Adsorption of free fatty acids on cells of certain microorganisms.

Information on the inhibitory effect of free fa t ty acids was sought by growing lactic streptococci in the presence of labeled fa t ty acids and collecting the cells on membrane filters. The distribution of the radioactivity indicated adsorption by the bacteria. As the number of cells in the growth medium increased, accumulation of radioactivity with the cells on the filter increased. The cells accumulated less than one-tenth as much radioactivity from sodium butyrate as from laurie acid. Washing the cells on the filter with water decreased radioactivity. More complete elution of the radioactivity was obtained by filtering the mixture af ter it was made alkaline. Though this implies that the association of fat ty acid and cells is pHdependent, adsorption was always evident in the p H range of normal growth. Gas-liquid chromatographic analysis indicated the fat ty acids were not metabolized. All results indicate weak adsorption at the bacteria: menstruum interface. The parallels of adsorption and inhibition indicate adsorption is associated with the inhibitory process. The presence of free fat ty acids inhibits the growth of laetie streptococci (1, 2, 7, 12). Fa t ty acids of intermediate chain length (capric and laurie) are most inhibitory. A similar inhibitory effect, however, is not apparent with most other bacteria. The mechanism of the inhibition has not been fully explained. Attempts to explain the process of inhibition have followed two main approaches: One understanding of the mechanism of inhibition was derived from the work on the bactericidal effect of the organic acids (3, 9, 10). The bactericidal activity was generally ascribed to the un-ionized fraction of the acids. Monocarboxylic acids were found to increase in activity with an increase in molecular weight. The work, however, was confined to caprylic and acids of shorter chain length. Received for publication October 20, 1966. Published with the approval of the Director as Paper no. 1810, Journal Series, Nebraska Agricultural Experiment Station, Lincoln. Another understanding of the mechanism of inhibition has been based on surface activity effects. As early as 1923, Ayers, Rupp, and Johnson (1) showed lactic streptococci to be inhibited in a medium to which certain materials had been added to depress the surface tension. Tarassuk and Smith (12), when working" with the inhibition of lactic streptococci by fa t ty acids, concluded that surface-tension depression was responsible for the inhibitory effect. They further excluded the possibility of metabolic interferences by concluding that lactic streptococci metabolized fat ty acids. Since surface activity arises prinmrily from the dipo l a r nature of ionized molecules of fat ty acids, inhibition by surface activity might be expected to be by a totally different mechanism than with inhibition by un-ionized molecules. Costilow and Speck (2) showed that inhibition was most pronounced in the presence of fat ty acids of intermediate chain length (eapric and laur ie) ; they attributed this effect to some unknown factor other than surface tension. This path of thinking follows well the explanation of the stimulatory effect of certain fat ty acids on growth of lactobaeilli and the close relation to metabolism (5, 6, 8). More recent work in our laboratory (7) has indicated surface activity at the bacterium: menstruum interface to be of I)rime importance. Surface active agents structurally unrelated to fat ty acids gave simi lar results to fat ty acids; effectiveness was proport ional to the surface-tension depression. In addition, preliminary observations showed that the fa t ty acids aceunmlated with the cells. The fate of these fa t ty acids, however, was not studied. The importance of fat ty acid inhibition of lactic streptococci is recognized in the dairy industI:g in present processes and as a limitation in developing new processes. To obtain additional information on the nature of the inhibition, with the eventual hope of eliminating the problem, observations on the accumulation and fate of fat ty acids were made.