Question

Which of the following cell wall components is unique to gram-negative cells? a. lipopolysaccharide b. teichoic acid c. mycolic acid d. peptidoglycan

Short answer

a. lipopolysaccharide

Step-by-step solution

Identify the Components

First, identify and understand each of the components listed in the options: lipopolysaccharide, teichoic acid, mycolic acid, and peptidoglycan.

Match Components to Cell Types

Next, match each of these components to either gram-positive or gram-negative cells. Peptidoglycan is present in both gram-positive and gram-negative cell walls but is thicker in gram-positive. Teichoic acid is unique to gram-positive cell walls. Mycolic acid is found in the cell walls of mycobacteria, which are neither truly gram-positive nor gram-negative.

Identify the Unique Component

Compare the remaining component, which is lipopolysaccharide. This component is unique to the outer membrane of gram-negative bacteria.

Key concepts

gram-negative bacteria

Gram-negative bacteria are a type of bacteria characterized by their cell wall structure. They have a thin layer of peptidoglycan between two membranes: an inner cytoplasmic membrane and an outer membrane.
This outer membrane is a defining feature and contains molecules like lipopolysaccharides (LPS).
Because of their cell wall composition, gram-negative bacteria do not retain the crystal violet stain used in the Gram staining procedure. Instead, they appear pink or red after being counterstained with safranin.

• Key identifier: Thin peptidoglycan layer
• Unique outer membrane containing LPS
• Gram stain response: pink/red

Examples of gram-negative bacteria include Escherichia coli and Pseudomonas aeruginosa.

lipopolysaccharide

Lipopolysaccharides (LPS) are large molecules found in the outer membrane of gram-negative bacteria. LPS play a crucial role in the structural integrity of the bacteria and protect against certain kinds of chemical attack.
The molecule is a combination of lipids and polysaccharides, which form a potent endotoxin, causing strong immune responses in animals.

• Key Parts of LPS:
• Lipid A: Anchors the LPS into the bacterial membrane and is responsible for the endotoxin activity.
• Core oligosaccharide: Connects Lipid A to the O-antigen and provides stability.
• O-antigen: Extends outward from the bacteria and can vary among different bacterial strains, aiding in immune system evasion.

LPS is a crucial factor in the pathogenicity of gram-negative bacteria.

gram-positive bacteria

Gram-positive bacteria have a thicker peptidoglycan layer which is vital for their survival. This thick layer is why they retain the crystal violet stain used in the Gram staining procedure and appear blue or purple under a microscope.
Another distinguishing feature is the presence of teichoic acids in their cell walls, which provide further strength and rigidity.

• Key identifier: Thick peptidoglycan layer
• Presence of teichoic acids
• Gram stain response: blue/purple

Examples of gram-positive bacteria include Staphylococcus aureus and Streptococcus pyogenes.

peptidoglycan

Peptidoglycan is an essential and highly conserved component of bacterial cell walls, made up of a polymer of sugars and amino acids. It provides structural strength and protects the cell from osmotic pressure.
In gram-positive bacteria, the peptidoglycan layer is thick, whereas, in gram-negative bacteria, it is much thinner.
The building block of peptidoglycan is a repeating disaccharide unit of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). These are linked by peptide chains, giving the structure its rigidity.

• Key Points: Thick layer in gram-positive bacteria
• Thin layer in gram-negative bacteria
• Made up of NAG and NAM
• Provides structural integrity

teichoic acid

Teichoic acids are unique to the cell walls of gram-positive bacteria. They are polymers of either glycerol or ribitol joined by phosphate groups.
These acids anchor the peptidoglycan layer to the cytoplasmic membrane.
Teichoic acids contribute to the cell wall's structural integrity and play roles in cell growth and division. There are two main types: wall teichoic acids (bound to the peptidoglycan) and lipoteichoic acids (anchored in the lipid membrane).

• Key Points: Found only in gram-positive bacteria
• Types: wall teichoic acids and lipoteichoic acids
• Roles: structural support, cell growth, and division.

mycolic acid

Mycolic acids are long-chain fatty acids found in the cell walls of mycobacteria, such as Mycobacterium tuberculosis. These acids are not present in standard gram-positive or gram-negative bacteria, making them unique.
Mycolic acids create a waxy, lipid-rich outer layer, which is hydrophobic and contributes to the bacterium's resistance to desiccation, detergents, and antibiotics. This makes infections caused by mycobacteria more challenging to treat.
Additionally, the presence of mycolic acids makes these bacteria resistant to the Gram staining procedure and require special staining methods such as the acid-fast stain.

• Key Points: Found in mycobacteria
• Creates a waxy, lipid-rich outer layer
• Contributes to resistance against environmental stresses and treatments