DEFICIENT CULTURES 101 



If washed suspensions of these deficient organisms are prepared 

 and their rate of oxidation of malic acid studied (malic acid 

 dehydrogenase requires coenzyme I), it is found that the 

 addition of coenzyme I to the suspension during the test 

 results in a greatly increased rate of oxidation. This demon- 

 strates that the organism has synthesised the enzyme malic 

 dehydrogenase, but the activity of the enzyme is not fully 

 effective as the coenzyme part of the system is deficient, and 

 the addition of coenzyme repairs the deficiency. 



In this last example it was possible to grow an exacting 

 organism in the presence of sub-optimal quantities of a growth 

 factor whose function could be guessed with reasonable 

 certainty, and then demonstrate a metaboHc impairment in 

 that function. A similar technique can be used to determine 

 the metabolic function of other growth factors; to do this a 

 culture is grown in the presence of sub-optimal quantities 

 of growth factor and a control culture in the presence of 

 excess growth factor. A survey is then made of the metabolic 

 activities of the two cultures in an attempt to discover an 

 activity affected by the deficiency of growth factor in the 

 deficient culture. If such an impaired activity is found, 

 the effect on the activity of adding growth factor to the washed 

 suspension of the deficient organism is studied. Staph, 

 aureus, for example, is exacting towards thiamin. Thiamin- 

 deficient organisms metabolise pyruvic acid at a rate signifi- 

 cantly less than that of normal organisms, and the addition 

 of thiamindiphosphate makes good the deficiency. It follows 

 that thiamin plays some part in the metabolism of pyruvic 

 acid by these organisms. 



Studies on the impairment of metabolism in growth-factor- 

 deficient cultures have assisted in the elucidation of the 

 function of pyridoxal which is the biologically active form 

 of pyridoxin (see Table VIII). If we study the growth of 

 streptococci in media containing increasing amounts of 

 pyridoxin, but otherwise fully nutrient, we get a curve relating 

 the amount of growth to the pyridoxin content of the medium 

 as shown in Fig. 8. We can distinguish three types of culture : 



