Bacterial Cell Wall Structure and Function

Main function of the bacterial cell wall

Maintains cell shape, counters osmotic pressure, protects cell contents, provides bacteriophage attachment sites, surface appendages, and antibiotic action site.

Is the bacterial cell wall selectively permeable?

No, it is porous and lets anything pass through that can fit its gaps.

Compound that uniquely characterizes bacterial cell walls

Peptidoglycan.

Two sugar molecules in the peptidoglycan structure

N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

Difference between Gram-positive and Gram-negative bacteria based on peptidoglycan layers

Gram-positive has many thick layers; Gram-negative has one or few thin layers.

Acids present in Gram-positive bacterial cell walls

Teichoic acids (wall and lipoteichoic acids).

Composition of the outer membrane of Gram-negative bacteria

Lipoproteins, proteins, lipopolysaccharides (LPS), and phospholipids.

Part of LPS responsible for its endotoxin properties

Lipid A.

Porins

Proteins in Gram-negative outer membrane allowing selective passage of molecules.

Periplasmic Space

The area between the outer and cytoplasmic membrane in Gram-negative bacteria, rich in enzymes and transport proteins.

Role of O antigen in LPS

Protection from host defenses.

How LPS contributes to Gram-negative bacteria's antibiotic resistance

It replaces phospholipids, creating a rigid, less penetrable barrier to large hydrophilic antibiotics.

Bacteria without cell walls

Mycoplasma, with stronger plasma membranes possibly containing sterols.

Characteristics of acid-fast bacterial cell walls

High lipid content (mycolic acid), poor permeability, and resistance to disinfectants and drying.

How archaeal cell walls differ from bacteria

They lack peptidoglycan; they may contain pseudopeptidoglycan/pseudomurein.

Effect of hypotonic solution on bacterial cells

Osmotic lysis, protected against by the cell wall.

Effect of hypertonic solution on bacterial cells

Plasmolysis, where water leaves the cell, cytoplasm shrivels and pulls away from the wall.

Practical importance of plasmolysis and osmotic lysis

Plasmolysis for food preservation; osmotic lysis as the basis for lysozyme and penicillin action.

Acid-fast staining process

Carbolfuchsin staining, heat application, acid alcohol decolorization, and methylene blue counterstain.

Bonds differentiating pseudopeptidoglycan in archaea from peptidoglycan in bacteria

Beta 1-3 glycosidic bonds instead of Beta 1-4.

Gram staining

It differentiates bacteria into Gram-positive (purple) and Gram-negative (red), based on cell wall composition.

Braun's lipoprotein

The most abundant protein in Gram-negative outer membranes, covalently linked to peptidoglycan, stabilizing the membrane.

Gram-negative outer membrane negative charge

Due to lipopolysaccharides (LPS), especially the core polysaccharide region.

Unusual sugars in core polysaccharide of LPS

KDO (2-Keto-3-Deoxyoctonate), heptulose (Hep), abequose (Abe).

Symptoms caused by Lipid A of LPS

High fever, low blood pressure, septicemia, toxic shock, possibly death.

Archaeal cell membrane lipids

They have glycerol diethers (bilayers) or glycerol tetraethers (monolayers), unlike bacteria.

Function of gas vacuoles in bacteria and archaea

To regulate buoyancy in aquatic environments.

Sensitivity to penicillin

Gram-positive bacteria are more sensitive to penicillin.

Backbone of teichoic acids

Ribitol (5-carbon) and glycerol (3-carbon).

Pseudomurein composition

N-Acetyltalosaminuronic acid instead of N-acetylmuramic acid, and L-amino acids instead of D-amino acids.

Protoplast

A bacterial cell completely lacking its cell wall.

Spheroplast

A bacterial cell with a partially degraded cell wall.

Acid-fast bacteria staining difficulty

Due to their thick lipid (mycolic acid) layer that prevents stain penetration and removal.

Phospholipid replacements in Gram-negative outer membranes

Lipopolysaccharides (LPS).

Thickness difference in cell walls

Gram-positive: 20-80 nm; Gram-negative: around 10 nm.

Key characteristic of mycoplasma bacteria

Lack of a cell wall and pleomorphic shape (variable shape).

Acid-fast bacteria resistance to disinfectants

Their thick lipid-rich cell walls prevent chemical penetration.

Lipid bilayer permeability of Gram-negative outer membrane

It is more permeable due to porin and transporter proteins.

Plasmolysis in food preservation

By causing water loss in bacteria, inhibiting their growth in hypertonic environments (e.g., dried foods, jams).

Structure of peptidoglycan

A lattice structure of repeating disaccharides (NAM and NAG) linked by polypeptides with alternating D- and L-amino acids.