Comparison of clinical manifestations, antimicrobial susceptibility patterns, and carbapenem resistance determinants between Acinetobacter seifertii and Acinetobacter nosocomialis isolated in Taiwan.

Background: Acinetobacter seifertii, a recently identified member of the Acinetobacter calcoaceticus-Acinetobacter baumannii (Acb) complex, has emerged as a cause of severe human infections. It is closely related to Acinetobacter nosocomialis, a major pathogen of the Acb complex. Here, we aimed to explore the clinical and molecular differences between these two species.

Methods: This retrospective study enrolled 83 adults with A. seifertii bacteremia and 402 adults with A. nosocomialis bacteremia from four medical centers over a 9-year period. Species identification was confirmed using matrix-assisted laser desorption ionization-time of flight mass spectrometry and rpoB sequencing. Clinical information, antimicrobial susceptibility, and carbapenem resistance determinants were analyzed.

Results: There were no significant differences in the underlying diseases or mortality between patients with A. seifertii and A. nosocomialis bacteremia. However, A. seifertii bacteremia was more frequently associated with intensive care unit (ICU) admission, recent ICU stay, central venous catheter use, ventilator use at bacteremia onset, and pneumonia as the primary infection source than A. nosocomialis bacteremia. A. seifertii exhibited significantly lower susceptibility to colistin, amikacin, gentamicin, ceftazidime, and cefepime than A. nosocomialis. Carbapenem resistance was primarily mediated by ISAba1-bla_OXA-51-like in A. seifertii and IS1006-ΔISAba3-bla_OXA-58-like in A. nosocomialis.

Conclusion: A. seifertii and A. nosocomialis exhibit distinct antimicrobial susceptibility profiles and carbapenem resistance mechanisms but share similar mortality rates. The ability of both species to act as reservoirs of carbapenem resistance highlights the importance of accurate identification, antimicrobial stewardship, and infection control strategies to mitigate the spread of resistant strains.