116 OXIDATION-REDUCTION POTENTIALS 



the medium to a level of potential of —0-4 v. or even lower. The preliminary 

 reduction of the medium may be effected by various methods, e.g., by : — 



(1) Boiling off the air. 



(2) Using 0-2% agar gives a " sloppy " consistency preventing the diffusion of 

 air back into the medium after expulsion of air. 



(3) Sealing the medium with a vaseline seal to exclude air after de-aeration. 



(4) Sealing with alkaline pyrogallol which absorbs oxygen. 



(5) Including a strip of iron which absorbs oxygen when it rusts. 



(6) Adding a reducing agent such as 0-1 '^,{, thioglycollic acid. Brewer's medium 

 consists of semi- " sloppy " agar ; it is boiled to expel air and thioglycollate 

 is added. By using such media strict anaerobes can multiply even under 

 conditions of ordinary aerobic incubation. 



(7) Growing an aerobic organism to lower the oxidation-reduction potential. 

 This method provides the explanation of symbiotic aerobic growth of 

 anaerobes in the presence of an aerobe. 



(8) Eemoving air by burning hydrogen and incubating in a hydrogen atmo- 

 sphere in a Mcintosh and Fildes jar. 



(9) Some anaerobes when inoculated heavily in aerobic media are able to 

 multiply in ordinary culture media. 



(10) Using meat media containing shreds of meat which de-oxygenate the 

 medium by mechanisms discussed in Chapter III. 



Limiting oxidation-reduction potentials for the growth of anaerobic bacteria are 

 discussed by Aubel, Rosenberg and Grunberg (1946). 



In fig. 27 are plotted the reducing activities at different En levels of a hypo- 

 thetical aerobe and obligate anaerobe. The range of reducing activities of the aerobe 

 is from about -f 0-4 v. to — 0-2 v., whilst that of the anaerobe is about + 0-05 to 

 — 0-4 V. Probably no hard and fast line can be drawn between " aerobes " and 

 " anaerobes," a continuous gradation of properties being observed. In media 

 exposed to general contamination from the air, one organism after another proliferates 

 freely as the Ej, level is depressed and reaches the optimum level for the reducing 

 activities of the different bacteria. This description is intended to give a clear picture 

 of the difference in behaviour of different organisms, without prejudice to the problem 

 of peroxide formation, and the definition of the basis of these differences is the next 

 important point. A reasonable explanation would seem to be that the reducing 

 powers of bacteria depend on the enzymes they are able to produce. Aerobic 

 organisms, such as staphylococci, are able to effect reversible oxidation-reduction 

 processes at a high level (oxidising level) of E^, but not at low levels ; whilst tetanus 

 is able to effect reductions at low E^ as its enzymes are able to catalyse reactions 

 which occur only under intense reducing conditions. A crude analogy might be made 

 to vacuum pumps — an ordinary water pump is analogous to the aerobes and can 

 reduce the pressure efficiently from 760 mm. to 20 mm. Mercury pumps are almost 

 useless when the pressure is 760 mm., but they are able to work efiiciently where the 

 water pump leaves off and reduce the pressure to 0-001 mm. 



Many biological processes for obtaining energy probably consist in large measure 

 of reactions of the type 



enzyme 

 AHg + B ^==^ A -f BH2 



