1542. The Evaluation of the In Vitro Synergy of Colistin in Combination with Meropenem and Tigecycline against 50 Multi-Drug-resistant Acinetobacter baumannii strains

BACKGROUND: Acinetobacter baumannii possess inherent and acquired antibiotic resistance mechanisms that have rendered most antibiotics, including carbapenems, inactive. Colistin (COL) has risen as salvage therapy against these organisms due to its retained activity against A. baumannii. However, COL monotherapy is often met with suboptimal outcomes. Recently, combination therapy with COL and meropenem (MEM) or tigecycline (TGC) has been shown to be effective in eradicating multi-drug-resistant A. baumannii infections. The objective of this study was to further evaluate the efficacy of COL in combination with MEM or TGC against 50 multi-drug-resistant A. baumannii strains. METHODS: Fifty carbapenem-resistant A. baumannii strains were evaluated using combination minimum inhibitory concentration (MIC) testing and time-kill analysis (TKA). Single-drug MIC testing was performed for each strain by broth microdilution. Combination MIC testing was performed for COL+MEM and COL+TGC. Each strain was evaluated via 24-hour TKA to assess the synergistic capabilities of COL+MEM, and COL+TGC. Synergy was defined as a ≥ 2-log reduction CFU/mL in either combination from the most active single agent, while bactericidal activity was defined as a ≥ 3-log reduction CFU/mL of either combination from the initial inoculum. RESULTS: All 50 strains were resistant to MEM and TGC with MICs ≥ 64 µg/mL and ≥ 4 µg/mL respectively; while 3 strains were resistant to COL, MICs ≥ 2 µg/mL. MEM and TGC MIC values were reduced as much as 128-fold (median 2-fold) and 32-fold (median 2-fold),, respectively, in the presence of subinhibitory COL. COL MIC values were reduced as much as 512-fold (median 4-fold) from baseline in the presence of subinhibitory MEM, and as high as 16-fold (median 2-fold) in the presence of TGC. In TKAs, COL+MEM was synergistic in 45/50 (90%) strains and bactericidal against 43/50 (86%) strains. COL+TGC TKAs revealed synergy in 32/50 (64%) strains, and bactericidal activity against 28/50 (56%) strains. CONCLUSION: The combinations of COL+MEM and COL+TGC demonstrate promise in combating highly resistant A. baumannii. Further research is mandated to explore other combinations that are capable of eradicating multi-drug-resistant A. baumannii. DISCLOSURES: All authors: No reported disclosures.