134 OXIDATION-REDUCTION POTENTIALS 



experimental groups. It is easier to prepare a standard bacterial culture than a 

 standard disease-free colony of rats. It is merely necessary to select a bacterium that 

 requires the given vitamin for growth and cultivate it in a medium adequate in all 

 other respects, but deficient in the vitamin ; and then add to different cultures 

 graded amounts of a standard vitamin preparation and the test sample. 



A similar method is used for amino-acid determination using an organism such 

 as Lactobacillus casei which is exacting in its amino-acid requirements. A series of 

 culture media is prepared, each deficient in one or other amino-acid ; addition of 

 graded amounts of the test sample to each of these media before inoculation again 

 gives a measure of the content of the various amino-acids. Results obtained are 

 comparable with those obtained chemically. 



Purines and many other compounds may be assayed by similar methods. The 

 use of the mould Neurospora crassa has proved a valuable tool and mutants obtained 

 from it by X-radiation deficient in power to synthesise a variety of compounds have 

 been used extensively, not only for assays but for tracking down pathways of synthesis 

 to investigate what intermediates or combinations of intermediates can act as 

 substitutes for the deficient component. 



BACTERIAL TOXINS 



The significance of soluble bacterial toxins to both the bacteria that produce 

 them and the animals they damage has received some elucidation within recent 

 years. The extreme potency of some toxins is quite remarkable : Pappenheimer 

 calculates that 1 mg. of tetanus or botuHnus toxin is sufiicient to kill more than 

 1,000 tons of guinea pig and that one molecule of toxin per cell is active. 



The a-toxin of CI. welchii has been shown to be a lecithinase and its effect in 

 haemolysing erythrocytes is explicable on these grounds (R. G. McFarlane). Oakley 

 and his colleagues have found evidence for a number of different proteinases in the 

 pathogenic Clostridia with toxic effects. 



An interesting suggestion has been made by Pappenheimer concerning the 

 significance of diphtheria toxin in the respiratory system of C. diphtJierice, although 

 he does not claim that the hypothesis is definitely established. He suggests that 

 diphtheria toxin, when inside the bacterial cell, forms the protein moiety of the 

 cellular cytochrome. Diphtheria toxin, on this theory, only occurs in the culture 

 filtrate as a soluble toxin when there is insufiicient iron in the culture medium to form 

 the iron porphyrin or protohaematin complex portion of the cytochrome molecules. 



One of the chief lines of argument in favour of this view is the inverse linear 

 relation between the iron content of the culture medium and the amount of toxin 

 present in the culture filtrate. Although not intended as a contradiction to the theory 

 some experiments of the author may be quoted as showing the limitations of the 

 evidence. 



In the curve is plotted the toxin found in the filtrates of a series of cultures of 

 C. diphthericB of the Park- Williams 8-strain (the same as that used by Pappenheimer). 

 It will be seen that as the iron content of the medium increases the toxin content falls, 

 and over the portion of the curve studied by Pappenheimer a reasonably straight line 

 is obtained for the relation between toxin production and iron content. If, however, 



