Meropenem/colistin synergy testing for multidrug-resistant Acinetobacter baumannii strains by a two-dimensional gradient technique applicable in routine microbiology

• Published in: Journal of antimicrobial chemotherapy 2015-01, Vol.70 (1), p.167-172
• Main Authors: van Belkum, Alex, Halimi, Diane, Bonetti, Eve-Julie, Renzi, Gesuele, Cherkaoui, Abdessalam, Sauvonnet, Véronique, Martelin, Roland, Durand, Géraldine, Chatellier, Sonia, Zambardi, Gilles, Engelhardt, Anette, Karlsson, Åsa, Schrenzel, Jacques
• Format: Article
• Language: English
• Subjects: Acinetobacter baumannii - drug effects; Anti-Bacterial Agents - pharmacology; Antibiotics; Antimicrobial agents; Colistin - pharmacology; Drug resistance; Drug Resistance, Multiple, Bacterial; Drug Synergism; Drug therapy; Gram-negative bacteria; Microbial Sensitivity Tests - methods; Microbiology; Thienamycins - pharmacology
• ISSN: 0305-7453; 1460-2091
• DOI: 10.1093/jac/dku342

Precise assessment of potential therapeutic synergy, antagonism or indifference between antimicrobial agents currently depends on time-consuming and hard-to-standardize in vitro chequerboard titration methods. We here present a method based on a novel two-dimensional antibiotic gradient technique named Xact™. We used a test comprising a combination of perpendicular gradients of meropenem and colistin in a single quadrant. We compared test outcomes with those obtained with classical chequerboard microbroth dilution testing in a study involving 27 unique strains of multidrug-resistant Acinetobacter baumannii from diverse origins. We were able to demonstrate 92% concordance between the new technology and classical chequerboard titration using the A. baumannii collection. Two strains could not be analysed by Xact™ due to their out-of-range MIC of meropenem (>128 mg/L). The new test was shown to be diagnostically useful, easy to implement and less labour intensive than the classical method.