In vitro activity of tobramycin, amilorides and other non-antibiotics against Pseudomonas aeruginosa and Burkholderia cenocepacia

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Chronic respiratory infection, mainly caused by Pseudomonas aeruginosa and Burkholderia cepacia complex, is the major cause of complications and eventually of death in patients with cystic fibrosis. These problems are exacerbated by drug resistance mechanisms induced in the infectious microorganisms, and by persistence of the microorganisms by sequestration in viscous mucus or biofilms. The sequestration prevents effective antibiotic access to the bacteria. Such problems have led to the search for alternative treatments and therapies, but none of these alternative techniques have yet been tested rigorously or successfully in clinical patients. In this project, we used a standard strain of P. aeruginosa (NCTC 10662) and a B. cenocepacia isolate from cystic fibrosis sputum to appraise tobramycin/amikacin efficacy in combination with clinically relevant concentrations of the adjunctive agents amiloride, benzamil hydrochloride, phenamil, salbutamol, verapmnil, and amlodipine. Altered conditions in the cystic fibro sis lung were simulated by using different concentrations of sodium chloride, potassium chloride, sodium gluconate, D-mannitol, and N-Methyl-D-glucamine. Benzamil hydrochloride was the most potent additive compound against the organisms tested; enhancing the antibacterial effect of tobramycin. A sub-inhibitory concentration of amlodipine was only marginally useful, even though its minimum inhibitory concentration (MIC) against both microbes was the lowest of all the nonantibiotic compounds tested. Conversely, salbutamol, verapamil, and amlodipine were antagonistic in some combinations with tobramycin. Amikacin was generally more potent than tobramycin. Sodium and potassium chlorides and sodium gluconate increased the tobramycin MIC up to 8-fold at salt concentrations fron1 50-400 mM. This antagonistic effect of cations appeared to be partially reversed by adding amiloride, verapamil, or salbutamol. This study needs to be extended by further assays with more clinical isolates, but it has shown that non-antibiotic adjunctive agents can be used with antibiotics to produce effective results in vitro; and potentially in vivo as an alten1ative regime for the treatment of chronic airway infections in cystic fibrosis patients.
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