ESMOND E. SNELL 443 



seen that the foUc acid requirement of Streptococcus faecalis varies re- 

 markably with the nature of the medium. In a medium which lacks serine 

 the folic acid requirement is over ten times that observed in the same 

 medium with added serine. This is interpreted to mean that folic acid 

 is required in relatively large amounts for synthesis of serine, and if 

 serine is supplied preformed, then substantially smaller amounts of 

 folic acid suffice to fulfill its remaining functions (6,ii). If to the 

 medium which contains serine, thymine is now added, the folic acid 

 requirement completely disappears (6,38). Under the latter conditions 

 no folic acid can be detected in the cells (58). Apparently the folic acid 

 requirement observed in curve 2 represents that required for synthesis 

 of thymine by these cells, and when thymine is supplied preformed the 

 requirement for this folic acid no longer exists. Traces of the vitamin 

 may still be required for other purposes, but if so, these traces can be 

 synthesized by the cells and are insufficient in magnitude to be detected 

 by present methods of assay- 

 Several other instances of similar effects are known which will not be 

 given in detail. The vitamin Be requirement of lactic acid bacteria, for 

 example, is greatly increased by omitting certain nonessential amino 

 acids from the medium (21) and can be eliminated entirely for some 

 lactic acid bacteria bv the addition of D-alanine to a medium which 

 contains a complete assortment of L-amino acids (32,10). Vitamin Be 

 is apparently required by these organisms primarily to permit synthesis 

 of amino acids (both l and d) which are essential for synthetic process 

 within the cell, and when all of these amino acids are supplied preformed, 

 the vitamin Be requirement is reduced to the point where it can no longer 

 be detected (10). Here, too, analyses have shown that the cells do not 

 synthesize increased amounts (if any) of the vitamin under conditions 

 where they grow without it. 



In a wholly analogous fashion the biotin requirement of many lactic 

 acid bacteria can be eliminated by addition of aspartic acid and oleic 

 acid (39,41), and the vitamin B12 requirement by appropriate desoxy- 

 ribosides or reducing agents (12,31). Similar explanations for these 

 results may hold, although further detailed investigations are necessary 

 to establish the mechanism of these effects. It will be apparent from the 

 above examples, however, that a given bacterial species does not neces- 

 sarily require a fixed and unchangeable assortment of growth factors, 

 but that different combinations, both quantitatively and quaUtatively, 



