The K77 capsular polysaccharides as determinants of virulence and resistance in hypermucoid clinical isolate Acinetobacter baumannii 10593

Abstract

Acinetobacter baumannii is an opportunistic pathogen and a major cause of hospital-acquired infections worldwide, with intrinsic antibiotic resistance and remarkable desiccation tolerance. Important virulence factor is the polysaccharide capsule which protects the bacteria from environmental stressors, including antimicrobials and the host immune response. With limited treatment options, capsular polysaccharides have become promising vaccine targets. To investigate the biological role and function of the capsule, the clinical isolate A. baumannii 10593, characterized by a hypermucoid phenotype, was selected from the laboratory’s collection. The capsule biosynthesis genes are clustered in the K locus. Following whole genome sequencing of the selected isolate, the KAPTIVE tool was used to analyse the K locus type, which was identified as KL77. Subsequently, the galU gene, involved in the biosynthesis of simple sugars, was selected for mutagenesis, and a corresponding knockout strain was successfully generated. The outcome of the selected gene deletion was assessed by comparing biofilm production between the wild type and the mutant strain using fluorescence microscopy, with the mutant showing a higher ability to form biofilm. The constructed deletion mutant was further employed to investigate the role of the capsule in several key aspects of A. baumannii pathogenicity, including antimicrobial resistance and tolerance to disinfectants. The knockout strain showed increased susceptibility to all tested disinfectants (benzalkonium chloride, benzethonium chloride, and chlorhexidine digluconate) and most antibiotics used in this study (meropenem, ciprofloxacin, gentamicin, amikacin, tobramycin, and colistin). Adhesion to extracellular matrix proteins (collagen type I and fibronectin) was examined and it was demonstrated that the mutant was able to adhere more strongly to these proteins compared to the wild type. Furthermore, LDH cytotoxicity assay using human keratinocyte line (HaCaT) showed that the mutant strain was less cytotoxic and therefore less virulent towards host cells than the wild type strain. Based on these findings, the K77 capsular polysaccharides play a significant role in the pathogenicity of A. baumannii 10593, as they enhance resistance to antimicrobials and disinfectants as well as cytotoxicity, whereas their disruption promotes bacterial adhesion.Book of abstract:6 BALKAN CONFERENCE ON BIOSCIENCES 30-31 OCT 2025 PLOVDIV, BULGARI