**Core Concept**
Gramicidin S is a type of antibiotic that belongs to the polymyxin class. It is a mixture of two cyclic decapeptides, gramicidin S1 and gramicidin S2, which are produced by the bacterium *Bacillus brevis*.

**Why the Correct Answer is Right**
Gramicidin S works by binding to the bacterial cell membrane, causing a disruption in the membrane potential and ultimately leading to cell death. This is achieved through the interaction of the antibiotic with the bacterial cell membrane's phospholipid bilayer, which is rich in negatively charged lipids. The positively charged gramicidin S molecules are able to insert themselves into the membrane, causing a disruption in the membrane's structure and function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because gramicidin S is not derived from *Staphylococcus aureus*, which is a different bacterium altogether.
**Option B:** This option is incorrect because vancomycin is a different type of antibiotic, derived from *Amycolatopsis orientalis*, and has a different mechanism of action.
**Option C:** This option is incorrect because polymyxin B is a different type of antibiotic, also derived from *Paenibacillus polymyxa*, and has a different structure and mechanism of action.

**Clinical Pearl / High-Yield Fact**
Gramicidin S is a narrow-spectrum antibiotic, meaning it is effective against a limited range of bacteria, primarily Gram-positive cocci. It is often used in combination with other antibiotics to enhance its effectiveness.

**Correct Answer:** D. *Bacillus brevis*