Mechanism of action of penicillin

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Mechanism of Action of Penicillin

1. Target: The Bacterial Cell Wall

The bacterial cell wall is a rigid, cross-linked polymer called peptidoglycan (also called murein). It consists of long chains of alternating amino sugars - N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) - with short pentapeptide side chains attached to the NAM residues. These side chains terminate in D-alanyl-D-alanine (D-Ala-D-Ala). For the wall to be structurally rigid and functional, these peptide side chains must be cross-linked to one another - a process called transpeptidation.

Bacterial cell wall showing NAG-NAM chains with PEP cross-links blocked by penicillin

Fig. 1 - The PEP side chains of adjacent peptidoglycan strands must be cross-linked as the final step in cell wall synthesis. Penicillin blocks this step. (Lippincott Illustrated Reviews: Pharmacology, Fig. 29.3)

2. Structural Mimicry - The Key to Penicillin's Action

Penicillin's core structure - the 4-membered β-lactam ring fused to a thiazolidine ring - closely resembles the terminal D-Ala-D-Ala dipeptide of the peptidoglycan strand. This structural mimicry allows penicillin to act as a competitive, irreversible inhibitor of the enzymes that carry out transpeptidation.

3. Penicillin-Binding Proteins (PBPs)

The enzymes responsible for transpeptidation are membrane-anchored enzymes called Penicillin-Binding Proteins (PBPs). They sit in the periplasmic space, embedded in the cytoplasmic membrane. PBPs normally:

Remove the terminal alanine from the D-Ala-D-Ala terminus
Form a covalent cross-link between adjacent peptide side chains

When penicillin (a structural analog of D-Ala-D-Ala) enters the cell, it covalently binds to the active site of PBPs, permanently inactivating them. This halts the final cross-linking step of peptidoglycan synthesis.

Cell wall structure showing porins, peptidoglycan layer, PBPs in the periplasmic space, and beta-lactamase

Fig. 2 - Cross-section of a gram-negative bacterial envelope. PBPs are located in the periplasmic space where they cross-link peptidoglycan. β-Lactamase, if present, also resides here and can destroy penicillin. (Katzung's Basic and Clinical Pharmacology, Fig. 43-3)

4. Consequences - Cell Lysis and Death

With PBPs inactivated:

New peptidoglycan strands cannot be cross-linked
The cell wall is weakened and loses structural integrity
Bacteria also activate their own autolysins (enzymes that normally remodel the cell wall during growth)
The combination of impaired wall synthesis + autolysin activity leads to cell lysis due to unopposed osmotic pressure
The result: bactericidal killing

Because this mechanism targets an actively synthesizing cell wall, penicillin kills only actively growing and dividing bacteria. It has no effect on dormant or non-replicating cells.

5. Pharmacodynamics

Penicillin is time-dependent (not concentration-dependent) in its killing. Efficacy is maximized by keeping drug concentrations above the MIC (minimum inhibitory concentration) for as long as possible during the dosing interval - not by using very high peak concentrations.

6. Gram-Positive vs. Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Cell wall thickness	Thick peptidoglycan layer, near surface	Thin peptidoglycan, deeper
Outer membrane	Absent	Present (LPS bilayer)
Drug access to PBPs	Easy - penicillin diffuses freely	Must pass through porins (water-filled channels)
General susceptibility	High (in absence of resistance)	Variable; depends on porin availability

7. Resistance Mechanisms

Three main ways bacteria evade penicillin:

β-Lactamase production - The most common mechanism. These enzymes hydrolyze the β-lactam ring, destroying penicillin's activity before it can reach PBPs. Gram-positive bacteria secrete them extracellularly; gram-negatives sequester them in the periplasmic space.
Altered PBPs - Mutations produce PBPs with low affinity for β-lactam antibiotics. MRSA (S. aureus) is the classic example: the mecA gene encodes a modified PBP2a that binds penicillin very poorly, conferring resistance to virtually all commercially available β-lactams.
Decreased permeability / efflux pumps - Gram-negative bacteria may lose or alter porins, reducing drug entry. Active efflux pumps (e.g., in Klebsiella pneumoniae, P. aeruginosa) can also pump the drug back out before it reaches PBPs.

Sources:

Lippincott Illustrated Reviews: Pharmacology, 7th ed., Chapter 29 - β-Lactam Antibiotics, pp. 941-948
Katzung's Basic and Clinical Pharmacology, 16th ed., Chapter 43 - Beta-Lactam & Other Cell Wall- & Membrane-Active Antibiotics, pp. 1239-1240

In short

Penicillin - Mechanism of Action (Short)

Penicillin inhibits bacterial cell wall synthesis by blocking the final step - transpeptidation (cross-linking of peptidoglycan strands).

How:

Penicillin mimics the D-Ala-D-Ala terminus of peptidoglycan
It covalently binds to Penicillin-Binding Proteins (PBPs) - the transpeptidase enzymes
Cross-linking is blocked → cell wall weakens
Autolysins degrade the defective wall → cell lysis and death

Key points:

Bactericidal, time-dependent killing
Works only on actively dividing bacteria
Gram-positive organisms are more susceptible (thicker, accessible peptidoglycan)
Main resistance: β-lactamase production (destroys the β-lactam ring)

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