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Peptidoglycan (PG) is a highly cross-linked peptide-glycan mesh that confers structural rigidity and shape to most bacterial cells. Polymerization of new PG is usually achieved by the concerted activity of two membrane-bound machineries, class-A penicillin binding proteins (aPBPs) and class-B penicillin binding proteins (bPBPs) in complex with shape, elongation, division, and sporulation (SEDS) proteins. Here, we have identified four phylogenetically distinct groups of bacteria that lack any identifiable aPBPs. We performed experiments on a panel of species within one of these groups, the Rickettsiales, and found that bacteria lacking aPBPs build a PG-like cell wall with minimal abundance and rigidity relative to cell walls of aPBP-containing bacteria. This reduced cell wall may have evolved to minimize the activation of host responses to pathogens and endosymbionts while retaining the minimal PG-biosynthesis machinery required for cell elongation and division. We term these "peptidoglycan-intermediate" bacteria, a cohort of host-associated species that includes some human pathogens. IMPORTANCE Peptidoglycan (PG) is a large, cross-linked polymer that forms the cell wall of most bacterial species and confers shape, rigidity, and protection from osmotic shock. It is also a potent stimulator of the immune response in animals. PG is normally polymerized by two groups of enzymes, aPBPs and bPBPs working together with shape, elongation, division, and sporulation (SEDS) proteins. We have identified a diverse set of host-associated bacteria that have selectively lost aPBP genes while retaining bPBP/SEDS and show that some of these build a minimal PG-like structure. It is expected that these minimal cell walls built in the absence of aPBPs improve the evolutionary fitness of host-associated bacteria, potentially through evasion of PG-recognition by the host immune system.

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Public Health Research Institute, Rutgers University, Newark, New Jersey, USA.