Antimicrobial Susceptibility Testing Performed in RPMI 1640 Reveals Azithromycin Efficacy against Carbapenem-Resistant Acinetobacter baumannii and Predicts In Vivo Outcomes in Galleria mellonella

Antimicrobial susceptibility testing (AST) in RPMI 1640, a more physiologically relevant culture medium, revealed that a substantial proportion of carbapenem-resistant Acinetobacter baumannii isolates were susceptible to azithromycin, a macrolide antibiotic not currently considered effective against A. baumannii. Experiments using Galleria mellonella validated these in vitro data. ABSTRACT Antimicrobial susceptibility testing (AST) in RPMI 1640, a more physiologically relevant culture medium, revealed that a substantial proportion of carbapenem-resistant Acinetobacter baumannii isolates were susceptible to azithromycin, a macrolide antibiotic not currently considered effective against A. baumannii. Experiments using Galleria mellonella validated these in vitro data. Our finding that RPMI 1640’s predictive accuracy for in vivo outcomes is superior to that of Mueller-Hinton II broth also supports the use of more physiologically relevant AST culturing conditions.