Bacillus solimangrovi is a flagellated (motile), Gram-positive bacterium isolated from mangrove soil in Micronesia. It grows fastidiously, requiring the presence of divalent cations, which allows it to build a protective protein coat, called a surface layer (S-layer). Although very distinct from the outer membrane of Gram-negative bacteria, the S-layer is likewise a strong barrier that protects the cell from harmful substances, while still allowing the passage of important nutrients.
In both Gram-positive and Gram-negative bacteria, the bacterial flagellum is anchored by a rotary motor comprising several rings (C, M, and S) to support the rotating rod through the centre. The MS-ring anchors the rotor to the cytoplasmic membrane, whereas the C-ring sits within the cytoplasm and is essential for torque generation. In Gram-negative bacteria, two additional rings are present that act as bushings to stabilise the flagellum and reduce friction during rotation: the P-ring (peptidoglycan) and the L-ring (outer membrane). While an equivalent bushing is not necessary for the flagellar of most Gram-positive bacteria, how is the bacterial flagellar secreted through the Gram-positive S-layer?