IHD 11

Antibiotics for the Treatment of Community-Acquired Infections

Learning Outcomes

At the end of this lecture, students should be able to:

1. Recall the principal components of the bacterial cell wall, particularly how the assembly of its components can be inhibited by β-lactam antibiotics.

2. Describe the biochemical mechanisms by which penicillin and other β-lactams inhibit their target enzymes.

3. Identify the common structural motif of all β-lactam antibiotics, highlighting the distinctions between the backbones of penicillins and cephalosporins.

4. Explain the therapeutic rationale for combining a β-lactam antibiotic with a β-lactamase inhibitor to effectively treat infections that have developed resistance.

Introduction

Antibiotics

Definition: Antibiotics are biological agents that act "against life," specifically targeting bacterial infection without affecting human cells directly.

Antibiotics = agents ‘against life’

Key Characteristics

• Antibiotics can be classified primarily as antibacterial agents, with a smaller subset classified as anti-fungal agents.

• These agents can be derived from natural sources or synthesized in a laboratory setting. The vast majority of clinically used antibiotics are based on structures derived from natural products.

Community-Acquired Infections

→ Infections acquired in the community (outside of hospitals)

Community-Acquired Pneumonia (CAP)

• CAP = one of the most common infectious diseases globally,

  • (lower respiratory tract infection)

• Lower respiratoy tract infections were responsible for > 1.5 million deaths associated w/ antibiotic resistance in 2019

• community-acquired pneumonia (CAP) has a very significant impact on health and society.

• CAP typically presents with a range of pathogens that may not be immediately identified, so empiric treatments are implemented.

Antibiotic Treatment for CAP

These options are only for patients without comorbidities.

Treatment Options

1. Oral Amoxicillin:

   • Type: Penicillin (β-lactam) antibiotic

   • Has broad applications

     • Effective against respiratory infections and also in treating conditions such as bronchitis, acute otitis media, and urinary tract infections.

2. Oral Doxycycline:

   • Type: Tetracycline antibiotic

   • Also broad spectrum

   • doxycycline works by inhibiting bacterial protein synthesis, making it effective against a variety of bacterial pathogens.

β-Lactam Antibiotics Target

Structural Components

• Bacterial cell wall

• Peptidoglycan

• Transpeptide linkages

Bacterial Cell Wall

Structure:

• Composed of layer of peptidoglycan

  • peptidoglycan is unique to bacterial cell walls

Function:

• To protect the cell and maintain it’s shape

Peptidoglycan

*Only found in bacteria

Structure of Peptidoglycan:

• Peptidoglycan is made up of long chains of two alternating sugar molecules:

  • N-acetylglucosamine (NAG)

  • N-acetylmuramic acid (NAM)

• These sugar chains are linked together by short peptides (chains of amino acids).

Cross-Linking - The Key to Strength:

• To create a strong and rigid cell wall, the peptide chains attached to the NAM sugars are cross-linked. This cross-linking involves the formation of peptide bonds between amino acids in adjacent peptide chains.

• Enzymes called transpeptidases (also known as penicillin-binding proteins or PBPs) are responsible for catalyzing these cross-linking reactions.

Importance of Cross-Linking:

• Cross-linking provides the peptidoglycan layer with its strength and rigidity. The degree of cross-linking can vary between bacterial species.

• Inhibition of peptidoglycan cross-linking weakens the cell wall, making the bacterial cell vulnerable to lysis (bursting) due to osmotic pressure.

Mechanism of Cross-Linking

Key Points

• Peptide Bonds: These bonds are stable and not easily hydrolyzed under normal physiological conditions.

• Transpeptide Linkage: This process involves forming new peptide bonds—critical for the structural integrity of the cell wall.

• The mechanism of action for β-lactam antibiotics is closely tied to the disruption of these cross-links.

