Activity of p-aminobenzoic acid compared with other organic acids against selected bacteria

The antibacterial activity of p-aminobenzoic acid against Listeria monocytogenes, Salmonella enteritidis and Escherichia coli was compared with the activity of commonly used acidulants: formic, propionic, acetic, lactic and citric acids. Viable count evaluations and MIC determinations indicated that p-aminobenzoic acid caused greater inhibitory effects than the other organic acids. The activity of p-aminobenzoic acid on the growth of the test organisms at selected pH values indicated that p-aminobenzoic acid was more active at low pH than at high pH. Uptake studies showed that the uptake of p-aminobenzoic acid by E. coli was markedly decreased as the pH values increased. Electron micrographs of E. coli cells grown in the presence of p-aminobenzoic acid indicate that p-aminobenzoic acid caused marked damage to the cell envelope. It is suggested that p-aminobenzoic acid has at least two mechanisms of action: one mechanism in common with other organic acids and the other mechanism by interfering with the synthesis of the peptidoglycan layer by an action on the dihydrofolate reductase enzyme.