**Question:** All of the following statements about Penicillin Binding Proteins (PBPs) are true, EXCEPT:

A. Penicillin binding to PBPs inhibits bacterial cell wall synthesis.
B. PBPs are involved in cross-linking peptidoglycan strands in the cell wall.
C. PBPs are transmembrane proteins that are essential for cell division.
D. Streptococcus pneumoniae is resistant to penicillin due to a single point mutation in PBP2x.

**Correct Answer: D. Streptococcus pneumoniae is resistant to penicillin due to a single point mutation in PBP2x.**

**Core Concept:** Penicillin Binding Proteins (PBPs) are enzymes that play a crucial role in bacterial cell wall synthesis and maintenance. PBPs are responsible for cross-linking peptidoglycan strands, providing structural integrity to the cell wall. Penicillin binds to these proteins, inhibiting their function and ultimately leading to cell wall disruption and death in susceptible bacteria.

**Why the Correct Answer is Right:** In the case of Streptococcus pneumoniae, penicillin resistance arises due to a single point mutation in PBP2x (β-lactamase-resistant PBP). This mutation alters the enzyme's binding site, rendering it incapable of interacting with penicillin and thus, the bacteria remains unaffected by penicillin therapy.

**Why Each Wrong Option is Incorrect:**

A. Penicillin binding to PBPs inhibits bacterial cell wall synthesis is true, as mentioned above. PBPs are essential for cell wall synthesis, and penicillin binding leads to inhibition.

B. PBPs are transmembrane proteins that are essential for cell division is partially correct. PBPs are indeed transmembrane proteins, but their primary function is to facilitate cross-linking of peptidoglycan strands, ensuring cell wall integrity and bacterial survival.

C. PBPs are involved in cross-linking peptidoglycan strands in the cell wall is also partially correct. PBPs do play a role in cross-linking peptidoglycan strands, but the statement is overly broad, encompassing both the primary function of PBPs (cross-linking peptidoglycan strands) and their role in cell division, which is not a direct function of PBPs.

**Core Concept:** Penicillin is a β-lactam antibiotic that is active against gram-positive bacteria, including Streptococcus pneumoniae. Its mechanism of action involves binding to PBPs, preventing cross-linking of peptidoglycan strands and resulting in cell wall instability and bacterial cell death.

**Why Each Wrong Option is Incorrect:**

A. Penicillin binding to PBPs is responsible for cross-linking peptidoglycan strands is true, as mentioned above.

B. PBPs are involved in cell division is incorrect as PBPs primarily function in cross-linking peptidoglycan strands, not cell division. While cell division is indirectly affected due to cell wall instability caused by penicillin binding, this option is too broad and does not specifically address the primary function of PBPs.