Colistin-resistant Acinetobacter baumannii (COLR-Ab) is an opportunistic pathogen commonly associated with nosocomial infections, and it is difficult to treat with current antibiotics. Therefore, new antimicrobial agents need to be developed for treatment. Based on this information, we investigated the antimicrobial, antibiofilm, and combination activities of p-coumaric acid (p-CA), ferulic acid (FA), and p-methoxycinnamic acid (p-MCA) against five COLR-Ab isolates. p-CA, FA, and p-MCA exhibited antimicrobial activity against COLR-Ab isolates, with minimum inhibitory concentration (MIC) values in the range of 256-128 µg/mL, 1024-512 µg/mL, and 512-128 µg/mL, respectively. The combination effects of the compounds with colistin (COL) were evaluated using a checkerboard synergy test. The combinations exhibited a synergistic effect and caused a 128- to 16-fold decrease in COL MIC values. In addition, the biofilm production capacities of the COLR-Ab isolates and the antibiofilm activities of the compounds were determined using the microtitre plate-based crystal violet (CV) technique. The compounds showed effective antibiofilm activity against strong and moderate biofilm-producing isolates, inhibiting biofilm formation by 77.5% and 19.7%. Spectrometric measurements were used to examine the effect of compounds on membrane permeability; 1.9-, 1.66-, and 1.34-fold increases in absorbance values were observed at MIC concentrations of p-CA, FA, and p-MCA, respectively. Furthermore, morphological changes caused by the compounds in the isolate were observed using scanning electron microscopy (SEM) micrographs. According to the WST assay, the compounds did not show any statistically significant cytotoxic effect on the cells (p > 0.05). These findings indicate that p-CA, FA, and p-MCA may be potential new alternative candidates against resistant A. baumannii.

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