Introduction: Carbapenem-resistant Acinetobacter baumannii (CRAB) infection is a growing healthcare problem worldwide. Management of CRAB infections is complicated, as most CRAB isolates exhibit resistance to multiple antibiotic classes. While additional antibiotics are on the horizon, optimal use of currently available treatment options remains an important goal.
Objectives: To conduct a retrospective review of laboratory data on A. baumannii isolates and identify antibiotic options for CRAB infections in an acute care hospital in Singapore.
Methods: The culture results and corresponding susceptibility profiles of clinical specimens sent for bacterial culture examination between February 2020 and January 2024 were reviewed. Species identity of bacterial isolates was determined by MALDI-TOF MS on a VITEK MS instrument (Biomérieux, France). Antibiotic susceptibility testing was performed on a VITEK 2 automated system (Biomérieux) for the following ten antimicrobial agents: amikacin, gentamicin, ciprofloxacin, ceftriaxone, ceftazidime, cefepime, meropenem, ampicillin/sulbactam, piperacillin/tazobactam, and trimethoprim/sulfamethoxazole (SXT). Additionally, concomitant disk diffusion testing of SXT was performed for isolates since July 2023. Susceptibility results were interpreted according to the CLSI breakpoints in M100-Ed33 and analyzed after de-duplication. Statistical analysis was performed in R version 4.3.2.
Results: A total of 438 A. baumannii isolates were recovered. Overall, 143 (32.6%) were CRAB isolates based on meropenem resistance, with a significantly increasing trend from 15/73 (20.5%) in 2020 (February to December) to 69/154 (44.8%) in 2023. As determined by the Vitek 2 system, SXT was the most active antibiotic against tested CRAB isolates with 112/137 (81.8%) susceptible. For meropenem-susceptible or -intermediate isolates, the SXT susceptibility rate was 263/276 (95.3%), which trailed behind the susceptibility rates of other antibiotics like amikacin and ampicillin/sulbactam. Furthermore, there was limited agreement between the methods for SXT susceptibility testing, with 20.0% very major errors (VMEs) and 10.5% minor errors (mEs) in the Vitek 2 results when disk diffusion was taken as the reference. All the VMEs (3/3) and most mEs (8/10) occurred when testing CRAB isolates.
Conclusion: Based on current data, SXT is a feasible option for antibiotic treatment of CRAB infections in our locality, though SXT-susceptible results of CRAB isolates tested by Vitek 2 should be confirmed by another method.