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A5486 - Synergistic Activity of Colistin in Combination with N-Acetylcysteine Against Colistin-Resistant Acinetobacter Baumannii Grown in Biofilms
Author Block: F. Sergio1, L. Pallecchi2, S. Pollini3, F. Blasi4, S. Boncompagni2, T. Di Maggio2, S. Aliberti5, G. Rossolini3; 1Global Respiratory Medical Affairs, Zambon SpA, Milan, Italy, 2Department of Medical Biotechnologies, University of Siena, Siena, Italy, 3Department of Experimental and Clinical Medicine, University of Florence, Florence, Italy, 4Istituto di Malattie Resp, Milano, Italy, 5University of Milan, Milano, Italy.
RATIONALE. Acinetobacter baumannii is a major respiratory pathogen, representing a leading cause of hospital-acquired pneumonia and being associated with higher morbidity and mortality rates, especially in critically ill patients. A. baumannii infections are often recalcitrant to antibiotic treatment, as a consequence of the propensity of this pathogen to grow in biofilms, and of the increasing dissemination of strains expressing extensively drug resistance (XDR) phenotypes. Colistin is a last-resort agent for the treatment of infections caused by XDR A. baumannii, and nebulized colistin (alone or in conjunction of intravenous colistin) has been increasingly used for the treatment of respiratory tract infections caused by those strains. In this perspective, great concerns arise from the rising trend of resistance to colistin observed in this pathogen over the last recent years. In a previous study, we reported that high N-acetylcysteine (NAC) concentrations (achievable by topical administration) were able to potentiate colistin activity against a collection of colistin-resistant A. baumannii strains grown in planktonic phase. Here we demonstrate that such synergism exerts also a relevant antibiofilm effect. METHODS. Two XDR A. baumannii clinical isolates expressing a colistin resistance phenotype were investigated (i.e. A. baumannii Z165 and A. baumannii Z167). Biofilm susceptibility testing was performed using the Nunc-TSP lid system. Biofilms were grown in cation-adjusted Mueller Hinton broth for 7 days, and successively exposed to NAC/colistin combinations. Three NAC concentrations (i.e. 1.6, 3.2 and 8 mg/ml) and three colistin concentrations (i.e. 2, 8 and 32 mcg/ml) were tested alone and in combination. Biofilm mass was determined by biofilm disruption and viable cell count (i.e. enumeration of colony forming units - CFUs) at time 0 (i.e. before exposure to NAC/colistin combinations) and after 24 hours of exposure. RESULTS. Under the experimental conditions adopted, biofilms were formed by an average of 6.08 ± 0.49 and 5.89 ± 1.06 log CFU/ml, for A. baumannii Z165 and A. baumannii Z167, respectively. Antibiofilm activity was not observed with NAC alone, or colistin ≤ 8 µg/ml alone. Colistin at 32 mcg/ml eradicated both biofilms. A relevant antibiofilm activity of NAC/colistin combinations was demonstrated for both tested strains. NAC potentiated colistin activity in a concentration-dependent way, but a relevant antibiofilm activity was already observed with colistin 8 mcg/ml plus NAC 1.6 mg/ml (i.e. reduction of >3 log CFU/ml compared to controls). CONCLUSION. Nebulized NAC/colistin combinations might represent a valid treatment option for biofilm-associated infections caused by colistin-resistant A. baumannii strains.