Horizontal gene transfer in Acinetobacter baumannii has been attributed to conjugative pili, transduction, and natural transformation. Here, we describe a previously unrecognized type of chromosomal DNA exchange mediated by cell-envelope conduits that establish direct cytoplasmic continuity between neighboring A. baumannii cells. Cryo-electron tomography reveals that these conduits, which were found in multiple clinical isolates, comprise an outer membrane, a peptidoglycan layer, and an inner membrane. The ~65 nm-thick conduits contain two ~2 nm-thick filaments, which super-resolution microscopy identifies as DNA. Interstrain horizontal gene transfer assays coupled with whole-genome sequencing demonstrate the transfer and homologous recombination of chromosomal segments up to 1.1 Mbp, corresponding to as much as 27% of the A. baumannii genome. Together, these findings provide direct structural and functional evidence for a conduit-mediated large-scale chromosomal DNA exchange, thus expanding the known repertoire of horizontal gene transfer strategies that may contribute to the remarkable genomic plasticity of A. baumannii.
Manger, S., Boernsen, C., Torres, S. G., Roth, P., Sommer, J., Balakrishnan, A., Langschied, F., Thiel, C., Haenelt, I., Heilemann, M., Goettig, S., Ebersberger, I., Pos, K. M., Frangakis, A.
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