130 OXIDATION-REDUCTION POTENTIALS 



occurs in liquid media. In these cases peroxide inhibits the proliferation of matt 

 variants more than glossy variants, so that with repeated subcultures there will be 

 a gradual predominance of the glossy variants. This method of selection is, however, 

 very slow, since peroxide does not accumulate in cultures until after the logarithmic 

 phase of growth, when proliferation is minimal. This process may, however, be 

 accelerated by addition of hydrogen peroxide to the cultures, in order that the 

 peroxide may exert its selective action during the phase of active proliferation. 



These constitute striking examples of the effect of high oxidation-reduction 

 potentials on the behaviour of bacteria, and Todd has made the following additional 

 observations. 



It is well known that organisms isolated from the blood stream of infected 

 animals are frequently of high virulence, but when subcultured in artificial media 

 the virulence falls. The virulence of hsemolytic streptococci may be maintained 

 by subculturing in broth containing 50 per cent, of blood serum. Todd has found 

 that this maintenance of virulence can be secured also by providing plain broth 

 cultures with an ample supply of oxygen by aeration, especially in the presence of 

 catalase which inhibits peroxide formation. The maintenance of virulence occurs 

 only when the cultures have free access to air. That the maintenance of virulence 

 is due to the same factors in 50 per cent, serum-broth and in aerated broth cultures 

 is suggested also by the similarity of the potential-time curves of the cultures (fig. 28), 

 which are entirely different from the potential curves of ordinary aerobic plain broth 

 cultures (Hewitt, 1930, 1). 



Elliott (1945), more recently, has carried the matter further and traced the course 

 of the fall in virulence of sub-cultured strains. He has isolated a proteolytic enzyme 

 from hsemolytic streptococci and this proteinase attacks the " M " specific substance 

 of Matt strains rendering them glossy. Hence a virulent strain under the action of 

 the proteinase will lose its matt-specific substance and hence its virulence. The 

 proteinase appears to require sulphydryl groups for its activation. This, therefore, 

 would appear to explain why matt virulent strains retain their virulence, under 

 highly aerobic incubation conditions, since the oxidation-reduction potentials are 

 then maintained at a high level, sulphydryl groups will be oxidised, activation of 

 the enzyme will thus be impossible and the matt specific substance will remain 

 unattacked. 



BACTERIAL VARIATION, MUTATION, ADAPTIVE ENZYMES, DRUG RESISTANCE 



Bacterial variation, mutation, adaptation, etc., may roughly be classified into 

 temporary effects that are readily reversed on sub-culture, and more permanent 

 mutations. By their nature and cultural conditions bacteria are peculiarly liable to 

 mutations and variations. As mentioned already bacteria are forced to adapt them- 

 selves to varying conditions and in the course of a single culture experience a wide 

 variety of conditions from an ample air supply to almost complete anaerobiosis, from 

 alkahnity to acidity, from abundance of carbohydrates to starvation. Cells grown in 

 the presence of one carbohydrate may readily ferment that carbohydrate, but not 

 another until gradually adapted by growing in the presence of the second carbo- 

 hydrate. In other cases it has not been found possible to adapt some bacteria to 

 some carbohydrates, although closely related strains ferment it quite readily. Again, 

 some types of bacteria are much more readily adaptable than others. 



