Biofilm formation in Acinetobacter baumannii isolates from companion animals and horses: associations with host, clinical origin, antimicrobial resistance, and clonal lineage

Acinetobacter baumannii is a major nosocomial pathogen in both human and veterinary medicine, in which biofilm formation (BF), multidrug resistance (MDR), and specific international clones (ICs) contribute to persistence and spread. In this study, 507 clinical A. baumannii isolates from cats, dogs, and horses, collected between 2008 and 2024, were analyzed for antimicrobial resistance (AMR), BF capacity, and genomic characteristics. Antimicrobial susceptibility was determined using the VITEK® 2 system, BF was quantified after 24 and 48 h by crystal violet assay, and genomic analyses included multilocus sequence typing, assignment to ICs, and detection of AMR and biofilm-associated genes (BAGs). Overall, 79.5% of isolates were classified as MDR, with significantly higher rates in cats and dogs than in horses. BF was widespread and showed host-specific dynamics: canine isolates exhibited progressive biofilm maturation over time, feline isolates remained largely stable, and equine isolates showed delayed but pronounced BF. BF also varied by clinical origin, with isolates from wounds/abscesses, urinary tract infections, and medical devices showing the strongest BF, compared, for example, with isolates from respiratory or genital tract infections. Among all clonal lineages, IC1 and IC2 were characterized by the strongest BF and the highest proportion of MDR isolates. Among the BAGs analyzed, blp1 was positively associated with BF, whereas bap showed an inverse association. These findings indicate that biofilm formation is a central feature of A. baumannii epidemiology in companion animals and is closely linked to clinical origin, MDR phenotype and clonal lineage. Improved understanding of these relationships may support the development of more targeted therapeutic strategies in veterinary medicine.