MICROBIOLOGICAL ASSAY 



111 



growth can occur. Many, if not all, of these growth factors 

 are of importance in mammalian nutrition, but are difficult to 

 estimate by chemical means owing to their complex structure 

 and the very small amounts in which they occur naturally. 

 If an organism is exacting towards a growth factor F, and is 

 inoculated under standardised conditions into a series of 

 media containing amounts of F varying from nil to sufficient 

 to give complete growth, then the growth is found to vary 

 with the amount of F in the medium as shown in Fig. 9. 



Over a certain range of concentrations of F there is a linear, 

 or approximately linear, relation between the concentration 



GROWTH 



(turbidity, 

 dry weight 



CELL-N, 



ACID FORMED 



ETC.) 



CONCENTRATION OF F 

 Fig. 9. 



and the amount of growth . If this relation can be standardised 

 consistently, then the growth can be used as a measure of the 

 amount of F in a given solution. This method of growth 

 factor or vitamin determination is known as microbiological 

 assay. For example, the growth of Pr. vulgaris can be used 

 for the assay of nicotinic acid. The method is open to many 

 sources of error: the basal medium must be completely and 

 easily freed from the factor to be assayed; the growth curve 

 must be accurately reproducible; and the growth must not 

 be affected by any other variable factor in the growth medium 

 or in the preparation of material added for assay. The 

 method can often be made to work with reasonable accuracy 



