Molecular epidemiology of carbapenem-resistant <i>Acinetobacter baumannii</i> group in Taiwan

<i>Acinetobacter,</i> particularly the <i>Acinetobacter baumannii</i> group, is a major cause of nosocomial infections, and carbapenem-resistant <i>Acinetobacter</i> spp. are important human pathogens. We collected 492 <i>Acinetobacter</i> spp. strains from two hospitals in Taiwan and classified them using MALDI-TOF MS and <i>bla</i>_OXA-51-like PCR; 94.5% were <i>A. baumannii,</i> and 5.5% were non-<i>A</i>. <i>baumannii</i> (NAB). We confirmed their identity by <i>rpo</i>B gene sequencing of 239 randomly selected <i>A. baumannii</i> strains and all 27 NAB strains. Our analysis revealed that the <i>rpo</i>B alleles of OXA51-like-negative strains matched those of two NAB species, <i>A. seifertii</i> and <i>A. nosocomialis</i>, while all OXA51-like-positive strains matched <i>A. baumannii</i>, as per the Pasteur MLST scheme database. Among the 492 strains, 240 exhibited carbapenem resistance, including 237 carbapenem-resistant <i>A. baumannii</i> (CRAB) strains and three CR-NAB strains. All CRAB strains were positive for <i>bla</i>_OXA-51-like; 72.6% also carried <i>bla</i>_OXA-23-like, 22.8% carried <i>bla</i>_OXA-24-like, 3.4% co-carried <i>bla</i>_OXA-23-like+<i>bla</i>_OXA-24-like, and 1.27% carried <i>bla</i>_OXA-51-like alone. Among the three CR-NAB strains, one carried <i>bla</i>_NDM-1, and two co-carried <i>bla</i>_OXA-58-like+<i>bla</i>_IMP. We also established a new multiplex PCR method for rapid screening of common capsular types (KL), which showed a difference between CRAB and carbapenem-susceptible <i>A. baumannii</i> (CSAB). KL2, KL10, KL22, and KL52 accounted for 76.6% of CRAB strains, whereas about half of the CSAB strains were other KL types. Of the remaining CSAB strains, KL14 was the most predominant type at 10.3%. We further conducted MLST Pasteur typing for 262 isolates and found that the carbapenemase genes were correlated with either ST or KL types. Additionally, KL types showed correlations with ST types, carbapenem resistance, and certain clinical records. Whole-genome sequencing of a <i>bla</i>_NDM-1-carrying <i>A. seifertii</i> strain revealed a plasmid transferable via <i>in vitro</i> conjugation, suggesting <i>A. seifertii</i> may be a reservoir for NDM-1 plasmids.IMPORTANCECarbapenem-resistant <i>Acinetobacter</i> spp. have been identified by the World Health Organization as a top priority for new antibiotic development. We established a rapid KL-typing method for efficient screening of <i>Acinetobacter baumannii</i> strains to enable epidemiological surveillance and provide a foundation for effective infection control. Our investigation of the molecular epidemiology of the <i>A. baumannii</i> group isolates revealed the prevalence of carbapenemase genes and major KL types among CR and CS strains of <i>A. baumannii</i> and NAB. We identified an <i>A. seifertii</i> strain carrying a Ti-type conjugative operon on a small plasmid that harbored genes encoding the NDM-1 carbapenemase alongside genes conferring resistance to aminoglycosides and bleomycin and closely resembled sequences detected in <i>A. soli</i> and <i>A. pittii</i> in Taiwan and China, respectively, suggesting its potential for transmitting multidrug resistance and contributing to the spread of antimicrobial resistance.