SUMMARY Peptidoglycan (PG) is a dynamic, load-bearing polymer whose crosslinking chemistry governs envelope mechanics, growth modes, and stress tolerance. For decades, PG crosslinking was viewed primarily through the lens of penicillin-binding proteins (PBPs). However, accumulating evidence over the past 2 decades has established LD-transpeptidases (LDTs) as important contributors to PG remodeling. Here, we organize the expanding LDT field into macro-domains bridging biochemistry, evolution, and ecology. We initially describe the reaction mechanisms, structural diversification, and convergent solutions and then explore the evolution across species. We highlight non-canonical LD-crosslinking chemistries, including L-Ala– meso -DAP (1–3) linkages, that broaden the design space of the sacculus. We then map functional repertoires across lineages—from reinforcement during envelope stress to outer membrane tethering, predation, specialized secretion, dormancy, and biofilms. In pathogens where LD-crosslinking is dominant or essential, carbapenems and penems remain particularly effective inhibitors of LDTs, yet family-wide diversity calls for structure-guided selectivity and ecological awareness. We also chart underexplored connections to microbiome ecology and propose LDT-derived biomarkers that report growth modes and dormancy. We integrate dispersed evidence into a complete landscape in which two-component systems and environmental cues coordinate LD pathways. Building on these threads, we propose a unifying model of LDTs as adaptive architects of PG whose acyl-enzyme intermediate and modular substrate gating endow reversibility, partner choice, and context-dependent outcomes—reinforcement, remodeling, anchoring, or controlled self-breach. Finally, we outline methods that enabled the discovery of LDTs and explore future directions. Together, these perspectives reframe LDTs from auxiliary enzymes to central designers of envelope architecture and bacterial fitness.
Walenkiewicz et al. (Mon,) studied this question.