Enzymatic Catalysis in Peptidoglycan Cross-Linking

Steps in the Reaction

1. Formation of Ester Bond: This reaction occurs with the glycopeptide transpeptidase enzyme.*

2. Second Peptidoglycan Strand: Amino group from the second peptidoglycan strand attacks the reactive ester

3. Transpeptide linkage forms

Penicillins Structure and Activity

Structure of penicillins and beta-Lactam antibiotics

Core Penicillin Structure

• All penicillin derivatives possess a core bicyclic ring structure complemented by a variable R-group, which determines their specific properties and efficacy.

• The penicillin core is characterized by Cys (cysteine) and Val (valine) combinations, contributing to their mechanism of action.

Characteristics of Amoxicillin

• Lactam Type: Contains cyclic amide

• Beta-Lactam: cyclic amide with 4 atoms in its ring

• Ring Strain: The cyclic structure results in significant ring strain due to bond angle deviations, enhancing the reactivity of the amide bond within the antibiotic.

  • ‘Antibiotic Warhead’

3-D structure of penicillin

Penicillin has a unique non-planar, half-open book shape, critical for its interaction with bacterial enzymes. Structural determination achieved by Dorothy Crowfoot Hodgkin earned her the Nobel Prize in 1964.

Mechanism of action

How Penicillin Targets Bacteria

• Penicillin mimics the terminal portion of peptidoglycan, with its specific structural shape allowing it to fit and inhibit the glycopeptide transpeptidase enzyme effectively.

• Penicillins as Suicide Inhibitors: This interaction with the enzyme leads to the formation of a new ester bond that irreversibly inactivates the transpeptidase, resulting in weakened bacterial cell walls and heightened susceptibility to osmotic pressure, making the bacteria more vulnerable to lysis.

Consequenses of Transpeptide Inhibition

• No crosslinking of peptidoglycan strands

• weak cell walls

• Bacterial cells become osmotically fragile

Other β-Lactam Antibiotics: Cephalosporins

Structural and Mechanism Differences

Found in:

• Cephalosporins were first sourced from the fungus Cephalosporium acremonium

  • Found in a group of microbes in sewage

Function:

• Same mechanism as penicillins

  • Inhibition of glycopeptide transpeptidase enzyme

• A cephalosporin can be prescribed in combination with another antibiotic (e.g., doxycycline) for outpatients with comorbidities

• Broad spectrum antibiotics

Penicillin vs. Cephalosporins:

• Penicillin - sulphur containing 5 membered ring

• Cephalosporins - sulfur containing 6 membered ring

Summary of Part 1

• Glycopeptide transpeptidase is required for the biosynthesis of the cell wall, a structure essential for bacteria survival.

Penicillin and Cephalosporin similarities:

• Both inhibit the bacterial enzyme glycopeptide transpeptidase

• Both mimic the natural substrate of glycopeptide transpeptidase because of their defined structures

• Both are reactive because of their strained structures (beta-lactam)

Resistance to β-Lactam Antibiotics

Mechanisms of Bacterial Resistance

Main mechanism of resistance in Gram-negative bacteria. The primary mechanism involves β-lactamases:

• β-Lactamases: Enzymes produced by certain bacteria that can hydrolyze penicillins and cephalosporins, neutralizing their antibiotic effects.

  • These mutated transpeptidases render the drugs ineffective against resistant bacterial strains.

Action of β-Lactamases

Bacteria produce β-lactamase enzymes that break the β-lactam ring, rendering the antibiotic inactive.

Hydrolysis of Key Bonds

• The hydrolysis of the β-lactam ring structure results in the loss of the antibiotic's effectiveness and its ability to react with the target enzymes, undermining treatment options for infections caused by resistant strains.

Combination of β-Lactam Antibiotic with a β-Lactamase Inhibitor

The β-Lactamase Inhibitor works to prevent the enzyme β-Lactamase breaking the β-Lactam ring of the antibiotic, and the antibiotic is able to carry out it’s function.

Innovative Approaches to Resistance Reduction

• The use of β-lactamase inhibitors, such as clavulanic acid, serves to block the active site of β-lactamases, thus reinforcing the efficacy of existing antibiotics like amoxicillin against resistant infections, allowing for better clinical outcomes in the treatment of infections that would otherwise evade standard antibiotic therapy.