298 THE BIOCHEMISTRY OF B VITAMINS 



a varied and extensive list of organisms which inhabit the intestinal tract, 

 frequently predominated by the coliform group, lactobacilli, enterococci, 

 and yeasts. 182 Intestinal bacteria which require nutritional sources of 

 B vitamins also exist, e.g., lactobacilli (p. 307), most frequently in rela- 

 tively small numbers although this fortuitous fact is seldom considered 

 and has been little investigated. The nature of the factors influencing the 

 intestinal flora, and its variation among species has been studied at 

 length, but the valid information derived from such studies is limited for 

 the most part to the influence of diet on the nature of the organisms 

 present. 182a 



The nature of the carbohydrate, protein, fat, and vitamin content of 

 the diet has a considerable effect upon the relative numbers of the various 

 types of organisms to be found in the intestinal lumen, depending upon 

 the selective effects of these dietary constituents in promoting or depress- 

 ing the growth of the various organisms. The phenomenon of "refec- 

 tion," 183 in which high starch diets protect experimental rats on a 

 B-deficient diet, has long been known, and is generally believed to be due 

 to the stimulation of thiamine-producing organisms in the rat intestine. 

 Various drugs also have pronounced effects, and antibiotic substances 

 may depress intestinal vitamin synthesis to a very low level. 



It is generally recognized that the entire B vitamin nutritional require- 

 ment of ruminants may be met by the microflora of the rumen, so that 

 B-avitaminoses are rare in this group of animals. In young ruminants, 

 before the microflora is well established, some deficiencies may occur, 

 however. 184 In most other higher animals, a lesser and more variable 

 part of the requirement is so met. Limited evidence suggests that the 

 diet of the pig is more subject to fluctuations in intestinal synthesis than 

 most other species, 185 although the reasons for this are unknown. Rela- 

 tively little is known at all concerning the effects or nature of intestinal 

 flora in the nonmammalian vertebrates, or in the invertebrates, although 

 there is ample evidence to suggest that bacterial vitamin synthesis may 

 be a critical factor in fulfilling nutritional requirements in the entire 

 plant and animal kingdoms. 



The study of the extent to which intestinal synthesis may affect the 

 nutritional requirement in any given case has been approached in several 

 ways. Most frequently sulfonamides and antibiotics have been fed, and 

 their effect on the nutritional requirement as judged by the various tech- 

 niques previously discussed (Chap. I C) has been observed. 186 Such 

 studies applied to man and domestic and laboratory animals have given 

 us most of the existing data regarding the intestinal synthesis of the B 

 vitamins in general, and biotin and folic acid in particular. Other special 

 diets have also been used from time to time with some success, when the 



