Synergistic effects of ethnomedicinal plants of Apocynaceae family and antibiotics against clinical isolates of Acinetobacter baumannii

Objective: To investigate the efficacy of 17 ethnomedicinal plants belonging to Apocynaceae family used in combination with 16 conventional antibiotics against non multidrug resistant, multidrug resistant (MDR), and extensive drug resistant (XDR) Acinetobacter baumannii A. baumannii).Methods: Antibacterial activity and resistance modifying ability of 272 combinations were determined by growth inhibition assays and further confirmed by time kill assay. Results: Among the combinations of the antibiotics with Apocynaceae ethanol extracts on this pathogen, 15 (5%) had synergistic effects, 23 (8%) had partial synergistic effects and 234 (86%) had no effects. Synergistic activity was observed mostly when the Apocynaceae extracts were combined with rifampicin or cefazolin. Interestingly, 10 out of 17 combinations between the extracts and rifampicin displayed synergistic or partial synergistic behaviors. Holarrhena antidysenterica extract was additionally tested to restore rifampicin activity against clinical isolates of MDR and XDR A. baumannii. With respect to total or partial synergy, 70% was XDR A. baumannii isolates and 66% was MDR A. baumannii isolates. Conclusions: Holarrhena antidysenterica extract clearly demonstrated the ability to restore rifampicin activity against both A. baumannii, ATCC19606 and clinically isolated A. baumannii. Additional studies examining its active principles as well as mechanisms of actions such as the effects on efflux pumps and outer membrane permeability alterations are recommended.