PEROXIDE FORMATION 



113 



An experiment was performed bearing on the sensitivity of organisms to per- 

 oxide. Hydrogen-peroxide disappears after a few hours when incubated in low 

 concentration with some samples of tryptic digest broth. A 0-006 per cent, solution 

 of hydrogen-peroxide in digest broth (8 c.c.) was inoculated with 0-1 c.c. of a 20-hour 

 broth culture of heemolytic streptococci. When the culture was incubated, the 

 potential (Fig. 24) fell gradually from its initial value of -(-0-48 volt and after 18 hours 

 had reached the value of Ei,+ 0-3 volt, which is the approximate level usually 

 obtaining in laboratory stocks of broth. From this time the potential-time curve 

 was similar to that of digest broth inoculated in the ordinary way with hsemolytic 

 streptococci. The incubation with hydrogen-peroxide had had no appreciable effect 

 on the behaviour of the organisms. Quastel and Stephenson (1926) found that 

 incubation with hydrogen-peroxide did not interfere with the ability of the anaerobe 

 B. sporogenes to proliferate when reducing conditions in the medium were estab- 

 lished. This suggests that the toxic effect of peroxide is not responsible for the 



Aerated Broth 

 +•4 ^ 



+•3 



+ 2 - 



o 



Aerobic Catalase Broth 



24 



Time (hours) 

 Fig. 25 

 Pneumococcus cultures 



failure of anaerobes to grow aerobically, but McLeod (1930) criticises their experiments 

 on a number of grounds. It would seem, therefore, that the question concerning 

 the essential nature of peroxide formation in inhibiting the aerobic growth of anaerobic 

 organisms must remain open (see also Johnstone, 1940 ; Kliger and Guggenheim, 

 1938 ; Holland, 1944 ; Reed and Orr, 1943). 



Further observations (Hewitt, 1931) on peroxide formation by pneumococci 

 indicate the important effect exerted by catalase on the electrode potential behaviour 

 of bacterial cultures. When an active catalase preparation was added to an aerobic 

 pneumococcus culture, the potential fell to the same level as in plain broth cultures, 

 but after the logarithmic phase of growth a marked difference was seen. In plain 

 broth cultures, the potential commences to rise after the cessation of active bacterial 

 proliferation, but in cultures containing catalase the potential remained at a low 

 level long after the cessation of active growth (see fig. 25). The rise in potential 

 after growth is apparently associated with peroxide formation, although no peroxide 



