Which of the following is the most characteristic morphological feature of Treponema pallidum that distinguishes it from other spirochetes?

Explanation

## Morphological Features of Treponema pallidum **Key Point:** *T. pallidum* possesses **4–8 periplasmic flagella** (endoflagella/axial filaments) inserted at each end of the organism, which overlap in the middle of the cell and are responsible for its characteristic corkscrew motility. ### Structural Characteristics | Feature | *T. pallidum* | Other Spirochetes | |---------|---------------|-------------------| | **Periplasmic flagella** | **4–8** (at each pole) | *Borrelia*: 15–20; *Leptospira*: 2 (1 per end) | | **Diameter** | 0.1–0.2 μm (very thin) | 0.2–0.5 μm | | **Coiling** | Tight, uniform, regular | Variable | | **Visibility** | Requires dark-field or silver stain | May be seen on routine staining | | **Cell wall** | Present (Gram-negative type) | Present in all spirochetes | **High-Yield:** According to standard microbiology references (Jawetz, Melnick & Adelberg; Ananthanarayan & Paniker), *T. pallidum* has **4–8 periplasmic flagella** per pole. This number is a defining morphological feature that distinguishes it from *Leptospira* (which has only 1 flagellum per end, giving it a hooked appearance) and *Borrelia* (which has 15–20 flagella). The figure of "8–12" cited in some sources refers to the total count across both poles in certain strains, but the standard textbook value for the characteristic flagellar number is **4–8**. **Clinical Pearl:** Because *T. pallidum* is extremely thin (0.1–0.2 μm) and cannot be stained by the Gram method, direct visualization requires **dark-field microscopy** or **silver staining (Warthin–Starry stain)**. The periplasmic flagella drive the corkscrew motility that enables penetration of intact mucous membranes and rapid dissemination in early syphilis. ### Why This Matters The number and arrangement of periplasmic flagella are key taxonomic and functional features of spirochetes. Knowing that *T. pallidum* has 4–8 flagella per pole (Option A) — as opposed to the 2–3 external flagella of non-spirochete bacteria (Option C) or the absence of a cell wall (Option D, factually incorrect) — is a classic NEET PG direct-recall point.