ABSTRACTThe treatment of extensively drug-resistant (XDR)A. baumanniihas emerged as a major problem. Tigecycline (TGC) and sulbactam (SUL) are both effective antibiotics against XDRA. baumannii. Here, we investigated the in-host evolution and mechanism of collateral sensitivity (CS) phenomenon in development of tigecycline resistance accompanied by a concomitant increase of sulbactam susceptibility. A total of four XDRA. baumanniistrains were sequentially isolated from the same patient suffering from bacteremia. Core-genome multilocus sequence typing separated all the strains into two clusters. Comparative analysis of isolate pair 1 revealed that multiplication ofblaOXA-23within Tn2006on the chromosome contributed to the change in the antimicrobial susceptibility phenotype of isolate pair 1. Additionally, we observed the emergence of CS to sulbactam in isolate pair 2, as demonstrated by an 8-fold increase in the TGC MIC with a simultaneous 4-fold decrease in the SUL MIC. Compared to the parental strain Ab-3557, YZM-0406 showed partial deletion in the two-component system sensoradeS. Reconstruction of theadeSmutant in Ab-3557in situsuggested that TGC resistance and CS to SUL were mainly caused by the mutation ofadeS. Overall, our study reported a novel CS combination of TGC and SUL inA. baumanniiand further revealed a mechanism of CS attributed to the mutation ofadeS. This study provides a valuable foundation for developing effective regimens and sequential combinations of tigecycline and sulbactam against XDRA. baumannii.IMPORTANCECollateral sensitivity (CS) has become an increasingly common evolutionary trade-off during adaptive bacterial evolution. Here, we report a novel combination of tigecycline (TGC) resistance and CS to sulbactam (SUL) inA. baumannii. TGC and SUL are both effective antibiotics against XDRA. baumannii, and it is essential to reveal the mechanism of CS between TGC and SUL. In our study, the partial deletion ofadeS, a two-component system sensor, was confirmed to be the key factor contributing to this CS phenomenon. This study provides a valuable foundation for developing effective regimens and sequential combinations of tigecycline and sulbactam against XDRA. baumannii.