The Impact of Peptidoglycan Structure on Immune Sensing

Peptidoglycan (PGN) is a mesh like polymer that builds and protects the bacterial cell wall. Across and even within species, PGN varies considerably in structure and composition, and this structural diversity shapes how released fragments are detected by the host. The varied structural features of PGN can also dictate its ability to act as a toxin or danger signal. Traditionally, bacterial cell wall components have been classified as toxic, based on their ability to cause extensive host cell damage, but the specific immune responses triggered by distinct PGN structural motifs when they are released into the extracellular space, and their potential direct role as toxins are not yet fully understood. Emerging evidence suggests that PGN is recognized by the host not only through its canonical di-/tripeptides, but also through novel structural motifs that are sensed by host pattern recognition receptors (PRRs). However, the mechanisms underlying the recognition of these structurally diverse fragments and their role in promoting immune activation, bacterial pathogenesis, and immune evasion remain poorly understood. In this review, we describe how structurally distinct muropeptides are synthesized, processed, and selectively sensed by host PRRs, and how differential immune recognition shapes PRR activation, thereby influencing host-pathogen interactions and infection outcomes.