ii8 BACTERIAL ASSOCIATIONS 



bacteria were. Tryptophane, as an example, is most toxic and affects the widest variety 

 of bacteria. Indol, a deaminization product of tryptophane, may account for the tox- 

 icity of tryptophane since it is more toxic than carboUc acid. Serum prevents to a de- 

 gree these inhibitory actions. Others are beneficial, such as taurine, aspartic acid, 

 and alanine. It is readily seen that we have here an additional explanation for certain " 

 of the phenomena being considered. Various bacteria will produce amino acids harm- 

 ful or beneficial to others, and on these products will depend the effect on the asso- 

 ciated bacteria in proportion to the relative sensitivity of the latter. The marked in- 

 hibitory effect of certain proteolytic bacteria on agar plates against the nearby col- 

 onies of other bacteria and the stimulating effect at some distance may well be due 

 to differences in diffusibility of the products formed. There are also, of course, direct 

 actions of proteolytic enzymes on the associated bacteria in certain cases as already 

 suggested, and the lack of agreement in many of the examples quoted above may well 

 be due to different factors having been at work. There is a pressing need to correlate 

 the wealth of available material on the various products formed by many bacteria 

 and the effect of these on associated forms. The work of Koser' and many others on 

 the utilization of the salts of organic acids by bacteria makes clear other groups of 

 phenomena and assists in the explanation of changes in H-ion concentrations as these 

 occur in the reversed reaction of single forms and in bacterial associations. The rel- 

 ative rapidity of growth of two forms, the continual alteration in their activity and 

 sensitivity, the adaptability of bacteria to the form of food material offered at dif- 

 ferent stages and under different conditions as aerobic or anaerobic, the changed 

 metabolism under acid and alkaline influences, and a host of other factors determine 

 the resultant metabolic products of mixed cultures. The analysis of these factors helps 

 in understanding the metabolic processes involved. 



CONCLUSION 



Bacterial association occurs under natural conditions, and it plays an important 

 part in many infections. At one time certain resistant but relatively harmless forms 

 may ward off the body defenses and allow a more sensitive microbe to become es- 

 tablished. At other times the reverse may occur and antagonistic bacteria may, and 

 no doubt frequently do, prevent numerous infections. We have in bacterial associa- 

 tion, then, a means of studying many natural phenomena and, as has been indicated, 

 we touch on many fields of bacteriological study. In the routine bacteriological diag- 

 nosis it must be always in mind that there are antagonisms in our media and methods 

 for preventing them; that pure cultures are essential, and that curious results occur 

 from mixed ones, often quite different from those with either culture alone; that the 

 beneficent associations are to be found and that it must be realized that bacteria com- 

 ing from varying environments may have been under inhibitory or stimulating in- 

 fluences which alter the results obtained in our test tubes. In artificial animal in- 

 jections and in natural human and animal infections and diseases bacterial associa- 

 tions as seen in lowered or raised virulence or pathogenicity; the presence of secondary 

 infections from the animal itself leading to faulty conclusions; the changes in bacterial 

 flora as the conditions alter, as seen in war wounds and intestinal infections; and many 



' Koser, S. A.: /. Bad., 8, 493. 1923. 



