GROWTH FACTORS 99 



animal nutrition and health. It is now becoming evident 

 that many bacterial vitamins are in fact identical with 

 those involved in animal metabolism. ]\Iany of these 

 substances, possibly all, are intimately connected with 

 enzyme or co-enzjrme systems, often constituting the 

 prosthetic group of such systems, and in other cases 

 serving as an essential intermediate step in a cycle of 

 reactions. 



Whereas, formerly, vitamins which were first dis- 

 covered in connection with animal nutrition were later 

 found to be necessary for the growth of micro-organisms, 

 the position is now rather the reverse. For instance, 

 p-aminobenzoic acid, biotin and riboflavin were first 

 investigated in connection with the metabolism of micro- 

 organisms and not until then was their activity in other 

 fields suspected. As the metabolism of the micro- 

 organisms is considerably easier to study than is that of 

 the much more complex animals it is very probable that 

 considerable improvement in our knowledge of general 

 metabolism will result from investigation along such 

 lines . 



The investigation of bios affords an example of the 

 way in which knowledge of gro\Hh factors evolves. 



In 1901 Wildiers showed that yeast would not grow 

 on synthetic media if sma.ll inocula were used, but that 

 the introduction of large inocula was followed by satis- 

 factory growth. He demonstrated that the addition 

 of boiled yeast to the synthetic medium permitted the 

 growth of small inocula. He attributed this phenomenon 

 to the presence in yeast of an essential gro\vth promoter 

 which he called bios ; he suggested that small inocula 

 did not contain enough of it to allow grov,i:h to start, 

 but large inocula carried sufficient bios into the new 

 medium for gro^^-th to occur. He showed that it could be 

 extracted from yeast with water, and that it was soluble 

 in 80 per cent, alcohol but not in absolute alcohol nor in 

 ether. It was stable to heat and moderately stable to 



