Lecture 3-3: Microbial cell biology

Cell Structure and Function

School of Life Science and Biotechnology

Shanghai Jiao Tong University

3.8 Cell Wall of Prokaryotes:Peptidoglycan and Related Molecules原核生物胞壁: 肽聚糖与相关分子

Fig 4.32 Protoplasts原生质体

(a) In dilute solution breakdown of the cell wall releases the protoplast, but it immediately lyses because the cytoplasmicmembrane is very weak. 稀溶液中原生质体会立即破裂.

(b) In a solution containing an isotonic concentration of a solute such as sucrose, water does not enter the protoplast and it remains stable.等渗溶液中, 水不进入原生质体, 原生质体稳定.

Shanghai Jiao Tong University

Shanghai Shanghai Jiao Jiao Tong UniversityTong University

What is a cell wall for?

A wall is for against pressure- 2 atmA wall is for maintaining cell shapeA wall is for giving cells rigidity

Shanghai Jiao Tong University

Shanghai Shanghai Jiao Jiao Tong UniversityTong University

Gram stain革兰氏染色

This staining method has long been used for classifying and identifying bacteria (For more detail: Lecture 3-1)

Fig 4-4 Steps in the Gram stain procedure.革兰氏染色过程

结晶紫染1分钟

碘液媒染3分钟

酒精脱色20秒

沙黄复染1-2分钟

FLASH

Examples of Gram Staining

Differences in cell wall structure are at the base of differences in the Gram-staining reaction. The G- cell wall is a multilayered structure and quite complex, whereas the G+ cell wall consists of primarily a single type of molecule and is often much thicker.

Fig 4.27 Cell walls of Bacteria FLASH

Shanghai Jiao Tong University

Shanghai Shanghai Jiao Jiao Tong UniversityTong University

4.8.1 Peptidoglycan 肽聚糖

Cell wall of Bacteria have one rigid layer to strengthen the wall.The rigid layer of G+ and G- Bacteria is peptidoglycan layer.肽聚糖层

In G- Bacteria, additional layers are present outside this rigid layer.

Fig 4.29

Peptidoglycan unit is composed of two suger derivatives, N-acetylglucosamine and N-acetylmuramic acid, and a small group of amino acids consisting of L-alanine, D-alanine, D-glutamic acid, and either lysine or diaminopimelic acid (DAP).肽聚糖由N-乙酰葡萄糖胺和-乙酰胞壁酸两种糖,加上一组氨基酸组成(DAP:二氨基庚二酸)

Fig 4-30 How the peptide and glycan units are connected to form the peptidoglycan sheet. (a) No interbridge in gram-negative Bacteria. (b) Glycine interbridge in gram-positive Bacteria.G+菌的肽聚糖单位之间有甘氨酸五肽桥, 而G-菌无五肽桥.

(c) Overall structure ofpeptidoglycan.

G, N-acetylglucosamine; M, N-acetylmuramic acid

Shanghai Jiao Tong University

Shanghai Shanghai Jiao Jiao Tong UniversityTong University

4.8.2 The structure of Gram positive bacterial cell walls

One thick peptidoglycan layer (25 sheets)Teichoic acids （磷壁酸）

• Polymers containing glycerolphosphate or ribitolphosphate residues 含在磷酸甘油及磷酸核糖醇的多聚物

• Lipoteichoic acids: teichoic acids that covalently bound to membrane lipids脂磷壁酸: 与膜脂类共价连接的磷壁酸

Fig. 4.31 Summary diagram of the gram-positive cell wall

The cell walls of gram-positive cells contain mostly peptidoglycan.

The cell walls of gram-positive cells are smoother than that of gram-negative cells.

Shanghai Jiao Tong University

Shanghai Shanghai Jiao Jiao Tong UniversityTong University

4.9 The Outer Membrane of Gram Negative Bacteria

Two membranes• Outer membrane外膜• Cytoplasmic membrane细胞质膜

One thin peptidoglycan layer一层肽聚糖层One out of cell bounded space

• Periplasm space周质空间

Porin

Fig 4.35 The gram-negative cell wall.Although the outer membrane is often called the "second lipid bilayer," the chemistry and architecture of this layer differs in many ways from that of the cytoplasmic membrane

Shanghai Jiao Tong University

Shanghai Shanghai Jiao Jiao Tong UniversityTong University

Structure of the lipopolysaccharide (LPS) of G- bacteria.脂多糖

•O-specific polysaccharide varies among species.O-特异多糖

•Core polysaccharide 核心多糖

•Lipid A 类脂A

Fig 4.36 Cell wall of G— bacteria under electron microscope

Fig 4.27 The surface texture of a G- cell is heavily folded when compared with a G+ cell.

Shanghai Jiao Tong University

Shanghai Shanghai Jiao Jiao Tong UniversityTong University

Relationship of Cell Wall Structure to the Gram Stain p.81

Phenomenon: In Gram stain, an insoluble crystal violet-iodine complex is formed inside the cell, and the complex is extracted by alcohol from G- but not from G+ Bacteria.Reason: G+ Bacteria, which have very thick peptidoglycanlayer, become dehydrated by the alcohol. This causes the pores in the walls to close, preventing the insoluble crystal violet-iodine complex from escaping. G+菌的厚肽聚糖层被乙醇脱水, 造成壁上小孔收缩,从而使不溶性的结晶紫-碘复合物不能洗出.Then, what happened to G- Bacteria?

Shanghai Jiao Tong University

Shanghai Shanghai Jiao Jiao Tong UniversityTong University

In G- Bacteria, alcohol readily penetrates the lipid-rich outer layer, and the thin peptidoglycan layer also does not prevent solvent passage, thus, the crystal violet-iodine complex is easily removed. G-菌中, 乙醇很容易穿透富脂的

外膜, 薄的肽聚糖壁也无法阻止溶剂的通过.

Shanghai Jiao Tong University

Shanghai Shanghai Jiao Jiao Tong UniversityTong University

How can cell wall become the target for killing a cell?

Peptidoglycan Structure• Lysozyme :β-1,4 bond is sensitive to lysozyme肽聚糖的β -1,4糖苷链对溶菌酶敏感

Peptidoglycan synthesis• Penicillin: The first commercialized and still

most important antibiotic青霉素

Transport of peptidoglycan precursors was send to the growing point of the cell wall across the cytoplasmic membrane

细菌萜醇

The transpeptidation reaction that leads to the final cross-linking of two peptidoglycan chains转肽作用

Shanghai Jiao Tong University

Shanghai Shanghai Jiao Jiao Tong UniversityTong University

Penicillin and lysozyme

Penicillin• inhibitor of transpeptidation 抑制转肽作用

• Create imbalance between the action of autolysin and transpeptidation 造成自溶与转肽作用的不平衡

• Can only kill actively growing cellsLysozyme• Break exisiting glycosidic bonds• Lead to cell burst• Create protoplast or spheroplast

Shanghai Jiao Tong University

Shanghai Shanghai Jiao Jiao Tong UniversityTong University