40 FATTY ACID METABOLISM IN MICROORGANISMS 



The biotin content of the cells which are grown on the 

 various fatty acids (cultures III to V) is significantly lower 

 than that of the cells grown in the presence of biotin (cul- 

 tures I and II). Thus, the growth-promoting potency of 

 unsaturated or cyclopropane fatty acids is not explicable 

 in terms of their serving as precursors for biotin biosynthesis. 

 Trace amounts of biotin (below the limits amenable to 

 quantitative determination) are always present in the fatty 

 acid grown cells; however, the metabolic role of these trace 

 levels of biotin is difficult to evaluate. Substitution of biotin 

 by cw-vaccenic acid (culture IV), although not affecting 

 significantly the "dihydroxy" acid level, practically doubles 

 the Ci9 acid content of the bacteria. Most remarkable is the 

 composition of the fatty acid mixture of the lactobacillic 

 acid grown cells (culture V). The lipids of these cells are 

 completely devoid of unsaturated fatty acids, as reflected by 

 the absence of "dihydroxy" fatty acids. As is to be expected, 

 the (Ci8 plus C19) and the C^g content of these lipids is 

 markedly higher than in the biotin cells. 



Lactobacillic acid grown cells of L. arahinosus and L. 

 casei (17) (Table 2.4) are also free of unsaturated fatty acids. 

 Biotin grown cells of these organisms contain a sizable 

 "dihydroxy" fraction. 



Lactobacillic acid appears to possess the ability to substi- 

 tute for c?\y-vaccenic acid in the metabolism of all three or- 

 ganisms. We concluded from these findings (15) that "the 

 biosynthesis of lactobacillic acid may involve the addition 

 of a 'Ci' fragment to the double bond of ciVvaccenic acid." 

 Experiments with labeled c^Vvaccenic acid and labeled one- 

 carbon donors, to be discussed in Chapter 2, section 5, 

 validate our hypothesis. 



