25Peptidoglycan, which is the main component of the bacterial cell wall, is a 26 heterogeneous polymer of glycan strands crosslinked with short peptides and is 27 synthesized in cooperation with the cell division cycle. Although it plays a critical 28 role in bacterial survival, its architecture is not well understood. Herein, we 29 visualized the architecture of the peptidoglycan surface in Bacillus subtilis at the 30 nanometer resolution, using quick-freeze, deep-etch electron microscopy. 31 Filamentous structures were observed on the entire surface of the cell, where 32 filaments about 11-nm wide formed concentric circles on cell poles, filaments 33 about 13-nm wide formed a circumferential mesh-like structure on the cylindrical 34 part, and a "piecrust" structure was observed at the boundary. When growing 35 cells were treated with lysozyme, the entire cell mass migrated to one side and 36 came out from the cell envelope. Fluorescence labeling showed that lysozyme 37 preferentially bound to a cell pole and cell division site, where the peptidoglycan 38 synthesis was not complete. Ruffling of surface structures was observed during 39 electron microscopy. When cells were treated with penicillin, the cell mass came 40 out from a cleft around the cell division site. Outward curvature of the protoplast 41 at the cleft seen using electron microscopy suggested that turgor pressure was 42 applied as the peptidoglycan was not damaged at other positions. When 43 muropeptides were depleted, surface filaments were lost while the rod shape of 44 the cell was maintained. These changes can be explained on the basis of the 45 working points of the chemical structure of peptidoglycan. 46 47 Bacteria, the major inhabitants of the Earth, are in a constant battle to outlast 49 3 their competitors in the environment and the immune system of host organisms. 50 Most bacterial cells are surrounded by a rigid shield called the "peptidoglycan 51 layer," which protects them from chemical agents, including lytic enzymes and 52 antibiotics, that are produced by their competitors [1-4]. 53 Peptidoglycan is an essential component of the bacterial cell wall that is found 54 on the outside of the cytoplasmic membrane of almost all bacterial cells except 55 the class Mollicutes. This polymer provides strength, rigidity, and shape stability 56 by maintaining turgor pressure. In peptidoglycan, the glycan strands comprise 57 alternating β-1,4-linked N-acetylglucosamine (GlcNAc) and N-acetylmuramic 58 acid (MurNAc), and the peptide stems are covalently linked to the glycan 59 strands with an amide bond to the carboxyl carbon of the MurNAc. The 60 peptidoglycan layer is also the site of action for antimicrobial agents. Lysozyme, 61 an antimicrobial enzyme critical in animal host defense, is one of the most 62 abundant proteins present on the mucosal surfaces and in body secretions, 63 such as saliva and tears [5, 6]. The epithelial cells secrete lysozyme to protect 64 the host's mucosal surfaces from infectious bacteria. Lysozyme is also pre...