Bacterial Mefabolhm 65 



radioactive isotopes of carbon, nitrogen, phosphorus, and 

 other elements has enormously simplified certain kinds of 

 metabolic studies. Tracer methods frequently permit the 

 direct observation of chemical processes that previously 

 could be detected and studied only, if at all, by much more 

 indirect and laborious procedures. For example, the carbon 

 isotopes made it possible to determine the fate of individual 

 carbon atoms during the metabolic transformation of a 

 multicarbon compound. This supplied new information 

 about the chemical reactions in living bacteria and enzyme 

 systems that could be used to determine the validity of pro- 

 posed metabolic pathways. Tracer methods also permitted 

 the study of the utilization of compounds that normally ac- 

 cumulate during metabolism. In this way the surprising dis- 

 covery was made that several common end products of 

 bacterial metabolism like acetate and carbon dioxide, pre- 

 viously regarded as stable and inert, are in fact inter- 

 mediates that can be further converted by many bacteria 

 into a variety of catabolic products and cellular con- 

 stituents. 



The balance and tracer methods give a picture of the 

 over-all chemical transformation in bacterial metabolism 

 and also provide a guide to the chemical mechanisms in- 

 volved. These techniques are limited, however, by the fact 

 that in living bacteria the metabolic processes are so per- 

 fectly coordinated that it is difficult to separate and identify 

 the individual reactions. Also, intact bacteria are imperme- 

 able to many compounds of biochemical importance such 

 as phosphate esters and coenzymes. Therefore a detailed 

 insight into the cellular chemical mechanisms, which are 

 responsible for substrate decomposition and the synthesis 

 of essential cellular components, requires the use of cellular 

 extracts and enzymatic techniques. This has been obvious 

 for a long time, ever since the discovery of the glycolytic 

 mechanism. But, the required combination of microbio- 

 logical and enzymological know-how was scarce, and so, 



