M. H. SOULE 261 



tion of the test cultures in air permitted excellent growth. However, Adams found 

 later that actinomyces and mycetoma were actually killed by exposure to the high 

 tensions of oxygen. 



Recently Karsner, Brittingham, and Richardson' streaked the surfaces of agar 

 plates with organisms suspended in salt solution. The plates were then stacked in a 

 jar, the atmosphere of which was replaced with high oxygen tensions at normal baro- 

 metric pressure. The growth of B. proteus, hemolytic streptococci, B. typhosus, and 

 B. mucosus capsulatus, four of the nine cultures tested by this method, was inhibited 

 by concentrations of over 80 per cent. Novy and Soule^ found that the human and 

 bovine strains of the tubercle bacillus were stimulated by tensions of 40-60 per cent 

 oxygen, but concentrations of 80 per cent or higher gave partial inhibition. No gas 

 other than CO, was produced under these conditions, and the respiratory quotients 

 were identical with the values given under ordinary air tensions. The observation 

 was made by Adams,-' and confirmed by Novy and Soule, that the colonies of the 

 tubercle bacillus developing in concentrations of oxygen above 80 per cent were iso- 

 lated and heaped up. It appeared that relatively few organisms were able to grow 

 under this extreme condition. The limiting tension for the growth of L. tropica'^ and 

 Tr. lewisi was found to be in the neighborhood of 60 per cent. Cleveland^ subjected 

 several species of parasitic and free living protozoa to atmospheres of pure oxygen 

 under high pressure, and found that exposure for forty minutes was usually sufficient 

 to kill the organisms. 



In making tests to determine the inhibiting tensions of oxygen it is of course nec- 

 essary to have a large volume of the gas available at the experimental pressure. This 

 will make it impossible for the initial growth of a few resistant cells to lower materi- 

 ally the tension to a concentration that will be easily tolerated by the other organisms 

 planted, and thus lead to erroneous conclusions. Conceivably, the initial growth in a 

 small container may result in the production of sufficient CO2 to counteract the in- 

 hibiting action of the remaining oxygen. 



Just how high tensions of oxygen inhibit the growth of organisms it is not possible 

 to state. It may be supposed that the oxidative changes within the cells are increased 

 to a point at which the vitality of the organism becomes exhausted. This may imply 

 inhibition or even destruction of a respiratory enzyme; or, it may mean the formation 

 of oxidative products which are directly injurious to the cell. 



Of more interest, perhaps, than the action of bacteria under increased oxygen 

 tensions is the growth of organisms under conditions where the oxygen pressure is 

 below that normally found in the air.^ 



In order to answer the question as to what the effect of decreased oxygen tension 

 may be on a given organism, it is obviously necessary to expose the organisms to 

 known concentrations of this gas. This can be effected by placing the inoculated agar 



' Karsner, H. T., Brittingham, H. H., and Richardson, M. L.: J. Med. Research, 44, 83-88. 1923. 



= Novy, F. G., and Soule, M. H.: ./. lufecl. Dis., 36, 168-232. 1925. 



3 Adams, A.: loc. cit. " Soule, M. H.: J. Infect. Dis., 36, 245-30S. 1925. 



s Cleveland, L. R.: Biol. Bull., 48, 455-68. 1925. 



* For a discussion of anaerobiosis, cf. chap, xiii in this volume. 



