Previously uncharacterized rectangular bacterial structures in the dolphin mouth

Much remains to be explored regarding the diversity of uncultured, host-associated microbes. Here, we report the discovery of unusual rectangular bacterial structures (RBSs) in the mouths of bottlenose dolphins. DNA staining revealed multiple paired bands within RBSs that suggested cells dividing along the longitudinal axis. Cryogenic transmission electron microscopy and tomography revealed parallel membrane-bound segments, suspected to be cells, encapsulated by an S-layer-like periodic surface covering. RBSs displayed novel pilus-like appendages with bundles of threads splayed at the tips. Multiple lines of evidence suggested that RBSs are bacterial and distinct from the Neisseriaceae genera Simonsiella and Conchiformibius, with which they share similar morphology and division patterning, including genomic DNA sequencing of micromanipulated RBSs, 16S rRNA gene sequencing, and fluorescence in situ hybridization. Our findings highlight the diversity of novel microbial forms and lifestyles that await discovery and characterization using tools complementary to genomics such as microscopy.