CULTURAL CONDITIONS 



119 



The amount of the oxygen-carrying effect varies somewhat for different samples 

 of sermn, but was observed with all the sera studied to some extent. This poising 

 effect of serum on the electrode potentials developed in bacterial cultures is of 

 considerable importance. It may account for the beneficial effect of serum on the 

 cultivation of organisms such as hsemo lytic streptococci and, above all, may account 

 for the special biological behaviour of organisms freshly isolated from the blood 

 stream. This subject is dealt with in the next section on bacterial variation. It 

 is possible that the poising effect of proteins accounts for the non-germination of 

 tetanus spores in the body. Fildes (1929) has shown that tetanus spores do not 

 germinate unless the Ef, level is adjusted below that of the subcutaneous tissue. 

 Fildes and Knight (1930) also investigated the levels of potential necessary for tetanus 

 spore germination. 



18 20 



26 as 



Time (hours) 

 Fig. 29 



Broth cultures of haemolytic streptococci 



Some samples of Douglas's tryptic digest medium have a quite considerable 

 oxygen-carrying effect preventing the level of potential of bacterial cultures in this 

 medium falling to the low level reached in infusion broth (fig. 29). It appears possible 

 that tryptic digestion of flesh may liberate oxygen-carrying substances similar to 

 those present in serum proteins. 



The presence of glucose produced characteristic effects on the electrode potentials 

 reached in bacterial cultures (fig. 30). With each organism studied the potentials 

 in 1 per cent, glucose broth fell rapidly at first, but after a few hours' growth the 

 potential curve showed a short period of rapid increase followed by a slight fall and 

 then a gradual rise. The sudden reversal in direction of the potential-time curve 

 occurs at about the time when acid production becomes appreciable, but it is doubtful 

 if the direct effect of pH upon the electrode potential accounts for the phenomenon, 

 although it is true that the metabolic processes which give rise to the production 

 of acid are also responsible for the changes in potential. In order to account for 

 some of the rapid potential changes it would be necessary to have a pH change of 

 the order of one unit within a few minutes. However, Gillespie and Rettger (1936) 

 state, without giving details, that w^ith one strain of a lactobacillus they are of opinion 

 that the inversion of potential might be accounted for on the grounds of pH change.. 



