AEROBIOSIS AND ANAEROBIOSIS 71 



to grow in the presence of oxygen, and to utilize it, depends on the possession 

 of a cytochrome-cytochrome oxidase system. The molecular oxygen may, however, 

 be utilized by other respiratory mechanisms, for some facultative anaerobes have 

 no cytochrome. AVhen the organisms are deprived of oxygen, the predominant 

 metabolic mechanisms change, with a consequent change in the nutritional require- 

 ments of the organism. It was Pasteur's observations on the different metabolic 

 products associated with the aerobic and anaerobic growth of certain bacterial 

 species that led him to place so great an emphasis on the absence of oxygen as 

 a determinant factor in bacterial fermentation. We have already noted some of 

 the changes in nutritional requirements consequent on anaerobic growth. The 

 change may concern energy sources or essential nutrients, or accessory growth 

 factors. Anaerobically, H. infliienzce can dispense with hsematin, since it has 

 no need of hsemochromogen catalysts for oxygen transfer. The pyrimidine base 

 uracil is required as an essential nutrient in addition to thiamin and nicotinic acid 

 when Staph, aureus is grown anaerobically. 



It is clear that the respiratory enzyme systems of bacteria may be adapted 

 to aerobic or anaerobic oxidations. It is much less clear, however, why the strict 

 anaerobes should be incapable of growth in the presence of oxygen. The explana- 

 tion of this inhibitory action of oxygen so far advanced has been made in terms 

 of oxidation-reduction potentials in the medium, or the production by the anaerobes 

 of toxic oxidation products when exposed to air. 



The electrical measurement of reducing intensity developed in a culture of bacteria 

 gives us a positive or negative value for the Eh, in volts. The greater the reducing intensity, 

 the more negative this value. The Eh level in a culture is dependent upon a large number- 

 of factors. Alterations of pH, for example, will alter the concentration of available 

 electrons, and so alter the Eh. Moreover, a particular Eh value cannot be ascribed to 

 a particular oxidation-reduction system in the culture, but may be the resultant of several 

 systems, each of which influences the observed level according to its characteristic E'o, 

 its relative predominance in the culture, its speed of action, and the " poising " effect 

 it has upon the culture as a whole. But though it has proved impossible to sort out 

 all these contributory factors, or to make any valid inferences about Eh levels inside 

 or at the siu-face of bacterial cells themselves, observations upon the Eh changes in culture 

 throw some light upon the conditions governing the growth of aerobic and anaerobic 

 bacteria. For instance, the aerobic growth of Sir. pyogenes in a medium having an initial 

 Eh of + 0-3 volt is accompanied by a gradual fall in potential to about 0-15-0-2 volt. 

 The spores of an anaerobe like CI. tetani will not grow in a similar medium unless the starting 

 Eh is as low as -f- 0-11 volt (Knight and Fildes 1930), though once established, the organism 

 itself reduces the Eh to still lower levels. The limiting potentials that will allow the 

 growth of certain bacterial species have been determined in a number of cases (see McLeod 

 1930, Hewitt 1936, Gillespie and Rettger 1938a, b, Gillespie and Porter 1938, Stephenson 

 1939). It appears that, in general, anaerobic organisms, unhke the aerobes, are unable 

 to reduce ordinary aerobic media to the Eh level at which they can germinate ; though 

 once growth is established, they can maintain it at a low level ; the medium must be 

 partially reduced for the culture of anaerobes. In bacteriological practice, the reduction 

 is usually achieved by the exclusion of molecidar oxygen from the culture. The reduction 

 of oxygen pressure by this means is not, however, a necessary feature of anaerobic culture 

 (see, for example, lOigler and Guggenheim 1938) ; anaerobiosis may be achieved, even 

 though molecular oxygen has access to the medium, by the addition of reducing sub- 

 stances Uke thioglycoUic acid (Quastel and Stephenson 1926), reduced iron (Hastings 

 and McCoy 1932), granules of cooked meat (Lepper and Martin 1929, 1930a, b), or by the 

 concomitant growth of an aerobic organism capable of bringing the Eh down to the 

 required level (see also Reed and Orr 1943). It may be that oxygen per se is inhibitory, 



