Question

Bacteria have cell wall made of a polymer of N-acetylglucosamine and N-acetylmuramic acid connected by $\beta$-1,4 glycosidic bonds. Lysozyme, an enzyme, cleaves the glycosidic bond and disrupts the cell wall structure. In an experiment, Gram-positive and Gram-negative bacteria, which differ in terms of cell envelope and peptidoglycan thickness, are treated with lysozyme and subsequently placed in a hypotonic solution. The correct statement about the effect of the treatment is:

• A. Gram-positive bacteria are lysed, but not Gram-negative bacteria.
• B. Both Gram-positive and Gram-negative bacteria are lysed.
• C. Only Gram-negative bacteria are lysed.
• D. Neither Gram-positive nor Gram-negative bacteria are lysed.

Hint:

The presence of the outer membrane in Gram-negative bacteria serves as an effective barrier to many large macromolecules and enzymes, including lysozyme, explaining their resistance under typical experimental conditions.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question describes the action of lysozyme on the $\beta$-1,4 glycosidic bonds in the peptidoglycan cell wall of bacteria. It asks for the outcome when Gram-positive and Gram-negative bacteria are treated with lysozyme and then placed in a hypotonic solution.

Step 2: Detailed Explanation:

• Lysozyme is an enzyme found in secretions like tears and saliva, acting as an antibacterial agent.

• It specifically targets and hydrolyzes the $\beta$-1,4 glycosidic bonds between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in peptidoglycan.

• Gram-positive bacteria have a thick, outer peptidoglycan layer that is directly exposed to the external environment, making it highly susceptible to lysozyme digestion.

• Gram-negative bacteria, however, have a thin peptidoglycan layer that is protected by an outer membrane composed of lipopolysaccharides (LPS) and proteins.

• Because of this outer membrane barrier, lysozyme cannot easily access or degrade the peptidoglycan of Gram-negative bacteria unless the membrane is first disrupted.

• When Gram-positive bacteria are treated with lysozyme, their cell walls are destroyed, turning them into protoplasts that lack structural integrity.

• Upon subsequent transfer to a hypotonic solution, water rapidly enters the protoplasts due to osmosis, causing them to swell and lyse.

• In contrast, Gram-negative bacteria remain largely unaffected because their outer membrane prevents lysozyme from reaching the peptidoglycan layer.


Step 3: Final Answer:
Therefore, Gram-positive bacteria are lysed, but not Gram-negative bacteria.