Synergistic and antibiofilm activity of colistin combined with cinnamaldehyde and eugenol against colistin-resistant Acinetobacter baumannii clinical isolates.

OBJECTIVE

This study aimed to evaluate the effectiveness of combinations of colistin (COL) with cinnamaldehyde (CIN) or eugenol (EUG) against planktonic and biofilm forms of colistin-resistant clinical Acinetobacter baumannii isolates.

METHODS

Minimum inhibitory concentration (MIC) values of COL and essential oil components in 10 colistin-resistant clinical isolates were evaluated by the broth microdilution method. Interactions were classified by the fractional inhibitory concentration index (ΣFICI) using the checkerboard method. Biofilm formation was detected using the crystal violet method and the inhibitory effects of selected combination ratios on biofilm formation were evaluated in isolates showing synergy. In addition, the time-dependent effects of synergy combinations on 48-h pre-formed biofilm biomass were examined.

RESULTS

Median MIC values of COL, CIN, and EUG were 8 (4-32), 128 (128-128), and 512 (256-1024) µg/mL, respectively. In combination, the MIC of COL decreased to 1 (0.5-16) µg/mL with CIN and to 2 (0.5-8) µg/mL with EUG (p = 0.002). Synergy was observed in half of the isolates in both combinations (median ΣFICI: 0.532 and 0.469). Three isolates formed strong biofilms and seven formed moderate biofilms. Biofilm inhibition ranged from 1.1% to 50.5% for COL-CIN combination and from 11.7% to 80.0% for the COL-EUG combination. In 48-h pre-formed biofilms, the residual biomass at 24 h was significantly lower than that at 1 h for both combinations (p < 0.05).

CONCLUSIONS

COL-CIN and COL-EUG combinations reduced MICs in colistin-resistant A. baumannii and decreased both biofilm formation and 48-h pre-formed biofilm biomass. These findings are promising for combination-based antibiofilm strategies.