Chromosomal co-carriage of bla_OXA-23 and bla_OXA-500 in a carbapenem-resistant Acinetobacter pittii isolate

Carbapenem resistance in non-baumannii Acinetobacter has been increasingly reported worldwide. This study investigated the genetic basis of carbapenem resistance in Acinetobacter pittii isolate KBN12P06770 carrying bla_OXA-23 and bla_OXA-500 using whole-genome sequencing. Two carbapenemase genes were located on the chromosome. Two copies of bla_OXA-23 located on Tn2006 were identified. Additional chromosomally encoded antimicrobial resistance genes, including bla_ADC-18 and ant(3")-IIc, as well as efflux pump genes (adeIJK, adeFGH, abeM, and abaQ), were identified, with the exception of plasmid-borne ant(2")-Ia. qPCR analysis revealed that the bla_OXA-23 was more highly expressed than bla_OXA-500. Notably, bla_OXA-23 featured the Ser225Pro substitution, first characterized in this study, which may enhance β-lactamase activity based on structural modeling. Furthermore, multilocus sequence typing (MLST) assigned KBN12P06770 to ST1611 within clonal complex 63 (CC63), with close clustering to ST2737 indicating recent divergence. The comprehensive genomic analysis of A. pittii isolate KBN12P06770 highlights the complex genetic architecture underlying carbapenem resistance.