All of the following statements regarding Gram-positive bacterial cell wall are TRUE EXCEPT:

Explanation

## Gram-Positive Cell Wall Structure ### Peptidoglycan Composition **Key Point:** Gram-positive bacteria have a thick peptidoglycan layer (20–80 nm) that comprises 40–50% of the cell wall dry weight, making it structurally dominant. ### Cross-Linking Mechanism **High-Yield:** Peptidoglycan cross-linking occurs between: - **D-alanine** (from one peptide chain) - **L-lysine** (from an adjacent peptide chain) — in *Staphylococcus aureus*, *Bacillus*, and most Gram-positive cocci - **Diaminopimelic acid (DAP)** — in some Gram-positive rods like *Bacillus* and *Clostridium* The statement claims D-alanine to L-lysine bridges, which is **partially correct but incomplete** — the actual cross-link is **D-alanine to the ε-amino group of L-lysine**, forming a peptide bond. However, the more critical error is that this is not the universal pattern; many Gram-positive bacteria use **DAP-to-D-alanine** cross-links instead. ### Teichoic and Lipoteichoic Acids **Key Point:** - **Teichoic acids** = polymers of ribitol or glycerol phosphate; **covalently linked** to peptidoglycan - **Lipoteichoic acids (LTA)** = anchored in the cell membrane via a lipid moiety; extend through the peptidoglycan layer ### Why Option 3 is Wrong The statement "Peptidoglycan is cross-linked via D-alanine to L-lysine bridges" is **misleading in its specificity**. While this cross-link exists in some Gram-positive cocci, it is NOT universal. Many Gram-positive bacteria (especially rods) use **diaminopimelic acid (DAP)** instead of L-lysine for cross-linking. The stem asks for the INCORRECT statement, and this overgeneralization makes it the best answer. **Mnemonic:** **PLTD** — Peptidoglycan, Lipoteichoic acids, Teichoic acids, DAP (or Lysine) — all present in Gram-positive walls. ## Comparative Table: Gram-Positive vs. Gram-Negative Cell Walls | Feature | Gram-Positive | Gram-Negative | | --- | --- | --- | | Peptidoglycan thickness | 20–80 nm (thick) | 2–3 nm (thin) | | % of cell wall dry weight | 40–50% | 5–10% | | Teichoic acids | Present | Absent | | Lipoteichoic acids | Present | Absent | | Outer membrane | Absent | Present | | Cross-link pattern | D-Ala–L-Lys or D-Ala–DAP | D-Ala–DAP (universal) | [cite:Prescott's Microbiology Ch 3]