358 



METABOLISM OF MICROORGANISMS 



Growth requirements 



Energy and Carbon. Because of the thousands of species of micro- 

 organisms, it is much more difficult to state their growth requirements 

 than it is those of higher animals, where only a few species have to be 

 considered. Perhaps the only general statement one can make is that 

 all require some source of energy. A few species of microorganisms can, 

 like plants, use light as a source of energy, but the vast majority obtain 

 their energy from chemical elements or compounds. Merely listing a 

 few examples shows the diversity of sources: elemental H, C, and S, 

 simple compounds such as HoS and NH3, carbon compounds ranging from 

 carbon dioxide and methane through the carbohydrates, lipides, and 

 proteins to such resistant materials as lignin and paraffin. 



Stephenson cites an example from the work of den Dooren de Jong 

 to illustrate the amazing synthetic powers of microorganisms. The 

 bacterium, Pseudomonas putida, can meet all its carbon requirements 

 from 77 different carbon compounds out of 200 tested. The utilizable 

 compounds included 6 carbohydrates, 10 alcohols, 13 fatty acids, 17 

 amino acids, 9 amides, and 7 amines. It would probably be a safe 

 statement to make that there is no form of combined carbon in the 

 world that cannot be utilized by some microorganism. 



Nitrogen. Since all living cells contain protein, some form of nitrogen 

 must be supplied. In some cases atmospheric nitrogen is utilized {e.g., 

 Azotobacter vinelandii) ; in others, inorganic nitrogen, such as nitrates 

 and ammonia, is adequate {e.g., yeasts, molds, and autotrophic bacteria) ; 

 but in many others only amino acids can meet the needs of the cell. 

 Lactic acid bacteria are conspicuous examples of cells that require pre- 

 formed amino acids for growth. One of these, Leuconostoc mesenteroides, 

 requires 17 amino acids, many more than are required by higher ani- 

 mals. 



The requirement for certain amino acids may depend upon the absence 

 of a vitamin. For example, some strains of propionic acid bacteria 

 require riboflavin if ammonium sulfate is the source of nitrogen, but if 

 amino acids are supplied, no riboflavin is needed. Another example is 

 Lactobacillus arabinosus, which grows without tryptophan if vitamin Be 

 is present, and without Be if tryptophan is present. In both cases it 

 synthesizes the compound that is omitted. Hence there is not an absolute 

 requirement for either compound, but the cell cannot make both com- 

 pounds simultaneously. 



The interesting phenomenon of imbalance among amino acids exists 

 in microorganisms, perhaps more markedly than with higher animals. 

 Thus a certain strain of Escherichia coli will grow in the absence of 

 tyrosine, but not in its presence unless phenylalanine is also present. In 



