聚糖的骨架（主链）； 一组相同的短肽侧链接于
NAM上；
相连主链上的短肽侧链通过一条肽桥、
或直接相连，从而形成网状分子结构。
G+菌肽聚糖单体
Gˉ菌肽聚糖单体
Figure 8. Schematic diagram of the peptidoglycan sheet of Staphylococcus aureus. G = N-acetyl-glucosamine; M = Nacetyl-muramic acid; L-ala = L-alanine; D-ala = D-alanine; Dglu = D-glutamic acid; L-lys = L-lysine. This is one type of murein found in Gram-positive bacteria. Compared to the E. coli peptidoglycan (Figure 7) there is L-lys in place of DAP (diaminopimelic acid) in the tetrapeptide. The free amino group of L-lys is substituted with a glycine pentapeptide (glygly-gly-gly-gly-) which then becomes an interpeptide bridge forming a link with a carboxy group from D-ala in an adjacent tetrapeptide side chain. Gram-positive peptidoglycans differ from species to species, mainly in regards to the amino acids in the third position of the tetrapeptide side chain and in the amino acid composition of the interpeptide bridge.
肽聚糖的特点:
1)为原核生物所特有(古细菌例外) 2)D-型氨基酸和DAP在真核细胞中未发 现 3)个别菌中DAP（二氨基庚二酸）可为 Lys代替
Figure 5 - A comparison of L and D amino acids. Note that while the structure is identical, it is impossible to superimpose them.
肽聚糖单体→网状结构
NAG 与 NAM 残基以ß-1,4糖苷键交替连接，形成
Figure 3 - Electron micrograph of a G+ cell wall.
细胞壁cell wall:
Gram Positive
Gram Negative
（1）细胞壁的基本骨架——肽聚糖（共有成分）
肽聚糖网格状结构
概念： 肽聚糖是由 N—乙酰胞壁酸（NAM） 和N—乙酰葡糖胺（NAG） 以及短肽链（主要是四肽） 组成的亚单位聚合而成的 大分子聚合物。
六、、细菌细胞结构
❖一般构造：如细胞壁、细胞膜、细胞质、核质体、核糖体等，是 所有细菌都有的构造
❖特殊构造：主要有鞭毛、菌毛、性菌毛、荚膜和芽孢等，并非所 有细菌都有的构造
Figure 1. Schematic diagram of a typical procaryotic cell. Minimally, a procaryote is composed of a cell wall and plasma membrane that surrounds its cytoplasm containing a chromosome, ribosomes, enzymes, several classes of RNA, and small molecules (precursors).
（一）细胞壁cell wall:
细胞壁是位于菌体的外层，内 侧紧贴细胞膜的一层无色透明， 坚韧而有弹性的结构。细胞壁 约占细胞干重的10%—25%。
细胞壁的观察方法： ① 质壁分离＋染色 ② 电镜观察
★共有组分— 肽聚糖 ★特有组分— G＋ 磷壁酸
Gˉ 脂多糖
G＋与Gˉ细菌cw 的of a Gcell wall.