Antibacterial activity of selected naturally occurring hydroxycinnamic acids

The hydroxycinnamic acids are naturally occurring phenolic compounds commonly found in most plants. They have been shown to possess lipid antioxidant properties, in addition to antifungal, antiviral, and antibacterial activities. The reports on the antibacterial activity have been conflicting. The purpose of this study was to further investigate the antibacterial effects of caffeic, p-coumaric, and ferulic acids, the three most common hydroxycinnamic acids, against a variety of gram-positive and gram-negative organisms. Bacillus cereus, Escherichia coli, Pseudomonas fluorescens, and Staphylococcus aureus were grown in Trypticase Soy Broth (TSB) at optimum incubation temperatures for 9 or 48 hours at pH's of 5.0, 6.0, 6.5, or 7.0 depending upon the organism. Growth was measured by plate counts in Trypticase Soy Agar (TSA). Limited inhibition of P. fluorescens with the compounds was found at pH 7.0, 6.5, and 6.0. Growth of E. coli was inhibited by the hydroxycinnamic acids at pH 6.0 and 5.0, but not at 7.0. Results suggested that inhibition was inversely related to the pH. The hydroxycinnamic acids were inhibitory to S. aureus at pH 7.0, 6.0, and 5.0 with the level of inhibition increasing at pH 5.0. The most effective acid was p-coumaric acid which exhibited lethal activity at pH 5.0. Bacillus cereus was sensitive to all of the acids, and a lethal effect was observed with p-coumaric acid. Higher concentrations of ferulic acid had increased activity while 1000 ppm of p-coumaric acid had no significant increase in activity over 500 ppm. Injury of S. aureus with the test compounds was monitored in TSA and Staphylococcus #110 medium. The most injury occurred during the lag phase of growth with 500 ppm p-coumaric acid at pH 7.0 and 6.0. No significant injury over time was observed with the other compounds. Escherichia coli was grown in a glucose salts medium containing a ¹⁴C-labeled amino acid mixture. Leakage of incorporated ¹⁴C-labeled compounds after exposure to 1000 ppm p-coumaric acid was detected with liquid scintillation counting. There was a maximum loss of 3.92% of incorporated ¹⁴C-labeled compounds, expressed as a percentage of the counts per min (cpm) of the whole cells, which suggested inhibition of E. coli by p-coumaric acid was at least partially due to leakage.