Preliminary Study on the Antibacterial Activity of Essential Oils Alone and in Combination with Gentamicin Against Extended-Spectrum β-Lactamase-Producing and New Delhi Metallo-β-Lactamase-1-Producing Klebsiella pneumoniae Isolates

Aim: The aim of the study was to investigate possible synergistic effects between several selected, commercially available essential oils and gentamicin against extended-spectrum -lactamase (ESBL)-producing and New Delhi metallo--lactamase-1 (NDM-1)-producing Klebsiella pneumoniae isolates. Materials and Methods: ESBLs production was confirmed by double-disk synergy test. Isolates positive for bla(NDM-1) gene were found among the tested strains. K. pneumoniae ATCC((R)) BAA-1705 strain was used as a control. The checkerboard method was applied to assess the synergistic and additive action of nine essential oils: caraway, fennel, peppermint, geranium, basil, clove, thyme, clary sage, and lavender, respectively, in combination with gentamicin. Results: Our results indicated that peppermint oil combined with gentamicin showed synergistic activity against both control, ESBL-producing and NDM-1-producing isolates. Caraway essential oil demonstrated synergy with gentamicin toward ESBL-producing and additionally gentamicin-resistant strains. The additive effect was observed for gentamicin combined with thyme, fennel, basil, and clary sage. Conclusions: Because of their synergistic activity with gentamicin, peppermint, and caraway oils in particular, can be considered as an alternative or an addition for the control of infections with limited therapeutic options due to multidrug resistance.