微生物-英语chater2

微生物学教案Chapter 2 Prokayotic Microorganism

Chapter 2 prokayotic microorganism（原核微生物）

第二章原核微生物

prokaryotic cells

The chief distinguishing characteristics of procaryotic cells are：

1. genetic material （DNA）is not enclosed within a membrane.

2. They lack other membrane bounded organelles.

3. Their DNA is not associated with histone proteins （special chromosomal proteins found in eucaryotes）。

4. Their cell walls almost always contain the complex polysaccharide peptidoglycan（肽聚糖）。

Bacteria（细菌）and archaeobacteria（古细菌）

Bacteria are small，single-celled，microorganisms that belong to a group called prokaryotes .

Bacteria are ubiquitous. They are a highly successful and diverse group of organisms that can obtain energy and carbon from a wide range of sources and therefore can colonize every niche on our planet from deep ocean trenches to volcanic craters. In the 1970s，using DNA sequencing information，it was found that the group we know as the bacteria could be split into two，the eubacteria and the archaeobacteria and it appears that these two groups evolved away from each other very early in the history of living things at about the same time that the first group of eukaryotic cells evolved. Members of the eubacteria include some of the more familiar bacteria such as Escherichia colt（大肠埃希氏菌）and Staphylococcus aureus（金黄色葡萄球菌）and are the prokaryotes that are best studied and understood. The archaebacteria are a very diverse group of organisms，which differ from the eubacteria（真细菌0 in a number of features having，in

particular，very different cell walls and membranes. This group includes bacteria that are capable of existing in extreme environments.

Size，Shape，and Arrangement of Bacterial Cells

1 Most bacteria are from 0.20 to 2.0 /μm in diameter and from

2 to 8μm in length.

2 The three basic bacterial shapes are coccus（spheres），bacillus（rods），and spiral （twisted）。

3 In addition to the three basic shapes，there are star-shaped cells （genus Stella）and recently discovered square，flat cells （halophilic（嗜盐的）archaeobacteria）and triangular cells （Haloarcula）

4 the shape of a bacterium is determined by heredity.However，a number of environmental conditions can alter that shape.Pleomorphic（多型性的）bacteria can assume several shapes.

Gram Stain：

The eubacteria are frequently divided into two groups on the basis of their reaction to a stain devised by Christian Cram in 1884. The differential reaction to the staining procedure is because of the structure of the cell wall in these two groups of bacteria. Gram-positive bacteria have a single membrane called the cytoplasmic （or plasma）membrane，surrounded by a thick layer of peptidoglycan （20-80 nm）。The Gram-negative bacteria have only a thin layer of peptidoglycan （1-3 nm）but on the outside of this there is a further outer membrane which acts as an additional barrier.

The procedure for the Gram stain is as follows. Fixed cells are stained with a dark stain such as crystal violet，followed by iodine which complexes with the stain in the cell wall of the bacteria. Alcohol is added，which washes the dark stain of crystal violet-iodine out of ceils that have thin cell walls but not from those that have thick cell walls. Finally，a paler stain such as carbol fuschin，called a counter stain，is added which stains the decolorized cells pink but is not seen on the dark