Outer membrane vesicles of carbapenem-resistant <i>Acinetobacter baumannii</i> derive an enhanced inflammatory response

Carbapenem-resistant <i>A. baumannii</i> (CRAB) is a critical nosocomial pathogen with limited treatment options. Although antibiotic resistance in CRAB is well-characterized, its interactions with host immunity and the contribution of outer membrane vesicles (OMVs) to pathogenesis remain poorly understood. We examined a clinical CRAB isolate and compared it with the reference strain A19606. Antimicrobial susceptibility testing revealed complete resistance of CRAB to commonly used antibiotics in clinical practice, while A19606 remained susceptible to most agents. In murine intranasal infection models and bone marrow-derived macrophages, CRAB induced significantly stronger activation of inflammatory signaling pathways and elevated levels of pro-inflammatory cytokines relative to A19606. Transcriptomic analysis of the infected lung tissue identified differentially expressed genes, enriched for inflammatory response pathways. Proteomics showed upregulated proteins in CRAB related to secretion systems. Characterization of OMVs showed that CRAB-derived vesicles were enriched in periplasmic and outer membrane proteins and elicited stronger inflammatory signaling in macrophages. CRAB displays expansive antibiotic resistance and enhanced pro-inflammatory potential mediated in part by unique OMVs properties. Targeting OMVs formation or host immune modulation may represent effective strategies for combating CRAB infections.IMPORTANCEIn this study, we comprehensively compare a clinical carbapenem-resistant <i>A. baumannii</i> (CRAB) with the reference strain A19606, examining antimicrobial resistance profiles, <i>in vivo</i> and <i>in vitro</i> immune activation, and the composition and function of extracellular vesicles (outer membrane vesicles, OMVs). We found that CRAB not only resists all tested antibiotics but also induces significantly greater activation of MAPK and NF-κB signaling and elevated cytokine production in macrophages and infected lungs. Proteomic analysis revealed that CRAB-derived OMVs are larger and enriched in proteins associated with secretion and immune activation, acting as potent pro-inflammatory mediators. These findings illuminate dual mechanisms driving CRAB pathogenicity-extensive drug resistance combined with heightened host inflammatory response-and suggest novel intervention strategies targeting OMVs-mediated immune modulation.