Antibacterial and antibiofilm effects of natural phenolic acids: Insights into structure-function relationships, extracellular pH, and morphological changes

Wound infections are a challenging healthcare burden against which treatment is hindered by the presence of biofilms and the rise in antibacterial resistance. To date, the potential of natural phenolic acids as topical antibacterial agents is under-researched. The aim of this study was to investigate and compare the antibacterial and antibiofilm effects of 5 hydroxycinnamic acids (caffeic acid, chlorogenic acid, p-coumaric acid, ferulic acid, and rosmarinic acid), 4 hydroxybenzoic acids (gallic acid, salicylic acid, syringic acid, and vanillic acid), as well as 2 related compounds (tannic acid and quinic acid) against a panel of 10 bacterial strains. The tested phenolic acids showed a bactericidal action with minimum inhibitory concentration (MIC) values in the range of 0.625 to 10.0 mg/mL. A negative correlation between an increase in the polarity of the molecule and antibacterial effect was observed. At minimum bactericidal concentration (MBC), phenolic acids prevent an increase in extracellular pH by halting bacterial metabolism. Under bacterial metabolism-induced alkaline conditions, caffeic, chlorogenic, rosmarinic, and gallic acids change color to brown or green at sub-MIC concentrations. Caffeic and chlorogenic acids induce filamentation of planktonic P. aeruginosa. Tested phenolic compounds reduce biofilm biomass in the range of 10% to 90%, with Gram-positive biofilms being more susceptible. SEM imaging revealed a significantly diminished biofilm EPS with damaged cell structure after phenolic acid treatment. Out of the tested compounds, p-coumaric acid and salicylic acid show the most promise as antibacterial agents against bacterial species characteristic of wound infections.

IMPORTANCE: The search for new antibacterial agents is one of the highest healthcare priorities. Phenolic acids are increasingly researched alone and as part of various drug delivery systems for wound care applications. However, to date, the available literature on the antibacterial effects of these compounds is rather fragmented and outdated, with unreproducible results. In this study, we link the physicochemical properties and structure of the molecules with their antibacterial and antibiofilm potential, investigate phenolic acid's impact on the bacterial metabolism-induced extracellular pH change, and use SEM imaging to characterize the morphological changes of both planktonic bacteria and biofilms in response to phenolic acid treatment. Additionally, we provide an assessment of the compound ADMET properties with respect to their potential topical application. The data can be used in the future to test potential synergic effects with antibiotics, develop drug delivery systems, and investigate the efficacy of these agents against multi-resistant bacterial strains.