Cross-reactivity and protective efficacy of antibodies raised to BamA for broad-spectrum antimicrobial resistant Acinetobacter baumannii

Outer membrane proteins (OMPs) are attractive vaccine targets due to their surface-exposed epitopes and essential roles in bacterial physiology. This study evaluated the protective efficacy of recombinant BamA, the most conserved component of the β-barrel Assembly Machinery (Bam) complex, as a broad-spectrum vaccine candidate against Acinetobacter baumannii. We combined in vivo murine immunization and sepsis challenge experiments with in vitro and in silico analyses to assess immunogenicity, protective capacity, and cross-reactivity of BamA-specific antibodies. BALB/c mice immunized with recombinant BamA generated high IgG titers and, upon bacterial challenge, showed significantly reduced bacterial loads in the spleen, liver, and lungs compared to controls. Whole-cell ELISA demonstrated that anti-BamA antibodies cross-react with multiple Gram-negative pathogens, including E. coli, S. flexneri, S. enterica, Enterobacter, and K. pneumoniae. Bioinformatic and molecular docking analyses revealed conserved extracellular loop motifs—particularly a VRGF motif in loop 6—mediating strong interactions with the innate immunity receptor CD89 through salt bridges and hydrogen bonds. These findings indicate that BamA elicits robust adaptive immune responses in vivo, possesses broad-spectrum immunogenic potential, and may also engage innate immune mechanisms, making it a promising universal vaccine target against multidrug-resistant Gram-negative bacteria.