A multilayered cell envelope of a member of the Chloroflexota offers an anchoring platform for the archaellum

In a previous study, we discovered that Litorilinea aerophila , a member of the bacterial phylum Chloroflexota, had acquired a bona fide archaellum gene cluster through horizontal gene transfer from Archaea, a surprising finding given that the archaellum had long been considered an archaeal-specific motility machinery. Here, we hypothesize that the distinctive multilayered cell envelope of L. aerophila provides the structural context that enables the integration and function of the archaellum motility machinery. Using fluorescence microscopy, thin-section electron microscopy, and cryo-electron tomography, we revealed the organisation of the L. aerophila envelope and propose a mechanism for how the archaellum can traverse the peptidoglycan of L. aerophila by using the Type IV pilus alignment complex proteins PilO and PilN. In addition, we identified two other cell surface appendages: (i) pilus-like structures consistent with Tad pili, and (ii) grappling hook-like structures. Structural analysis of the grappling hook by CryoEM revealed an architecture that possibly plays a role in cell–cell interactions. Together, these findings imply that the evolution of a complex, multilayered cell envelope in Chloroflexota has facilitated the functional adaptation of archaeal surface machineries, allowing these bacteria to exploit the archaellum as a simpler, more energy-efficient motility system than the bacterial flagellum.