A 32-year-old farmer from Tamil Nadu presents with fever, myalgia, and jaundice on day 7 of illness. Blood culture grows a spirochaete later identified as Leptospira interrogans. Regarding the structural and antigenic features of Leptospira, all of the following are correct EXCEPT:

Question

Accepted Answer

C. Leptospira contains a peptidoglycan layer typical of gram-positive bacteria. ## Structural Features of Leptospira

### Gram-Negative Spirochaete Architecture

**Key Point:** Leptospira is a gram-negative spirochaete with a unique cell wall structure that differs fundamentally from gram-positive bacteria.

### Cell Wall Composition

Leptospira possesses:
1. **Outer membrane** — contains lipopolysaccharide (LPS) endotoxin [cite:Park 26e Ch 34]
2. **Peptidoglycan layer** — **thin and sparse**, not the thick layer seen in gram-positive bacteria
3. **Inner (cytoplasmic) membrane** — surrounds the protoplasm
4. **Periplasmic flagella** — 2 axial filaments located in the periplasmic space between outer and inner membranes; these enable motility in viscous environments

### Why Option 3 is Wrong

**High-Yield:** The critical distinction is that Leptospira, like all spirochaetes, has a **gram-negative cell wall architecture**. While it does contain peptidoglycan, the amount is **minimal and atypical** — far less than gram-positive bacteria. The question states "peptidoglycan layer typical of gram-positive bacteria," which is **factually incorrect**. Leptospira's peptidoglycan is neither typical of gram-positive bacteria nor present in the same quantity.

### Antigenic Determinants

| Feature | Role |
|---------|------|
| LPS (somatic antigen) | Serovar differentiation; elicits IgM response |
| OMPs (outer membrane proteins) | Major antigens; used for serovar classification |
| Flagellar antigens | Less prominent in serovar typing |

**Clinical Pearl:** The two major serogroups are *Leptospira interrogans* (pathogenic) and *Leptospira biflexa* (saprophytic), with >250 serovars identified based on LPS and OMP variation.

### Summary Table

| Structural Component | Present in Leptospira? | Characteristic |
|---|---|---|
| LPS endotoxin | Yes | Gram-negative feature |
| Periplasmic flagella | Yes | 2 axial filaments |
| Peptidoglycan (typical gram-positive) | **No** | Minimal, atypical; gram-negative pattern |
| OMPs | Yes | Serovar determinants |

Answer

A. Leptospira possesses a lipopolysaccharide (LPS) endotoxin in its outer membrane

Answer

B. The organism has periplasmic flagella located between the outer and inner membranes

Answer

D. The major outer membrane proteins (OMPs) are important for serovar differentiation

A 32-year-old farmer from Tamil Nadu presents with fever, myalgia, and jaundice on day 7 of illness. Blood culture grows a spirochaete later identified as Leptospira interrogans. Regarding the structural and antigenic features of Leptospira, all of the following are correct EXCEPT:

Explanation

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