104 OXIDATION-REDUCTION POTENTIALS 



This oxygen deficit at the time of active growth accounts for the success, for 

 example, of aerated submerged cultures in the production of penicilHn, diphtheria 

 toxin and streptomycin. Formerly, all these products of micro-organisms were 

 obtained on a large scale by incubating shallow layers with its growth of organisms at 

 the surface with maximal access of air. Frequently incubation had to continue for 

 7-14 days for the completion of the process, but in modern submerged culture plants, 

 rapid production in 1 or 2 days is secured in cultures receiving an abundant air supply 

 by a forced pressure feed of air. The oxygen demand of sewage is another example of 

 the same phenomenon. 



There should, however, be a word of warning here and that is the danger of 

 generalisation. Bacteria vary enormously in the enzyme systems they possess and 

 hence in their reaction to oxygen supplies and in the oxidation-reduction potentials 

 developed in their cultures under different conditions. As will be seen, an abundant 

 oxygen supply at the wrong time may stop the growth of anaerobes ; and lead to 

 sterihsation of cultures through peroxide formation in organisms deficient in catalase. 



Again, it must be emphasised that reducing conditions per se are not the essentials 

 of a medium suitable for bacterial growth, although, of course, growth may be 

 impossible when oxidation-reduction conditions are unsuitable. To consider an 

 analogous case, growth of bacteria is impossible in too acid a medium, but mere 

 adjustment of any fluid to a neutral reaction does not constitute a satisfactory 

 culture medium. There has been a tendency rather to over-emphasise the importance 

 of reducing conditions in the initiation of bacterial growth and to attribute reducing 

 properties to growth adjuvants on rather slender evidence. In the case of the aerobic 

 organism M. lysodeikticus better growth and actually more highly reducing conditions 

 were observed when the oxygen supply was increased by aerating the cultures 

 (Hewitt, 1931). 



In the presence of an abundant oxygen supply, the free oxygen available may be 

 sufficient for preliminary oxidation processes, but when more anaerobic conditions 

 are established other means of oxidation must be available ; hydrogen acceptors 

 must be present, which in the presence of cell enzymes oxidise nutrient materials by 

 removing hydrogen from them. Anaerobic growth, therefore, requires a more 

 carefully constituted medium than aerobic cultures. Hence it is frequently difficult 

 to obtain luxuriant growth of certain organisms anaerobically in a culture medium 

 which is quite suitable for aerobic growth. It is often observed that anaerobic 

 cultures are more successful in the presence of glucose. This has been ascribed to 

 the reducing properties of glucose, but a more probable explanation is that glucose 

 may be fermented anaerobically, and the energy liberated in this reaction is available 

 to the organisms, although it is only a fraction of the energy made available in 

 aerobic oxidation. 



Relatively little was known of the oxidation-reduction equilibria in bacterial 

 cultures, and the study of the electrode potentials developed in cultures under 

 different conditions has provided a most promising method of studying bacteria, and 

 the results obtained are proving of value in the case of tissue and otlier cells. 



Reducing conditions are established in bacterial cultures during growth owing 

 to the metabolism of the organisms as mentioned above. The organisms use oxygen, 

 oxygen donators and hydrogen acceptors to effect the oxidation of nutrient materials 



