Undergraduate microbiologists are taught that tripartite efflux pumps are exclusive to Gram-negative bacteria. This is because the third component (e.g. TolC) will sit within the outer membrane that is exclusive to diderms. Surprisingly, Gram-positive bacteria also encode what appears to be tripartite efflux pumps. In these monoderms, rather than the outer membrane, the third component sits within a protective protein layer, called the Surface Layer (S-layer).
Bacillus solimangrovi is a flagellated (motile), Gram-positive bacterium isolated from mangrove soil in Micronesia. It grows fastidiously, requiring the presence of divalent cations in order to build a robust 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 acting like a molecular sieve to ensure the passive diffusion of important nutrients, but what do they do once this S-layer is breached by harmful substances?