Associative Bacterial Action in the Propionic Acid Fermentation

Among the most interesting problems in bacteriology are those that have to do with the mutual relations of the organisms under natura l conditions. Numerous notations of effects, both beneficial and detrimental, of on e organism upon another are to be found in bacteriological literature. The beneficial action on a specific organi s m of the associative growth of another organism is well illustrated by such well known examples as the helpful effect on the tetanus bacillus of associated aerobes; and the increased vigor of the lactic acid fermentation when conducted in impure culture, which has been studied extensively by Marshall and his associates. Antagonisms, aside from those which may be readily explained upon a purely physidal or chemical basis, have been noted in many connections. An example of this nature is the recent report of Smith and Smith 1 on the inhibiting action of the paratyphoid organism upon the gas production of Bacterium coli. In the present paper we wish to call attention to a beneficial associative effect of greater magnitude than is frequently observed, and which involves the little studied propionic acid fermentation. The full details of this work will be included in a forthcoming paper on the propionic acid fermentation of lactose. The propionic acid-producing organism used in these experiments was of the type which has been tentatively designated as Bacterium acidi propionici (d) and which has been shown to be the essential organism for the production of "eyes" and the characteristic flavor in Swiss cheese3 The other organisms employed were Streptococcus