Antimicrobial synergy between mRNA- and protein-level inhibitors

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Abstract
Background: The few available distinct classes of antimicrobials limits the scope for single and combination drug treatment of resistant infections. Objective: To evaluate antimicrobial effectiveness from combinations of protein-specific drugs and mRNA-specific antisense inhibitors. Methods: Interactions between conventional antimicrobial drugs and mRNA-specific translation inhibiting antisense peptide nucleic acids were assessed in Escherichia coli and Staphylococcus aureus cultures using pairwise combinations in a chequerboard arrangement. Fractional inhibitory concentration indices (FICIs) were calculated and grouped according to the functional relationship between the inhibitor targets. Antisense specificity controls included different antisense sequences targeting the same mRNA, as well as biochemical quantification of active protein expressed from the essential fabI gene and from the lacZ reporter gene after single and combined inhibitor treatment. Results: FICIs were higher for inhibitor combinations with unrelated targets than for combinations with functionally related targets. Inhibitor combinations with shared genetic targets displayed the lowest FICIs, with several qualifying for the conservative definition of antimicrobial synergy (FICI ≤ 0.5). Furthermore, low FICIs arise as the hyperbolic dose–response curves for each separate inhibitor are maintained in combination. Conclusion: Interactions between mRNA- and protein-level inhibitors with the same genetic target can be synergistic and may provide a strategy to improve antimicrobial efficacy, facilitate drug mechanism of action studies and aid the search for new antimicrobials.