Whole genome analysis revealed the role of blaOXA-23 and blaOXA-66 genes in carbapenem resistance of Acinetobacter baumannii strains.

Acinetobacter baumannii is a multidrug-resistant bacterium that has emerged as a significant nosocomial pathogen globally and renowned for its ability to acquire antimicrobial resistance (AMR) genes. However, understanding of its resistance mechanisms to certain drug classes remains limited. This study focused on four bacterial strains (AB863, AB889, AB930, and AB960) exhibiting carbapenem resistance. They demonstrated high minimum inhibitory concentration (MIC) (128 mg/L) to meropenem and were categorized as extensively drug-resistant strains. Subsequently, they were identified as A. baumannii through 16S rRNA gene sequence analysis and species-specific PCR targeting the bla_OXA51-like gene. Three strains were sequenced for their genomes to study the genetic determinants and functional relevance of carbapenem resistance. The draft genome length of the strains ranged from 3.8 to 4.0 Mbp. A total of 16 antibiotic resistance genes including the genes bla_OXA-23 and bla_OXA-66 which mediate carbapenem resistance were identified in the genomes. A comprehensive multilocus sequence typing analysis involving 95 A. baumannii strains from different Asian countries assigned the four strains to sequence type 2 (ST2), the most predominant ST circulating in Asia. Comparative genome analysis also revealed bla_OXA-66 as the most dominant variant of bla_OXA-51-like gene and also a widespread distribution of bla_OXA-23 gene. In addition, various mobile genetic elements associated with AMR genes and three efflux pumps families were detected in the genomes of the strains. Transformation of bla_OXA-23 and bla_OXA-66 genes resulted in meropenem resistance in the transformant which exhibited a MIC of 2 mg/L, thus confirming direct involvement of both genes in carbapenem resistance.