Phenotypic and genotypic characterization of Acinetobacter baumannii clinical isolates: yesterday and today

Background: Acinetobacter baumannii has become notable as a cause of nosocomial infections. It showed remarkable genomic plasticity due to mutational variations and horizontal gene transfer to increase its fitness in hospital environment. Therefore, as a step forward in understanding its evolution under antibiotic selective pressure, this study aims to compare the resistome and virulome of A. baumannii clinical isolates collected from the last decade. The phenotypic properties of the A. baumannii clinical isolates will also be addressed. Methods: A total of 30 clinical A. baumannii isolates were collected from a public hospital in Rome during 2010-2023. Isolates were collected mainly from respiratory and urine specimens. Antibiotic susceptibility was determined by minimum inhibitory concentration according to EUCAST. Biofilm-forming ability was evaluated by crystal violet staining; motility and protease assays were also performed. The ability to adhere to human alveolar and bladder epithelial cells is ongoing. Results: The majority of the isolates were nonsusceptible to various agents of carbapenem, aminoglycoside, fluoroquinolones, β lactamase, and folate pathway inhibitors, while less than 10% of strains were colistin-resistant. Preliminary results showed that 60% of strains isolated from respiratory infections displayed moderate to strong biofilm-forming activity compared to those from urinary sites. This ability allows bacteria to promote their lifespan on abiotic surfaces, i.e., ventilators, catheters, or surgical tools. Motility is involved in persisting in a stressed environment; only 10% of the isolates exhibited marked twitching motility on soft agar plates, 35% had some degree of motility, whereas the remaining 55% were not motile. All strains showed low protease activity on skim milk agar plates. Conclusions: Current work with whole-genome sequencing is ongoing; these analyses will provide the genomic profile associated with antimicrobial resistance, virulence factors, multi-locus sequence types, and the relationship among isolates. Characterizing their chromosomal and plasmid-encoded resistance and virulence traits could help us understand the mechanisms behind the genetic mobilization and spread of these genes among these opportunistic pathogens and clues about their evolution as a response to environmental stress conditions. This comparative study could spotlight valuable information on preventing transmission in a healthcare environment.