The Role of Natural Flavonoid Quercetin in Combating Tet(X3)/Tet(X4)-Positive Bacterial Infections.

The increased occurrence of tetracycline resistance (conferred by the tet(X3)/tet(X4) resistance genes) poses a major challenge to the clinical treatment of infections using tetracyclines, including tigecycline, which is the "last-resort" therapeutic agent. Thus, there is an urgent need to develop inhibitors targeting Tet(X3)/Tet(X4) to address the crisis of tetracycline resistance. This study revealed that quercetin is an inhibitor of Tet(X3)/Tet(X4), which, when combined with different tetracyclines, can exert synergistic growth-inhibitory effects against Tet(X3)/Tet(X4)-positive bacteria (fractional inhibitory concentration < 0.5). Quercetin + tigecycline significantly decreased the biofilm-forming ability of bacteria. Mechanistic investigations revealed that quercetin inhibits the catalytic activity of Tet(X4) against tetracycline antibiotics by binding to the catalytic active site. In the Galleria mellonella larval infection model of Tet(X4)-positive E. coli J53p47EC or Tet(X4)-negative E. coli J53, tigecycline + quercetin enhanced the survival rate, decreased melanization, and suppressed bacterial load. Thus, the findings of this study will enable the development of novel Tet(X3)/Tet(X4)-targeted therapeutic strategies for clinical infections caused by drug-resistant Enterobacteriaceae and Acinetobacter baumannii, especially Tet(X3)/Tet(X4)-positive pathogens.