considered to be a chemical 

 process which follows in all respects the laws of chemistry. That we 

 are not familiar with all the changes taking place in the cell is not 



is8 



