RESPIRATORY METABOLISM 391 



1. Oxygen is directly lethal to the cell. 



2. Anaerobes do not contain catalase and therefore are incapable of 

 destroying the toxic HoO^ which is formed by reduction of oxygen (see 

 equations given above). 



3. Growth of anaerobes is dependent upon the presence of a low 

 oxidation-reduction potential in the medium, the attainment of which 

 is prevented by oxygen. 



4. Oo forms a loose chemical complex with the respiratory system of 

 obligatory anaerobes, and thereby inhibits its activity. 



The relative merits of these theories, as applied to bacteria, are dis- 

 cussed by Hewitt (1936) and Broh-Kahn and Mirsky (1938). The first 

 theory is certainly not true for those anaerobic organisms which will 

 grow under anaerobic conditions after exposure to oxygen. The second 

 theory is supported by considerable evidence, in that most anaerobes do 

 not contain catalase, and in that some bacteria (e.g., pneumococci) will 

 grow aerobically until they are killed by the accumulation of HgOo re- 

 sulting from their metabolism (so-called "suicide" of cultures). How- 

 ever, some anaerobes do contain catalase, and apparently it has not been 

 definitely demonstrated that strict anaerobes consume O, in order to 

 produce HgO,, or even that obligatory anaerobes do produce H.Og (Broh- 

 Kahn and Mirsky). The theory, however, might still be applicable to 

 organisms such as penumococci and hemolytic streptococci, and to 

 Escherichia coli in the presence of HCN and methylene blue. In these 

 cases appreciable amounts of HgO, can be detected. 



Among the Protozoa we have very little evidence of the relative merits 

 of the first two of these theories. It is shown by the work of Cleveland 

 on termites and on xylophagous cockroaches (Cleveland, Hall, Sanders, 

 and Collier, 1934, include citations of earlier papers) that the symbiotic 

 Protozoa which inhabit the digestive tracts of these organisms are prob- 

 ably strict anaerobes. At least the O2 tension of their normal environment 

 is extremely low, and they are rapidly killed by appreciable quantities 

 of molecular O.,. Cleveland found that at 23° C. the time necessary for 

 death of the symbionts of termites was an inverse function of oxygen 

 tension (e.g., in Termopsis, all Protozoa were dead in 72 hours at one 

 atmosphere, in 30 minutes at 3.5 atmospheres). This is apparently due 

 to an increase in Oo concentration in the digestive tract, with increased 

 O2 pressure in the atmosphere, and this could easily be explained on the 



