Synergy Testing between Sulbactam and Meropenem/ Colistin in MDR Acinetobacter baumannii-calcoaceticus Complex Isolated from VentilatorAssociated Pneumonia
2016
Background: A. baumannii-calcoaceticus (Abc) complex has surfaced as a major nosocomial pathogen causing
blood stream infection and ventilators associated pneumonia (VAP). Carbapenems have come to be the cornerstone
of treatment for Abc complex. However, there has been an increased incidence of infections with carbapenem
resistant strains. To validate the clinical practice of combination antibiotic therapy, in-vitro combinations of antibiotics
have been examined using checkerboard methods, E-tests, and the reference, time-kill assay.
Method: A prospective pilot study was conducted for the duration of one year. Twenty five isolates of
carbapenem resistant Abc complex cultured from endotracheal aspirates of patients admitted in medical and
surgical intensive care units diagnosed with ventilator associated pneumonia were collected. Isolates were tested for
MIC (Minimum inhibitory concentration) by micro-broth dilution method for meropenem, sulbactam and colistin.
Synergism between sulbactam plus meropenem and sulbactam plus colistin was tested by micro-broth
checkerboard assay and the reference, time kill assay.
Result: Minimum inhibitory concentration ranges (μg/ml) for sulbactam, meropenem, and colistin were 16-512,
16-256, and 0.5-64, respectively. MIC50 for sulbactam, meropenem, and colistin was 128, 128 and 1,
correspondingly, and MIC90 for sulbactam, meropenem, and colistin was 256, 256 and 2, respectively. In the
checkerboard assay and time-kill assay, a higher percentage of synergy was noted for the combination of sulbactam
plus meropenem.
Conclusion: Against multi-drug resistant (MDR) isolates of Abc complex, commendable synergy was seen with
time kill assay for sulbactam plus meropenem combination. Therefore, in-vitro combinations of antimicrobial agents
are most effective than the single agent against multidrug resistant organism.
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