Abstract: Objective To study the in vitro antibacterial activity of meropenem combined with doxycycline, ciprofloxacin,
sulbactam or cefoperazone/sulbactam against clinically isolated extensively drug-resistant Acinetobacter baumannii (XDRAB).
Methods Using a checker board synergy design, the minimal inhibitory concentration (MIC) of antibiotics against 50 isolates
of XDRAB was determined by broth microdilution antifungal susceptibility test. The fractional inhibitory concentration (FIC)
index was calculated to determine the combined effect of the antibiotics. Results Meropenem showed significantly reduced
MIC50 and enhanced antimicrobial activities when combined with doxycycline, sulbactam or cefoperazone/sulbactam. The
FIC results suggested that the main actions of doxycycline, sulbactam, and cefoperazone/sulbactam were synergistic (38%,
26%, and 10%, respectively) and addictive (62%, 74%, and 90%, respectively) without indifferent or antagonistic effects. The
main actions of meropenem combined with ciprofloxacin were additive (56%) and indifference (44%) with synergistic and
antagonistic effects. Conclusion Meropenem combined with doxycycline, sulbactam or cefoperazone/sulbactam shows
excellent activity against clinical isolates of XDRAB.