**Core Concept**
The difference between gram-positive and gram-negative bacteria is primarily based on the composition and structure of their cell walls, which affects their staining properties and response to antibiotics. This distinction is critical in identifying and treating bacterial infections.

**Why the Correct Answer is Right**
Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, which retains the crystal violet stain used in the Gram staining procedure. In contrast, gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane containing lipopolysaccharides, which prevents the stain from penetrating. This difference in cell wall structure also affects the bacteria's susceptibility to antibiotics, with gram-positive bacteria generally being more susceptible to cell wall inhibitors like penicillin.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the difference between gram-positive and gram-negative bacteria is not solely based on their shape.
* **Option B:** This option is incorrect because the difference in cell wall structure is not the primary factor in the distinction between gram-positive and gram-negative bacteria.
* **Option C:** This option is incorrect because the presence or absence of a capsule is not a defining characteristic of gram-positive versus gram-negative bacteria.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the Gram staining procedure is a rapid diagnostic tool used to identify the presence of bacteria in a specimen. However, it's not a definitive test and should be used in conjunction with other diagnostic methods to confirm the identity of the bacteria.

**Correct Answer: C.** The difference between gram-positive and gram-negative bacteria lies in their cell wall structure.