118 



OXIDATION-REDUCTION POTENTIALS 



metabolic reactions. An indirect effect of deficiency of " oxygen enzymes " may be 

 even more important in that inhibitors of important aerobic fermentations may 

 accmnulate (e.g., peroxide cannot be rapidly removed in the absence of catalase) and 

 activators, such as SH compounds, may not be formed in sufficient concentration. 



It should perhaps again be emphasised that different classes of bacteria and even 

 different strains vary so much that all anaerobes do not behave identically and many 

 shades of difference can be detected from the very strictest anaerobes to the micro- 

 aerophile, which can multiply slowly even in the presence of air. At the other extreme 

 to the anaerobes are the obligate aerobes which flourish in an oxygen-rich environ- 

 ment and are well provided with iron-porphyrin and " oxygen " enzymes, but are 

 singularly deficient in available fermentation enzymes and in the absence of abundant 

 oxygen cannot multiply. 



EFFECT OF CULTURAL CONDITIONS ON POTENTIALS 



In the presence of serum, the electrode potentials of bacterial cultures frequently 

 do not fall to as low a level as in plain broth (Hewitt, 1930 ; 1, 2, 3, 4). A typical experi- 

 ment is illustrated in fig. 28. In plain broth, the potential of the culture of haemolytic 

 streptococci fell to E^ — 0-16 volt, but in 50 per cent, serum-broth the potential 

 did not fall below + O'^O volt. This represents less than 50 per cent, of the fall in 

 potential in plain broth. The same oxygen-carrying effect was found to be produced 

 by the serum proteins prepared by salting out the proteins by sodium chloride and 

 acetic acid. It has been tentatively suggested that sulphydryl groups in the protein 

 molecule may have an oxygen-carrying effect. 



o 



ja 



B 





Aerobic plain broth^ 



Sealed serum -broth 



10 12 14 16 18 20 22 24 26 28 



Time (hours) 



Fig. 2S 



Cultures of hpemolytic streptococci, showing effect of serum 



The presence of sulphydryl groups in proteins has been demonstrated by 

 Hopkins. In certain cases, these groups can be detected chemically only after 

 denaturation and reduction (1930), but it is possible that their effect on bacterial 

 respiration may be manifested in the native proteins. In serum proteins, the sulphur 

 compounds appeared to be in the oxidised condition, in agreement with observations 

 of the poising effect mentioned above. 



Lepper and Martin (1929) have shown that the unsaturated fatty acids in lipins 

 and also hsematin compounds are of importance in the reducing effects observed in 

 culture media. These compounds may well occur in serum together with traces of 

 haemoglobin, thus making a very complex system. 



