Lecture 9: Prokaryotic Cell Walls

What is the function of the cell wall?

Provides structural support, protection, and acts as a filtering mechanism

What is the main structural component of bacterial cell walls?

Peptidoglycan

Why do bacterial cells need a cell wall?

Prevents bursting due to osmotic pressure.

What are the two sugars that make up peptidoglycan?

• N-acetylglucosamine (NAG)

• N-acetylmuramic acid (NAM)

How are NAG and NAM linked?

By a β-1,4 glycosidic bond

What enzyme breaks the glycosidic bond in peptidoglycan?

Lysozyme

What bonds hold peptidoglycan layers together?

Covalent peptide cross-links

How does penicillin affect bacterial cell walls?

It inhibits transpeptidase, preventing cross-linking and causing osmotic lysis

How much of the Gram-positive cell wall is peptidoglycan?

About 90%

What is the thickness of Gram-positive peptidoglycan layers?

20-80 nm

What are teichoic acids?

Polymers that provide structural support and maintain a negative charge

What are the two types of teichoic acids?

• Lipoteichoic acids

• Wall teichoic acids

What are lipoteichoic acids linked to?

to the membrane

What are wall teichoic acids linked to?

to peptidoglycan

What do Gram Positive/Negative look like when stained?

• positive stain purple

• negative stain pink/red

How much of the Gram-negative cell wall is peptidoglycan?

About 15-20%

Where is peptidoglycan located in Gram-negative bacteria?

In the periplasm, between the inner and outer membranes

What is the outer membrane composed of?

• Lipopolysaccharide (LPS)

• Phospholipids

• Proteins

What is the periplasmic space?

The space between the inner and outer membranes where peptidoglycan is found

What role do porins play in Gram-negative bacteria?

They allow transport of molecules through the outer membrane

What are the three main components of LPS?

• Lipid A

• Core polysaccharide

• O-polysaccharide

What is the function of Lipid A?

Acts as an endotoxin and anchors LPS to the membrane

What is the function of the core polysaccharide?

Provides structural stability

What is the function of the O-polysaccharide?

Functions as an antigen and distinguishes bacterial species.

How does LPS protect Gram-negative bacteria?

Blocks hydrophobic antibiotics, detergents, and dyes.

What happens when LPS is released from dead bacteria?

It can cause fever, blood vessel leakage, and toxic shock

Why do Gram-positive bacteria survive better in dry conditions?

Their thick peptidoglycan layer prevents water loss.

Why are Gram-negative bacteria well-adapted to host environments?

Their outer membrane provides protection against the immune system

How do Mycobacterium and Nocardia differ from other bacteria?

They have an acid-fast cell wall with mycolic acid.

Which bacteria lack a cell wall?

Mycoplasma species

What helps Mycoplasma resist lysis?

Sterols in their plasma membrane

Do Mycoplasma stain with Gram stain?

No, because they lack a cell wall

What is pseudomurein?

A peptidoglycan-like structure found in some Archaea

What is the major component of acid-fast cell walls?

Mycolic acid, a waxy lipid

What is the structure of an acid-fast cell wall?

A thin peptidoglycan layer covered by a mycolic acid layer

What function does mycolic acid serve?

Provides resistance to desiccation, antibiotics, and immune attacks.

Do archaea have peptidoglycan in their cell walls?

No, they have pseudomurein or other unique polymers.

What are archaeal cell walls composed of if not pseudomurein?

• Polysaccharides

• Glycoproteins

• S-layer

Do Mycoplasma species have a cell wall?

No, they lack a cell wall entirely, so there plasma membrane contains sterols for stability

Do archaea Gram stain?

No, because their cell walls differ significantly from bacteria