Antibacterial activities of phenolic benzaldehydes and benzoic acids against Campylobacter jejuni, Escherichia coli, Listeria monocytogenes, and Salmonella enterica

We evaluated the bactericidal activities of 35 benzaldehydes, 34 benzoic acids, and I benzoic acid methyl ester against Campylobacter jejuni, Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella enterica when these compounds were substituted on the benzene ring with 0, I, 2, or 3 hydroxy (OH) and/or methoxy (OCH3) groups in a pH 7.0 buffer. Dose-response plots were used to determine the percentage of the sample that induced a 50% decrease in CFU after 60 min (BA(50)). Of the 70 compounds tested, 24 were found to be active against all four pathogens, and additional 4, 10, and 12 were found to be active against three, two, and one of the pathogens, respectively. C. jejuni was similar to100 times as sensitive as the other three pathogens. The 10 compounds that were most active against the four pathogens (with average BA(50) values ranging from 0.026 to 0.166) and are candidates for studies of activity in foods or for disinfections were 2,4,6-trihydroxybenzaldehyde, 2,5-dihydroxybenzaldehyde, 2,3,4-trihydroxybenzaldehyde, 2-hydroxy-5-methoxybenzaldehyde, 2,3-dihydroxybenzaldehyde, 2-hydroxy-3-methoxybenzaldehyde, 4-hydroxy-2,6-dimethoxybenzaldehyde, 2,5-dihydroxybenzaldehyde, 2,4-dihydroxybenzaldehyde, and 2-hydroxybenzaldehyde. Comparison of the chemical structures of the test compounds and their activities revealed that (i) the aldehyde (CHO) group was more active than the carboxyl (COOH) group whether or not OH groups were present; (ii) compounds were most active with trisubstituted OH > disubstituted OH > monosubstituted OH; (iii) for disubstituted derivatives, 2-OH enhanced activities were exhibited by benzaldehyde but not by benzoic acid; (iv) compounds were more active with OH than with OCH3, irrespective of the position of substitution on the benzene ring; (v) compounds with mixed OH and OCH3 groups exhibited variable results, i.e., in some cases OCH3 groups enhanced activity and in other cases they did not; (vi) methoxybenzoic acids were largely inactive; and (vii) gallic acid was 20 times as active against S. enterica at pH 7.0 as it was at pH 3.7, suggesting that the ionization of its OH groups may enhance bactericidal activity.