Background and purpose: Antibiotic combinations are used to enhance antibacterial efficacy and to prevent the development of resistance. In this study, the in vitro activities of antibiotic and phytochemical combinations against Pseudomonas aeruginosa were tested by the fractional inhibitory concentration method, derived from the minimal inhibitory concentrations (MICs) of the agents in combination.
Methods: The antimicrobial activity of phytochemicals, alone and in combination with antibiotics, was evaluated using the checkerboard assay and time-kill curve methods.
Results: There was synergism between gentamicin and caffeic acid, and sulfadiazine and the 3 phytochemicals under investigation (protocatechuic acid, quercetin, caffeic acid). The MIC of sulfadiazine was 256 microg/mL, and of gentamicin was 2 microg/mL. When gentamicin was combined with one-quarter the MIC of caffeic acid, the MIC of gentamicin was reduced 4-fold. When sulfadiazine was tested with one-quarter the MIC of protocatechuic acid, quercetin, and caffeic acid, the MIC was reduced 4-fold in combination with each of the drugs.
Conclusions: These results indicate the potential efficacy of phytochemicals in combination with antibiotics for enhancing total biological activity.