4-Hydroxybenzoic acid, a phenolic derivative of benzoic acid, could inhibit most gram-positive and some gram-negative bacteria, with an IC50 of 160 μg/mL.
Most of the gram-positive and some gram-negative bacteria are sensitive to trans 4-Hydroxycinnamic acid (4-HBA) and 4-Hydroxybenzoic acid at IC50 concentrations of 100-170 and 160 μg/mL, respectively. The antimicrobial activities of 4-Hydroxycinnamic acid and t4-HCA against 11 food pathogenic bacteria, 6 plant pathogenic bacteria, 2 yeasts and 15 plant pathogenic fungi are tested by the paper disc method. These compounds inhibit the growth of most of the bacteria and yeasts at concentrations of 200-400 μg. However, the inhibition is more effective against most of the gram-positive bacteria. When tested by the paper disc method, 4-Hydroxycinnamic acid has stronger antimicrobial activity than t4-HCA against S. aureus, L. mesenteroides, S. cerevisiae and C. albicans at a concentration of 50 μg. However, no inhibitory effect against fungi was observed at concentrations even up to 1000 μg[1].
References:
[1]. Cho JY, Antimicrobial activity of 4-hydroxybenzoic acid and trans 4-hydroxycinnamic acid isolated and identified from rice hull. Biosci Biotechnol Biochem. 1998 Nov;62(11):2273-6.