<<–2/”>a href=”https://exam.pscnotes.com/5653-2/”>p>The cell wall is a crucial structure in bacterial cells, providing shape, protection, and support. Bacteria are broadly classified into two groups based on their cell wall composition and staining characteristics: Gram-positive and Gram-negative. This Classification is based on the Gram stain method, developed by Hans Christian Gram in 1884. The primary distinction lies in the structural differences of their cell walls, which affect their staining properties, susceptibility to antibiotics, and overall behavior in different environments.
| Feature | Gram-Positive Cell Wall | Gram-Negative Cell Wall |
|---|---|---|
| Peptidoglycan Layer | Thick, multilayered (20-80 nm) | Thin, single-layered (7-8 nm) |
| Teichoic Acids | Present, embedded in the peptidoglycan layer | Absent |
| Outer Membrane | Absent | Present, contains lipopolysaccharides (LPS) |
| Periplasmic Space | Generally absent or very thin | Present, between the outer membrane and peptidoglycan |
| Lipid Content | Low | High, due to the presence of the outer membrane |
| Porins | Absent | Present, allow the passage of small Molecules |
| Sensitivity to Penicillin | More sensitive | Less sensitive |
| Resistance to Physical Disruption | More resistant | Less resistant |
| Gram Staining | Retains crystal violet stain, appears purple | Does not retain crystal violet, appears red/pink |
| Flagella Structure | 2 rings in the basal body | 4 rings in the basal body |
| Lysozyme Sensitivity | High | Low |
| Toxins Produced | Exotoxins | Endotoxins (LPS) and exotoxins |
| Example Organisms | Staphylococcus aureus, Streptococcus pneumoniae | Escherichia coli, Salmonella typhimurium |
Q1: What is the main structural difference between Gram-positive and Gram-negative cell walls?
A: The main structural difference is the thickness of the peptidoglycan layer and the presence of an outer membrane. Gram-positive bacteria have a thick peptidoglycan layer and no outer membrane, while Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane.
Q2: Why do Gram-positive bacteria retain the crystal violet stain?
A: Gram-positive bacteria retain the crystal violet stain due to their thick peptidoglycan layer, which traps the dye during the Gram staining process.
Q3: How does the outer membrane of Gram-negative bacteria contribute to antibiotic resistance?
A: The outer membrane of Gram-negative bacteria acts as a barrier to many antibiotics and detergents, limiting their access to the inner peptidoglycan layer and the plasma membrane, thereby contributing to antibiotic resistance.
Q4: What role do teichoic acids play in Gram-positive bacteria?
A: Teichoic acids play several roles in Gram-positive bacteria, including contributing to the rigidity of the cell wall, regulating ion passage, and playing a role in the bacterial pathogenicity.
Q5: Why are Gram-negative bacteria more likely to cause septic shock?
A: Gram-negative bacteria are more likely to cause septic shock because their outer membrane contains lipopolysaccharides (LPS), which are potent endotoxins that can trigger severe immune responses in the host.
Q6: Can Gram-positive bacteria produce endotoxins?
A: No, Gram-positive bacteria typically produce exotoxins, which are proteins secreted by the bacteria. Endotoxins are specific to Gram-negative bacteria and are a component of the outer membrane.
Q7: Why are Gram-negative bacteria generally more resistant to penicillin?
A: Gram-negative bacteria are generally more resistant to penicillin because their outer membrane provides an additional barrier that prevents the antibiotic from reaching the peptidoglycan layer effectively.
Q8: What types of infections are commonly caused by Gram-positive bacteria?
A: Gram-positive bacteria commonly cause skin infections, respiratory infections, and soft tissue infections. Examples include Staphylococcus aureus causing skin abscesses and Streptococcus pneumoniae causing pneumonia.
Q9: How do porins function in Gram-negative bacteria?
A: Porins are proteins located in the outer membrane of Gram-negative bacteria that form channels, allowing the passive diffusion of small molecules and nutrients into the cell.
Q10: Are there any bacteria that do not fit into the Gram-positive or Gram-negative categories?
A: Yes, some bacteria do not fit neatly into these categories. For example, Mycobacterium species have a unique cell wall structure with a high lipid content, making them acid-fast rather than Gram-positive or Gram-negative.