Reversal of tetracycline resistance by clove and peony extracts in a multi-drug resistant Escherichia coli

Infections with Gram-negative bacteria that are resistant to multiple antibiotics are increasingly prevalent and challenging to treat. We sought to identify compounds with potential to reverse resistance to tetracycline and tobramycin in a multi-drug resistant isolate of Escherichia coli (NCTC 13400). A screen of 800 extracts of traditional herbs and medicines revealed that polar extracts of cloves (Syzygium aromaticum) and Peony flowers (Paeonia lactiflora) significantly increased sensitivity to both antibiotics in this strain. Fractionation of clove and peony extracts by high performance liquid chromatography revealed activity within the fractions comprising mainly phenolic acids and flavonoids, respectively. Sampling candidate compounds from these fractions revealed that while no tested compound enhanced the activity of tobramycin, myricetin significantly increased the sensitivity of this strain to tetracycline. Myricetin alone inhibited growth, but was not bactericidal and did not induce membrane permeability or inhibit Nile red efflux. In vitro MTT assays using mammalian HEK-293 cells revealed no significant toxicity of myricetin, or most of the other tested compounds, at up to 100 µM. The results suggest that the screening of plant secondary metabolites has potential for the identification of antibiotic resistance reversers with potentially favourable in vitro toxicity profiles.