In vitro antimicrobial synergy studies of carbapenem-resistant Acinetobacter baumannii isolated from intensive care units of a tertiary care hospital in Egypt

Summary The increasing prevalence of multidrug-resistant and pan drug-resistant Acinetobacter baumannii as a cause of nosocomial infections has led to the need for the reassessment of novel combinations of antibiotics as our only current viable option for handling such infections until a new therapeutic option becomes available. Two of the most commonly used methods for testing antimicrobial synergy are the Time-kill assay method and the E-test method, and these were the methods used in this study. Antibiotic combinations tested in this study were azithromycin and polymyxin, tobramycin and polymyxin, polymyxin and rifampicin, and tobramycin and rifampicin. The azithromycin and polymyxin combination showed synergy, while the rifampicin and polymyxin combination showed antagonism. The synergy was achieved at lower MIC values than using each of the single agents alone against the same isolates. Synergy testing results varied according to the method used, and it is difficult to establish which method is more accurate. The use of these lower MIC values as a guide to determine effective therapeutic doses used in combination therapy can help to decrease the emergence of resistance and can also minimize the side effects associated with using a single agent at a higher dose. Further research is still required to predict in vivo efficacy of such combinations.