bacterial cell wall and envelope

7/31/2019 Bacterial Cell Wall and Envelope

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13/09/20

3b. The Prokaryote cell

3a) Prokaryote vs. Eukaryote cell

● How cell parts are studied

● The plasma membrane and transport

3b) The cell wall and envelope (other outer layers)

● Eukaryote cell walls

Readings: Ch 3 & pg 142‐143

3c) ● Specialized structures / appendages, including pili & stalks, spores, VBNC

● Bacterial flagellum (flagella) and chemotaxis

• includes the cell wall & extracellular components (e.g.

slimes & capsules)

• Varies in composition; but the cell wall is always present

• Envelope composition defines:

‐ Gram‐positive bacteria (thick cell wall)

(e.g. phylum Firmicutes)

‐ Gram‐negative bacteria (thin complex cell wall)

‐ e.g, the phylum Proteobacteria)

The bacterial cell

envelope

Figure 3.16

murein sacculus

Gives shape & rigidity to the cell

Helps cell to withstand turgor

pressure

Long polymers of two sugars,

N‐Acetylglucosamine & (b) the peptide

containing N‐acetylmuramic acid

‐ Connected by Peptide cross‐bridges

Most Bacteria have a cell wall composed of

Peptidoglycan (Murein)

Figure 3.17

Most Archaea have a cell wall; BUT , the cell wall is

composed of Pseudomurein / Pseudopeptidoglycan

NOT Peptidoglycan!

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‐ Pseudopeptidoglycan composition

‐ N‐acetyl glucosamine ‐ N ‐acetyltalosaminuronic acid (NOT N‐acetylmuramic acid)

‐ Archaea are resistant (not affected) by penicillin or vancomycin

‐ S‐layers in most

• ‘Capsule’

• S‐Layer (+/‐)

• Thick cell wall

– Thick peptidoglycan,

– Teichoic acids

– Lipoteichoic acid (polymers)

The Gram‐Positive Cell Envelope

Figure 3.18a



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Mycobacteria have v. complex Cell Envelopes

• E.g. Mycobacterium tuberculosis &

Mycobacterium leprae

• Multi‐layered v. thick cell

envelope

– Glycolipid capsules

– Mycolic acid lipid bilayer

– Arabino‐galactan layers

– Thick Peptidoglycan

Figure 3.21

Gram‐Negative envelope is ‘thinner’ but complex

Figure 3.18b

(+/‐) Capsule & S‐layers

Outer membrane:Lipopolysaccharide (major lipid)

Proteins e.g. porins &

lipoproteins

Periplasm, containing the

Very thin peptidoglycan layer

(1‐2 sheets), located in the,

Figure 3.22

Lipid A is an Endotoxin,

‐ disaccharide with

attached (multiple lipids)

and phosphate groups

LPS is composed of 3 regions

S (surface)‐layers are also called A‐

layers

‐In Archaea & some Bacteria

‐Extremely stable to chemicals

‐ Crystalline arrays of proteins or

glycoproteins subunits

Fig. 3.20, TEM micrograph of the S‐layer

of Thermoproteus tenax ; a cell wall‐less

Archaea

Capsules of Acinetobacter sp‐

colonies on agar are stained with

Indian Black Ink; capsules exclude

the stain

Electron micrograph of a thin

section of Rhizobium trifolii;

capsules are stained with

ruthenium red (an electron‐dense

dye) dye

• Some prokaryotes are naturally lacking a cell wall

‐Genus Mycoplasmas (domain Bacteria)

‐in Domain Archaea

genus Thermoplasma, genus Picrophilus & genus Ferroplasma

(all are Thermoacidophilic Archaea)

? Do they have a ‘regular cell membrane? If so, how do they survive in the environment?



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Figure 19.14A

Cells of the acidophilic

hyperthermophile (80‐

90◦C, pH 3) Sulfolobus

(Archaea) cells lack a cell

wall but have a thick S‐

layer.

Examples & shapes of cell wall‐less archaea

Picrophilus oshimae,

Ferroplasma

acidarmanusAn extreme

acidophile

See fig 19.35

Discussion:

Based on these cell wall

structure, what can we

deduce ,

a) Permeability to polar &

non‐polar compounds

b) General permeability

c) Surface charge

properties

d) Resistance to penicillin /

vancomycin

e) Resistance to (or

protection against)

unfavourable

environmental factors

f) Habitat preference?

[+/ ‐ ] capsules / slimes

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The essential & protective properties of the

bacterial Cell wall & Cell envelope

Can be deduced from studies on bacterial genetics,

pathogenesis, physiology and ecology

1)

2)

3)

4)

5)

6)

Eukaryotic Microbes

All possess a cell membrane

• Algae form cell walls of cellulose.

• Fungi form cell walls of chitin.

• Diatoms form exoskeletons of silicate.

• Paramecia possess a contractile vacuole to

pump water out of the cell.

• Protists have pellicles

The gram‐stain differentiates prokaryotes into 2 groups

Gram‐positives & Gram‐negatives (lecture set 2)

Critical thinking Questions:

a) Why do Gram‐positives stain purple and G‐negatives stain

pink? (i.e. differential staining) (Hint: review Text pg 53‐54)

b) Are mycobacteria & Archaea Gram‐positives or Gram‐

negatives?c) What about Mycoplasma or Thermoplasma cells?

Concept Questions:

1. An extension of the cytoplasm that attaches bacteria to a surface

is called a ________________

2. In archaeal membranes, the glycerol is linked to the hydrocarbon

chains by _____ bonds.

bacterial cell wall and envelope

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