434 



PLANT GROWTH SUBSTANCES 



a glucose-salts medium (37). The closely related Clostridium acetobuty- 

 licum requires biotin and /7-aminobenzoic acid for growth under similar 

 conditions (15), whereas Clostridium perfringens requires several different 

 vitamins (pyridoxamine, biotin, riboflavin, and pantothenic acid), 

 adenine and uracil, and thirteen different amino acids for growth (5). 

 Somewhat similar variations, if not so marked, may be found in the 

 requirements of different representatives of other groups of bacteria, 

 for example, the lactic acid bacteria, some of which have nutritive 

 requirements which surpass those of Clostridium perfringens in complexity 



(24.33) • 



It would be of little interest here to tabulate the growth-factor re- 

 quirements of various bacteria which have so far been investigated. For 

 each organism such a tabulation would show one, a few, or many of the 

 compounds listed in Table i as being required. Summary articles which 

 contain this valuable information have appeared (14,24,33). Instead, 

 certain other aspects pertinent to this problem may be discussed. 



The nature of the response to an essential growth factor is shown in 

 Figure i, which depicts the response of Lactobacillus casei to additions 

 of riboflavin. Within hmits, the amount of growth is dependent only 

 upon the concentration of the vitamin. If, at these growth-Hmiting 

 concentrations, one measures the amount of the vitamin present in the 

 medium and the cells after growth has ceased, it is found that all of the 

 vitamin (within experimental error) has been absorbed from the medium 



^0.4 



0.05 0.1 



0.15 0.2 0.25 0.3 0.35 0.4 



0.45 0.5 055 



MICROGRAMS RIBOFLAVIN PER 10 ML. OF ME DIUM 



Figure i. Relationship between the concentration of riboflavin and growth 

 of Lactobacillus casei. 



