Inhibition of Bacterial Growth by Auxins

We have observed that a number of synthetic unsaturated ring-containing acids endowed with auxin (plant hormone) activity can exert a bacteriostatic effect on the growth of certain bacterial species. The following substances were selected as representatives of different chemical types of auxins: indole-3-acetic acid, β-(indole-3)-propionic acid, -(indole-3)-n-butyrie acid, α-naphthalene-acetic acid, β-naphthoxyacetic acid, 2,4-dichlorophenoxyacetic acid, 2,3,5-triiodoben-zoic acid. 1

The order of activity of these different substances on a few bacterial species is illustrated in Table I in which are given the concentrations of auxins required to cause a 50% inhibition of growth in a medium containing by weight 0.1% enzymatic hydrolysate of casein and 0.02% yeast extract. It should be emphasized that the inhibitory concentrations recorded in Table I have only a relative value since, as will be indicated later, tryptophane and peptone can antagonize the bacteriostatic action of auxins. It is also worth recording that the antibacterial activity of 2 of these substances had been recognized before anything was known of their plant hormone activity. Thus, it was known that indole acids (indole acrylic acid in particular) inhibit the growth of certain Gram-negative bacilli in synthetic media, and that this bacteriostatic effect can be reversed by tryptophane; 2 the growth of mycobacteria in synthetic Long's medium has been shown to be inhibited by 2,3,5-tri-iodobenzoic acid although the same substance stimulates oxygen uptake of these organisms when their metabolism is studied by the conventional Warburg technic. 3

As shown in Table I the bacteriostatic activity of the different auxins varies greatly from one bacterial species to another, each substance appearing to exhibit a selective inhibitory effect against one or several groups of organisms. With the strains and preparations used in our tests, indole-3-acetic acid is the most active substance against the Gram-negative bacilli, whereas mycobacteria are peculiarly susceptible to β-(indole-3)-propionic acid; triiodobenzoic acid exhibits highest activity against certain strains of streptococci.