**Core Concept**
Penicillin G is a narrow-spectrum beta-lactam antibiotic that acts by inhibiting cell wall synthesis in susceptible bacteria. Its mechanism of action involves binding to penicillin-binding proteins (PBPs) in the bacterial cell wall, leading to a disruption in the cross-linking of peptidoglycan layers.

**Why the Correct Answer is Right**
The correct answer is **Listeria monocytogenes**. This is because penicillin G is effective against Listeria monocytogenes due to its ability to inhibit the synthesis of the bacterial cell wall. Listeria monocytogenes is a gram-positive bacterium that causes listeriosis, a serious infection that can be life-threatening in immunocompromised individuals. The use of penicillin G is recommended for the treatment of listeriosis, particularly in pregnant women and individuals with compromised immune systems.

**Why Each Wrong Option is Incorrect**

* **Option A:** Staphylococcus aureus - This is incorrect because Staphylococcus aureus has developed resistance to penicillin G through the production of beta-lactamase enzymes, which break down the beta-lactam ring of penicillin G.
* **Option B:** Pseudomonas aeruginosa - This is incorrect because Pseudomonas aeruginosa is resistant to penicillin G due to its ability to produce beta-lactamase enzymes and its outer membrane porin proteins, which prevent the entry of penicillin G into the bacterial cell.
* **Option C:** Escherichia coli - This is incorrect because Escherichia coli is often resistant to penicillin G due to the production of beta-lactamase enzymes and the presence of efflux pumps, which reduce the intracellular concentration of penicillin G.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the use of penicillin G in the treatment of listeriosis should be guided by susceptibility testing and should be used in conjunction with supportive care, such as hydration and management of symptoms.

**Correct Answer: C. Escherichia coli is often resistant to penicillin G and is not typically treated with it, however the correct answer is not listed.