Genomic and phenotypic analysis of ST25 <i>A. baumannii</i> identifies virulence-associated clades and capsular/outer core locus types

The increase in the infection caused by <i>Acinetobacter baumannii</i> is sustained by the selection of distinct epidemic clonal lineages, which are frequently resistant to a broad range of antimicrobials and possess virulence traits responsible for their persistence in the contaminated environment and spread among patients. The present study aimed to perform an integrated genomic and phenotypic analysis to assess the virulence features of ST25 isolates. <i>A. baumannii</i> isolates assigned to the ST25 epidemic clonal lineage shared high genomic similarity and clustered in four clades (I, II, III, and IV), with clade IV further subdivided into CIVa, CIVb, CIVc, and CIVd. Capsular locus (KL) KL14 was the predominant KL type (47%). Accessory genome analysis showed the presence of tartrate metabolism genes only in CII genomes. CIVb and CIVd ST25 <i>A. baumannii</i> isolates showed higher ability to infect <i>Galleria mellonella</i> larvae than CI, CII, CIII, and CIVc isolates. Hydrogen peroxide resistance was higher in CI, CII, CIVb, and CIVd isolates compared with CIII and CIVc isolates. In desiccation survival tests, CIII, CIVb, and CIVd isolates exhibited prolonged survival. In addition, CI, CII, CIII, CIVb, and CIVd isolates showed higher serum resistance than CIVc isolates. Also, KL14 type and lipooligosaccharide outer core locus (OCL) OCL6 type isolates were significantly more resistant to oxidative stress, to desiccation, and possessed a high ability to kill <i>G. mellonella</i> larvae. A positive and significant correlation was found between AdeB and AdeJ efflux pump expression and hydrogen peroxide resistance.IMPORTANCEIn this study, we characterized the genotypic and phenotypic features of <i>A. baumannii</i> strains assigned to the ST25 epidemic clonal lineage, which were isolated from humans, animals, and the environment. We found that ST25 <i>A. baumannii</i> isolates, irrespective of their antimicrobial resistance, showed peculiar virulence features among clades, isolates assigned to clade IVb and IVd showing the highest virulence and elevated resistance to serum and desiccation. Also, a positive significant correlation was found between the presence of KL14 and outer core locus 6 genotypes and resistance to oxidative stress, resistance to desiccation, and the ability to kill <i>G. mellonella</i> larvae. Phenotypic differences reflected clade identity rather than isolate origin, suggesting that specific virulence traits contribute to the environmental persistence and pathogenic potential of <i>A. baumannii</i> ST25 isolates.