112 growth: synthesis of bacterial protoplasm 



for solutions of the assay factor, but difl&culties arise when 

 attempts are made to assay the factor in biological materials, 

 food-stuffs, etc., as, unless it is possible to extract the factor 

 easily and quantitatively, addition of the preparation con- 

 taining the factor is certain to involve the addition of salts, 

 amino-acids, and other growth factors which may affect the 

 growth and so vitiate the assay. The problem is to obtain a 

 satisfactory basal medium such that the addition of the assay 

 factor alone has any effect on growth. G-rowth may be 

 estimated turbidimetrically, but in the case of the ^omolactic 

 fermenters such as Lactobacilli or S. lactis, it is possible to 

 obtain a measure of the growth by titration of the acid formed 

 in the medium. Many workers prefer the titration method 

 to the turbidity measurement. Microbiological assay is 

 used for the estimation of nicotinic acid, pantothenic acid, 

 bio tin, pyridoxin, riboflavin, folic acid, thiamin, and certain 

 amino-acids. 



NUTRITIONAL ANTAGONISM 



We have seen that many pathogenic organisms have become 

 nutritionally exacting as a consequence of parasitic existence. 

 Such organisms are unable to grow in media which do not 

 contain certain growth factors, or in fully nutrient media if 

 the utilisation of the growth factors is prevented. The possi- 

 bility of preventing growth by interference with growth factor 

 utilisation was brought into prominence by the work of Woods 

 on sulphanilamide action. 



Sulphanilamide, or ^-amino-benzene-sulphonamide, in low 

 concentrations prevents the growth of certain bacteria, 

 particularly the Gram-positive cocci. It can be shown that 

 sulphanilamide does not immediately kill bacteria in these 

 low concentrations, but prevents their division. Organisms 

 whose growth has been checked by sulphanilamide can 

 proceed to grow and multiply normally after removal of the 

 sulphanilamide. This can be demonstrated by inoculating a 

 sensitive organism into medium containing just sufficient 

 sulphanilamide to prevent its growth ; if the static culture is 



