Bacteriostatic action of sorbic acid: a hypothesis induction of a stringent-type response in putrefactive anaerobe 3679 by the effects of the protonophoric activity of sorbic acid

Abstract

The inhibitory effect of potassium sorbate on bioenergetics, amino acid uptake, protein synthesis, cell regulation, and morphology was examined in Putrefactive Anaerobe (PA) 3679. Undissociated sorbic acid appeared to act as a protonophore, lowering the intracellular pH and dissipating the protonmotive force of the membrane. Sorbate and benzoate inhibited uptake of the amino acid, phenylalanine. Further studies on the effects of sorbate on PA 3679 revealed altered patterns of phosphorylated nucleotide accumulation with increased rates of formation of GTP, ppGpp, and an unidentified compound, possibly pppGpp, and a decreased rate of protein synthesis. The addition of a noninhibitory amount of tetracycline released the sorbate-induced inhibition of growth in PA 3679. Based on these results, it is concluded that inhibition of PA 3679 by sorbate resulted from a stringent-type response to the protonophoric activity of sorbic acid. Similar morphological changes were observed when PA 3679 was treated with sorbate or inorganic acid, HCl. In both cases, the cells became filamentous with bends and bulges and had thickened cell walls. Sorbate-induced filaments had fewer septa than did the acid-treated cells. In contrast nitrite-treated cells also became filamentous with bends, but the cells had many septa, did not have thickened cell walls, and did not bulge. Low concentrations of sucrose and sodium chloride antagonized the inhibitory effects of sorbate on the growth of PA 3679; however, a low concentration of nitrite acted synergistically with sorbate. Nitrite alone increased the intracellular pH and protonmotive force in untreated as well as sorbate-treated cells.