MICRO-ORGANISMS 



6. VITAMIN Bi2 AND 2VUCRO-ORGANISMS 



Vitamin B12 as Growth Factor 



As already mentioned (page 534), vitamin B^g is an essential growth 

 factor for Lactobacillus lactis Dorner and L. leichmannii, and both 

 these organisms have been used for its assay. In the absence of 

 vitamin B^g, thymidine and several purines stimulated the growth of 

 both organisms, although only in much higher concentrations.^ It 

 has been suggested that vitamin Bj^g ^^^-Y function as a coenzyme in 

 the conversion of thymine into thymidine, and that the biochemical 

 lesion in pernicious anaemia may be an inability to synthesise certain 

 nucleosides, especially thymidine, from purines or pyrimidines. The 

 curative effects of pteroylglutamic acid may well be due to increased 

 thymine synthesis (page 527), which by a mass action effect yields 

 additional amounts of thymidine. The hypothesis that in Lactobacilli 

 vitamin B^g is involved in nucleic acid synthesis is supported by the 

 observation that it causes an increase in the phosphorus uptake of 

 L. leichmannii and an increase in the desoxyribonucleic acid fraction. ^ 

 Although ascorbic acid is not a growth factor for L. leichmannii, it 

 augments the growth-promoting action of casein hydrolysate on this 

 organism, an effect shared with other reducing agents such as thio- 

 glycolic acid and glutathione. It is believed that these substances 

 protect the small amounts of vitamin B^g in trypsin and casein from 

 destruction by oxidation diuring the autoclaving of the medium. ^ 

 Vitamin B^g combines with a non-dialysable, heat-labile substance in 

 normal gastric juice to form a complex, erythrein, which is non-dialys- 

 able and not dissociated by dialysis. In this form vitamin Bjg is not 

 available to L. lactis, L. leichmannii or E. coli, but is released by heat.* 

 The heat labile factor, apoerythrein, also appears to be present in hog 

 gastric mucosa and may be Castle's intrinsic factor (page 498). It 

 can be assayed by measuring the growth inhibition of E. coli produced 

 by the addition of known amounts of the juice to cultures containing 

 vitamin B^g. Each ml. of normal gastric juice was found to be capable 

 of combining with 15 to 60 m/xg. of vitamin B^g, whereas each ml. of 

 gastric juice from pernicious anaemia patients combined with only i 

 to 5 m^Ltg. of vitamin B^g. 



Vitamin B^g is not essential for the growth of 5. faecalis R or 

 Leuconostoc citrovorum.^ 



Vitamin B^g is essential for the growth of the alga, Euglena gracilis 

 var. bacillaris, another organism that has been used for the assay of 

 vitamin B^g ; it does not respond to thymidine.^ 



Vitamin B^g appears to be one of the rate-limiting factors in the 

 synthesis of bacteriophage T4r, but it was not apparently utilised as 



543 



