114 OXIDATION-REDUCTION POTENTIALS 



can be detected chemically under these conditions. In specially aerated pneumo- 

 coccus cultures, the potential fell to a much lower level when catalase was present. 

 After a time, however, the potential in these aerated cultures commenced to rise, 

 and this was due, in part at leasfc, to destruction of catalase under the oxidising 

 conditions established. This was shown by the fact that addition of a fresh catalase 

 preparation resulted in an immediate fall in potential (see fig. 25). 



The rapid fall in potential on adding catalase to cultures containing peroxide 

 was noted on several occasions with cultures of pneumococci and of hsemolytic strepto- 

 cocci. This fall in potential is the more remarkable since, at the very time the 

 potential is falling, the culture is being oxygenated by bubbles of oxygen liberated 

 throughout the culture from the decomposed peroxide. Oxygen itself seems, in fact, 

 to have a stimulating effect on the growth of both pneumococci and heemolytic 

 streptococci as long as the accumulation of peroxide is prevented. Thus an aerated 

 fifty-hour culture of pneumococci was completely sterile, whereas in an otherwise 

 similar culture to which catalase had been added growth was particularly good and 

 the bacteria proliferated actively when sub-cultured. From the evidence presented 

 it is clear that the harmful effects of peroxide on some bacteria at any rate are not 

 due to the tendency to liberate molecular oxygen. The catalase preparation used in 

 most of these experiments was prepared from liver, but similar results were obtained 

 using red blood corpuscles and killed aerobic bacteria {M. lysodeikticus) as the source 

 of catalase ; allowance must be made, of course, for the differences in the gross 

 properties of the enzyme preparations. Similar results were obtained with both 

 pneumococci and hsemolytic streptococci. 



The experiments present new evidence of the formation of hydrogen-peroxide 

 in cultures of hsemolytic streptococci and pneumococci, and it is obvious that the 

 presence or absence of catalase from bacteria has marked effects on their electrode 

 potential behaviour. Some of the differences between G. diphiJiericB and staphylo- 

 cocci on the one hand and h8emol}i:ic streptococci and pneumococci on the other 

 are evidently due to the presence of catalase in the former and its absence from the 

 latter bacteria. It is probable, however, that there are other differences between 

 the reducing activities of the two classes of bacteria. 



Experiments bearing on the subject of bacterial peroxide formation by bacteria 

 are described in a later section on bacterial variants. 



ANAEROBIOSIS 



Given the two postulates : — 



(i) Anaerobes are very sensitive to peroxide which inhibits their growth ; 

 (ii) They produce peroxide when exposed to air, since they do not contain 



catalase ; 

 an explanation is provided of why anaerobes do not proliferate in the presence of 

 air. It is difficult, however, to devise experimental proofs for or against this explana- 

 tion which would be absolutely convincing and free from criticism. 



In any case, the problem of anaerobiosis may be regarded from a different point 

 of view. It has been mentioned that aerobic bacteria such as staphylococci, 

 streptococci, pneumococci or C. diphtheric^ (Hewitt, 1930, 1, 2, 3, 4) reduce the 

 medium in which they are cultivated, so that the original level of electrode potential 

 of the medium (E^==-f 0-3 v. circa) falls to about — 0-15 to — 0.20 volt. When 



