Interplay between bacterial outer membranes and S-layers

Bacteria can adopt a variety of strategies to shield themselves from environmental insult. Among these, the outer membrane (OM) and S-layers are both surface-exposed layers with overlapping functions. Despite their existence and composition being described decades ago, recent advances have led to a revised view of their biological functions, structural organisation and spatiotemporal dynamics. Here, we review recent advances in our understanding of these envelope layers. We focus on new insights into the biological significance, molecular composition, surface architecture and spatiotemporal patterns underlying their biogenesis. By comparing these systems, we show that the OM and the S-layer share notable features, including aspects of surface symmetry and the spatiotemporal coordination of their assembly. We discuss outstanding questions related to each of these envelope layers and the potential interplay between them. Understanding this interplay promises to reveal fundamental principles of bacterial cell envelope assembly and organisation, opening new avenues for antimicrobial targeting.