110 growth: synthesis of bacterial protoplasm 



Fourthly, we can demonstrate the presence of the growth 

 factors in the constitution of non-exacting organisms by 

 the technique of microbiological assay (see below). For 

 example, if we make up a salt-ammonia-glucose medium and 

 carefully free it from nicotinic acid before inoculating with 

 Pr. vulgaris then no growth will occur. Consequently we 

 can use the appearance of growth as a test for the presence 

 of nicotinic acid and, over a certain range, the growth is 

 proportional to the amount of nicotinic acid added. In this 

 way we can show the presence of nicotinic acid in the proto- 

 plasm of non-exacting bacteria by boiling the cells and adding 

 the sterile extract to the basal medium. The test is highly 

 specific and, in an analogous manner, we can show the presence 

 of all these growth substances in the protoplasm of non- 

 exacting organisms (e.g. autotrophes). 



So we get knowledge concerning the synthetic abilities of 

 nutritionally non-exacting organisms from studies of the 

 disabilities of exacting types. Autotrophic bacteria must be 

 able to synthesise all their amino-acids from ammonia and 

 carbon dioxide; the amino-acids are then condensed in various 

 stereochemical combinations to form proteins; some of these 

 proteins require the synthesis of complex prosthetic groups 

 before becoming active as enzymes; some enzymes are not 

 complete without carrier systems of coenzyme I nature; the 

 enzyme systems break down energy-yielding substances in 

 the environment with the production of acid, alkaline, and 

 toxic end-products, and further enzymes are synthesised to 

 neutralise or detoxicate such products. At each stage of 

 synthesis some organism finds the task too difficult, so we get 

 differentiation into autotrophic, heterotrophic, anaerobic, 

 exacting, parasitic, pathogenic, etc., organisms, and in each 

 case the differentiating property is a reflection of the synthetic 

 abilities of the organisms concerned. 



Microbiological assay 



In this chapter organisms have been mentioned which require 

 the presence of certain growth factors in the medium before 



