Chapter 4.2 Bacterial Cell Walls

What are the three possible layers of the bacterial cell envelope?

- Cell membrane (all bacteria)

- Cell wall (most bacteria)

- Outer membrane (only Gram-negative bacteria)

What is the primary function of the bacterial cell wall?

Provides shape and structure, protects from osmotic pressure changes, prevents cell lysis in hypotonic environments

What happens to bacterial cells in hypertonic solutions?

dehydration, membrane collapse;

What happens to bacterial cells in hypotonic solutions?

water influx, potential lysis (cell wall helps prevent this)

Who developed the Gram stain and when?

Hans Christian Gram, a Danish scientist, in 1884

First steps of the Gram stain procedure in order.

Crystal violet (primary stain)

Second step of the Gram stain procedure in order.

Iodine (mordant)

Third step of the Gram stain procedure in order.

Alcohol (decolorizer)

Forth step of the Gram stain procedure in order.

Safranin (counterstain)

What is the function of iodine in the Gram stain?

Acts as a mordant - fixes the crystal violet-iodine complex to the cell envelope

Why do Gram-positive bacteria remain purple after the Gram stain?

Their thick peptidoglycan cell wall retains the crystal violet-iodine complex even after alcohol decolorization

Why do Gram-negative bacteria appear pink/red after the Gram stain?

The alcohol dissolves their outer membrane, washing away the crystal violet-iodine complex, so they take up the safranin counterstain

What colors do Gram-positive bacteria appear under the microscope?

purple

What colors do Gram-negative bacteria appear under the microscope?

pink/red (often appears pink due to microscope lighting)

Describe the cell envelope structure of Gram-positive bacteria.

Inner cell membrane + thick peptidoglycan cell wall (20-80 nm thick)

Describe the cell envelope structure of Gram-negative bacteria.

Inner cell membrane + thin peptidoglycan cell wall (1-3 nm thick) + outer membrane of lipopolysaccharide

What additional components are found in Gram-positive cell walls besides peptidoglycan?

• Teichoic acid

• lipoteichoic acid (aid in cell wall maintenance, enlargement during division, provide acidic charge)

The thickness of peptidoglycan in Gram-positive bacteria

20-80 nm thick

The thickness of peptidoglycan in Gram-negative bacteria

1-3 nm thick

What is peptidoglycan composed of?

Glycan chains (sugars) cross-linked with short peptide fragments

What makes peptidoglycan unique to bacteria?

It's a unique macromolecule that can only be produced by bacteria (not found in archaea or eukaryotes)

How does the structure of peptidoglycan provide both strength and flexibility?

Its mesh-like organization creates a strong but flexible support framework

What is the significance of peptidoglycan being unique to bacteria?

It's a major difference between bacteria and archaea, and makes it a good target for antibiotics

What is the primary component of the Gram-negative outer membrane?

Lipopolysaccharide (LPS)

What are the three main functions of lipopolysaccharide?

• Provides receptors for environmental interaction

• Acts as antigens (O antigen triggers immune response)

• Can function as endotoxin

What is endotoxic shock and when does it occur?

Life-threatening condition with fever and low blood pressure; occurs when large amounts of LPS are released into bloodstream (e.g., when antibiotics kill many Gram-negative bacteria)

What anchors the outer membrane to the peptidoglycan cell wall?

Specialized lipoproteins

What are porin proteins and what is their function?

Proteins that span the outer membrane; allow small molecules to pass through but can alter size to block harmful chemicals and certain antibiotics

How do porin proteins help Gram-negative bacteria resist antibiotics?

They can change size to prevent certain antibiotics from entering the cell

What is mycolic acid and which bacteria contain it?

A very long-chain fatty acid (wax-like) found in Mycobacterium cell walls; contributes to pathogenicity

What diseases are caused by mycolic acid-containing bacteria?

Tuberculosis and leprosy (caused by Mycobacterium species)

What staining technique is used to identify mycolic acid-containing bacteria?

Acid-fast stain

What is unique about Mycoplasma bacteria regarding cell walls?

They naturally lack a cell wall entirely (cannot produce peptidoglycan)

How do Mycoplasma bacteria compensate for lacking a cell wall?

Their membrane is stabilized by high amounts of sterols, making it resistant to lysis

What does "pleomorphic" mean and which bacteria exhibit this property?

Having many different shapes; Mycoplasma bacteria are pleomorphic because they lack the rigid cell wall that determines bacterial shape

What is the size range of Mycoplasma bacteria?

0.1-0.5 μm (very small, even for bacteria)

What is the composition of bacterial cell membranes?

30-40% phospholipids, 60-70% proteins (higher protein content than eukaryotic membranes)

How do Mycoplasma cell membranes differ from other bacteria?

They contain high amounts of sterols to provide stability without a cell wall

How do archaeal cell membranes differ from bacterial membranes?

Archaea use branched hydrocarbons instead of fatty acids

What are the main functions of the bacterial cell membrane?

Energy reactions, nutrient processing, transport (nutrients in, wastes out), selective permeability, secretion

Which molecules can pass freely through the cell membrane?

Water and small uncharged molecules (like O₂ and CO₂)

What is required for most larger or polar molecules to cross the cell membrane?

Special carrier mechanisms or transport proteins

Why are Gram-negative bacteria generally more difficult to kill than Gram-positive bacteria?

Their outer membrane makes them impervious to many antimicrobial chemicals

How does the outer membrane contribute to the pathogenicity of Gram-negative bacteria?

Acts as a virulence factor, provides protection, helps evade immune system, and can cause antibiotic resistance

Why are infections with Gram-positive bacteria treated differently than Gram-negative infections?

The structural differences (presence/absence of outer membrane) affect which antibiotics will be effective

How can proteins in Gram-positive cell walls be harmful?

Some proteins can be toxic to the host

How can lipids in Mycobacterium cell walls harm human cells?

The mycolic acids can be harmful/deadly to human cells

Why can bacterial cell wall components stimulate antibody production?

They are large macromolecules that the immune system recognizes as foreign antigens

Define "cell envelope"

The boundary layers of bacteria that separate internal cellular reactions from the external environment

Define "mordant" in staining

A substance (like iodine) that helps fix dye to cellular structures

Define "selective permeability"

The ability of a membrane to control what substances can pass through it

Define "endotoxin"

A toxic component (LPS) of Gram-negative bacterial cell walls that can cause fever and shock when released into the bloodstream

Define "virulence factor"

A component that enhances a microorganism's ability to cause disease

What does "homogenous" mean when describing Gram-positive cell walls?

Uniform in composition; made up of the same material (peptidoglycan) throughout

Concentration of Hypertonic solutions

high solute concentration (causes cell dehydration)

Concentration of Hypotonic solutions

low solute concentration (causes cell swelling)

Cell wall thickness of Gram-positive bacteria

thick (20-80 nm), homogenous peptidoglycan

Cell wall thickness of Gram-negative bacteria

thin (1-3 nm) peptidoglycan layer

What gives Gram-negative bacteria greater flexibility but also more sensitivity to lysis?

The thin peptidoglycan layer provides flexibility but less structural protection

How do the protective mechanisms of Mycoplasma compare to other bacteria?

Other bacteria rely on peptidoglycan cell walls; Mycoplasma uses sterols in their membrane for protection since they lack cell walls

Membrane components in bacteria

fatty acid-based phospholipids

Membrane components in archaea

branched hydrocarbons instead of fatty acids