W. L. HOLMAN 117 



microscopic form, a strict parasite of bacteria, and therefore the most striking example 

 of microbic association. This is discussed in other chapters of this book. 



There have been two main explanations offered in the interpretation of most of 

 the phenomena of bacterial association which I have mentioned. The first is the effect 

 of changes in reaction. Usually one of two organisms in a mixture may produce un- 

 favorable H-ion concentration for the continued metabolism of the other organism, 

 and indeed the latter bacterium may be killed. At other times these reaction changes 

 may only alter the degree or kind of metabolism taking place, or the change may be of 

 benefit to one or both. The other explanation is the production of enzymes of various 

 kinds which directly affect the second organism in a mixture. This is undoubtedly a 

 prime factor in many of the examples I have cited. 



A distinct advance has been made toward other explanations for the facts given 

 above by M'Leod and Gordon' in grouping bacteria under their relative sensitivity 

 to, and power to produce, hydrogen peroxide and a corresponding catalase. Certainly, 

 if we take their table, with due consideration for variations in different strains we shall 

 find a rather satisfactory explanation for many of the antagonistic and beneficial re- 

 sults of bacterial associations. Burnet^ has analyzed many of these relationships, and 

 his contributions have further made clear their application to our study. Bacteria 

 sensitive to hydrogen peroxide will be inhibited by the presence of a strong hydrogen 

 peroxide producer, and an organism with a well developed catalase production will 

 assist another which forms much hydrogen peroxide to which it is itself sensitive. 

 Anaerobes are very sensitive to this substance, and therefore a variety of aerobes 

 producing catalase will benefit them in this respect as well as do those strongly aerobic 

 forms which help the anaerobic conditions, 



A number of workers have stressed the role of carbon dioxide in the inhibition phe- 

 nomena of bacterial cultures. Sierakowski and Zajdel' were able to show, by sealing cul- 

 tures of various bacteria, that the H-ion concentration alone did not account for the 

 growth inhibition, but they believed it due to the retention in the tubes of carbon diox- 

 ide. Valley and Rettger-" showed that increasing amounts of CO2 raise the acidity and 

 lessen the oxygen tension, and that the complete absence of CO2 in the atmosphere 

 stops the growth of many bacteria. Other bacteria such as B. acidophilus were bene- 

 fited by an increase in the atmospheric CO2. I^ have shown that anaerobes will form 

 surface colonies on solid media in experiments in which their own gas production dis- 

 places the fluid medium in inverted tubes. This may be a factor in wound infections. 



A further valuable contribution to our knowledge of these mutual relationships 

 is to be found in the work of Gordon and M'Leod^ on the inhibition of growth by some 

 amino acids. They found that the effect differed markedly as tested on different bac- 

 teria, B. coli and staphylococcus were not at all affected while other more delicate 



' M'Leod, J. W., and Gordon, J.: /. Palh. cr Bad., 26, 326. 1923. 



^ Burnet, F. M.: Aiislnilian J. Exprr. Biol, iy M. Sc, 11, 65, 77. 1925; J. Path, b' Bad., 30, 21. 

 1927. 



^ Sierakowski, S., and Zajdel, R.: Compl. rciid. Sac. dc hioL, 90, 1108. 1924. 



'•'Valley, G., and Rettger, L. F.: Ahstr. Bad., 9,344. 1925. 



sHolman, W. L.: Illinois M. J., 35, 289. 36, 10. 1919. 



^ Gordon, J., and M'Leod, J. W.: /. Path, of Bad., 29, 13. 1926. 



