**Core Concept**
Phage typing is a method used to classify bacteria based on their susceptibility to specific bacteriophages. This technique relies on the ability of certain phages to infect and lyse specific bacterial strains, making it a valuable tool for distinguishing between different serotypes of a given species.

**Why the Correct Answer is Right**
Staphylococci, particularly Staphylococcus aureus, have been extensively typed using phage typing due to their clinical significance and the need for precise identification of strains. This method has been widely employed to differentiate between various epidemic and pandemic strains of S. aureus, which can cause a range of infections from mild skin conditions to life-threatening diseases like septicemia. The ability of phage typing to identify specific strains of S. aureus is crucial for epidemiological investigations and infection control measures.

**Why Each Wrong Option is Incorrect**
* **Option B:** E. coli, while an important human pathogen, is not typically classified using phage typing. Instead, other methods such as serotyping, biotyping, and molecular typing are commonly employed to identify and differentiate E. coli strains.
* **Option C:** Klebsiella pneumoniae can be typed using various methods, including capsular typing and molecular techniques, but phage typing is not a primary method used for this species.
* **Option D:** Pseudomonas aeruginosa has been typed using several methods, including biotyping, serotyping, and molecular techniques, but phage typing is not a widely used method for this species.

**Clinical Pearl / High-Yield Fact**
Phage typing is an example of a serotyping method that relies on the interaction between a bacteriophage and its bacterial host. This interaction is highly specific, making phage typing a valuable tool for distinguishing between different strains of a given species.

**✓ Correct Answer: A. Staphylococci**