Synergy between amikacin and Protium heptaphyllum essential oil against polymyxin resistance Klebsiella pneumoniae

We investigated the chemical composition and the in vitro and in vivo antibacterial effects of Protium heptaphyllum essential oil (PHEO) alone and in combination with antibiotics against polymyxin-resistant Klebsiella pneumoniae isolates.Hydrodistillation was used to obtain PHEO, and gas chromatography coupled with mass spectrometry revealed α-pinene, δ-3-carene, and β-pinene as major components present in PHEO. Minimum inhibitory concentration was determined using the broth microdilution technique and ranged from 256 to 512 µg ml-1. The checkerboard method showed synergy with the combination of PHEO and amikacin (AMK) against the polymyxin-resistant K. pneumoniae isolates. In 8 of the 10 isolates tested, the fractional inhibitory concentration indexes (FICIs) ranged from 0.06 to 0.5, while in the remaining two isolates, the combination exerted an additive effect (FICI of 0.6 and 1.0), resulting in AMK dose reduce of range 2- to 16-fold, in the presence of PHEO. Analysis using zero interaction potency revealed high synergy score (63.9). In the in vivo assay, the survival of Caenorhabditis elegans was significantly improved in the presence of PHEO (1 µg ml-1) + AMK (µg ml-1) combination as compared to 32 µg ml-1 AMK alone. Furthermore, PHEO concentrations of 256 and 512 µg ml-1 were found to be non-toxic on the experimental model.To our knowledge, this is the first report of such type of synergism demonstrating an antimicrobial effect against polymyxin-resistant K. pneumoniae isolates.