METABOLISM OF MICROORGANISMS 



359 



this instance tyrosine and phenylalanine are antagonistic to one another. 

 There are many other examples of imbalance between amino acids. 



Growth of some bacteria requires, or is increased by, purines and 

 pyrimidines in the culture medium (e.g., Staphylococcus aureus and 

 Clostridium tetani) . Some bacteria use uric acid and other purines as 

 their sole source of carbon and nitrogen (e.g., Clostridium acidiurici) . 



Growth Factors (Vitamins, etc.). The growth factor requirements of 

 microorganisms vary over a wide range of compounds. For example, 

 E.coli can grow in a medium containing no B vitamins, whereas Lacto- 

 bacillus casei requires at least seven of these vitamins and, in addition, 

 some growth factors of undetermined nature. In some cases only a part 

 of the vitamin molecule is required preformed in the medium; e.g., the 

 ^-alanine jiart of pantothenic acid by yeast and the pantoyl part of it 

 by Acetobacter suboxydans. In other cases, a combined form of the 

 vitamin is required; e.g., nicotinamide riboside by Hemophilus parain- 

 jluenzae and pantetheine (pantothenic acid-/3-aminoethanethiol) by 

 Lactobacillus bulgaricus. An example of a progressively more complex 

 series of compounds, and bacteria requiring them, follows: 



Bacteria Compounds required 



Clostridium acetobutylicum p-aminobenzoic acid 



Streptococcus fecalis pteroic acid 



Lactobacillus casei pteroyl glutamic acid 



Lactobacillus citrovorwn formyltetrahydropteroj'lglutamic acid 



The probable explanation of this series is that the last compound in 

 the series is either the one that functions in metabolism or is nearer 

 to it than the earlier members. The bacteria that need only the simpler 

 compounds probably synthesize the complex compound from the simpler 

 ones. For example, CI. acetobutylicum can perform all the syntheses 

 between p-aminobenzoic acid and formyltetrahydropteroylglutamic acid, 

 and L. citrovorum cannot perform some, if any, of them. 



Another such progressively complex series starts with pantothenic 

 acid, proceeds to pantetheine, and ends with coenzyme A. A third series 

 can be formed from pyrimidine (or thiazole), thiamine, and lipothiamide. 

 The existence of these series of compounds suggests that the compound 

 actually functioning in the metabolism of the cell is the complex com- 

 pound and not the siinple one. 



The fat-soluble vitamins (A, D, E, and K) , so essential for animals, 

 are not required by microorganisms. Many microorganisms synthesize 

 K and carotene and ergosterol, the precursors of A and D. Ascorbic 

 acid stimulates the growth of some bacteria, but it seems to act as a 

 reduction-oxidation compound rather than as a vitamin. On the other 

 hand, many other compounds not required in the diet of animals serve 

 as growth factors for microorganisms. Examples of such compounds, 

 in addition to those already named, and the associated microorganisms 



