Bacterial Cell Wall and Gram Staining

Questions and Answers

What distinguishes the outer membrane of Gram-negative bacteria from that of Gram-positive bacteria?

Absence of a peptidoglycan layer

Presence of lipopolysaccharides (correct)

Presence of porins only

Presence of teichoic acids

Which component of the Gram-negative cell wall is responsible for its resistance to certain antibiotics?

Peptidoglycan layer

Lipoprotein

Outer membrane (correct)

Teichoic acids

What is the primary toxic component of lipopolysaccharide (LPS) in Gram-negative bacteria?

Core unit

Outer leaflet

Polysaccharide chain

Lipid A (correct)

How does lipopolysaccharide (LPS) contribute to the immune response in vertebrates?

By activating the immune cells through O antigen Signup and view all the answers

Which characteristic is NOT associated with the structure of the peptidoglycan layer in Gram-negative bacteria?

Presence of teichoic acids Signup and view all the answers

The lipoproteins in the Gram-negative cell wall perform which crucial function?

Link the outer membrane to the peptidoglycan layer Signup and view all the answers

Which feature of the Gram-negative cell wall allows for the passive diffusion of small molecules?

Porins in the outer membrane Signup and view all the answers

In Gram-negative bacteria, the lipopolysaccharide (LPS) structure is primarily composed of which elements?

A core polysaccharide and a series of repeat units Signup and view all the answers

Why do Gram-negative bacteria generally have a higher resistance to antibiotics?

They possess an outer membrane that slows penetration Signup and view all the answers

Which component of Gram-negative bacteria’s outer membrane is primarily responsible for its toxic effects?

Lipopolysaccharide (LPS) Signup and view all the answers

Study Notes

Bacterial Classification and Cell Wall Structure

Bacteria are divided into Gram-positive and Gram-negative based on their cell wall structure and response to Gram staining, developed by Hans Christian Gram.
The Gram-staining procedure helps identify bacteria in infected tissues.
The cell wall offers osmotic protection and is crucial for cell division and its own biosynthesis.
Peptidoglycan, a key component of the bacterial cell wall, consists of three parts:
- A backbone of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by β1→4 bonds.
- Identical tetrapeptide side chains attached to NAM, with L-alanine in position 1, D-glutamate in position 2, and D-alanine in position 4.
- Variable amino acid at position 3: diaminopimelic acid in Gram-negative bacteria, L-lysine in Gram-positive bacteria.
Gram-positive bacteria typically have multiple layers (up to 40) of peptidoglycan, which constitutes 50% of their cell wall, while Gram-negative have 1-2 layers, making up only 5-10%.

Bacterial Cell Morphology

Bacteria are classified based on shape:
- Cocci: spherical or oval-shaped cells, can be arranged in clusters, pairs, or chains.
- Bacilli: rod-shaped cells, can also be arranged in chains or pairs, with some being curved or spiral.
- Coccobacilli: intermediate between cocci and bacilli, requiring careful differentiation.
Further classification by Gram property:
- Gram-positive cocci, Gram-negative cocci, Gram-positive bacilli, Gram-negative bacilli.
Some bacteria are weakly stained and require special techniques:
- Spirochetes: thin, spiral-shaped bacilli.
- Mycoplasma: lack a stable cell wall; appear as variable shapes.
- Rickettsiae and chlamydiae: obligate intracellular bacteria.

Components of Gram-Negative Cell Walls

Gram-negative bacteria have three critical components outside the peptidoglycan layer:
- Outer Membrane:
  - Bilayer structure with an inner leaflet similar to the cytoplasmic membrane and an outer leaflet containing lipopolysaccharide (LPS).
  - Contains porins that allow passive diffusion of small molecules; contributes to antibiotic resistance.
- Lipopolysaccharide (LPS):
  - Consists of lipid A (toxic component) tethered to a polysaccharide chain; acts as an endotoxin.
  - The O antigen contributes to immune response variability among species and strains.
- Lipoprotein:
  - Links the outer membrane to the peptidoglycan layer, consisting of 57 amino acids forming a unique sequence linked to DAP residues in the tetrapeptide chains.

Description

This quiz explores the classification of bacteria based on their Gram-staining response, highlighting the significance of the cell wall structure. Learn about the Gram procedure and its implications in identifying bacterial infections. Understand the role of peptidoglycan in cell division and its biosynthesis.