JAC Advance Access originally published online on May 24, 2005
Journal of Antimicrobial Chemotherapy 2005 56(1):97-103; doi:10.1093/jac/dki173
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Antimicrobial synergy between mRNA- and protein-level inhibitors
Center for Genomics and Bioinformatics, Karolinska Institutet, Berzelius väg 35, 171 77, Stockholm, Sweden
Received 9 March 2005; returned 12 April 2005; revised 13 April 2005; accepted 19 April 2005
* Corresponding author. Tel: +46-8-524-87934. Fax: +46-8-323950; E-mail: rikard.dryselius{at}cgb.ki.se
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.
Keywords: antimicrobial treatment , antisense , chequerboard , inhibitor combinations , PNA
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