ß-Lactams: Mechanisms of Action and Resistance

Scientific Concepts & Nature Phenomena Presented

Bacterial Cell Structure

Bacteria have:

• A cell membrane
• A cell wall
• In some bacteria, an additional outer layer

Internal components include:

• Cytoplasm containing ribosomes
• A nuclear region
• Sometimes granules/vesicles

External structures may include:

• Capsule
• Flagella
• Pili

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Gram-Positive vs Gram-Negative Differences

• Gram-negative bacteria
  • The space between the cell membrane and cell wall is the paraplasmic space.
• Gram-positive bacteria
  • Generally no paraplasmic space
  • “Paraplasm” is described as an area for metabolic digestion and where new peptidoglycan is attached.

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Peptidoglycan (Cell Wall) Chemistry

Peptidoglycan is described as a polymer of:

• Alternating N-acetylmuramic acid
• and N-acetylglucosamine

Key features:

• Cross-linking is done by chains of four amino acids.
• Function:
  • Maintains cell shape
  • Prevents osmotic bursting by resisting water influx

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Peptidoglycan Synthesis / Assembly Pathway (Methodology)

Steps outlined in sequence:

1. Add five amino acids to N-acetylmuramic acid.
2. Add N-acetylglucosamine to form a peptidoglycan precursor.
3. Transport the precursor across the cell membrane into the paraplasm.
4. Bind precursors to cell wall acceptors in the paraplasm and perform extensive cross-linking.
5. Form multiple peptidoglycan layers that are cross-linked to build the wall.

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Cross-Linking Enzymes / Penicillin-Binding Proteins (PBPs)

Two major enzymes are described:

• Transpeptidase (cross-linking)
• (D)alanyl carboxypeptidase (spelled variably in subtitles)

They are also called penicillin-binding proteins (PBPs) because they bind:

• Penicillins
• Cephalosporins

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β-Lactam Antibiotics: Mechanism of Action

• β-lactams include penicillins and cephalosporins containing a β-lactam ring.
• The β-lactam ring binds to PBPs (transpeptidase and carboxypeptidase), blocking cross-linking of peptidoglycan.

Outcome:

• Failure of cell wall synthesis → bacterial damage/lysis
• In Gram-positive bacteria (high internal osmotic pressure):
  • Loss of a rigid wall → bursting in a hypotonic environment
• The PBP complex is described as stimulating autolysins that digest existing cell wall.

Conclusion: β-lactams are described as bactericidal agents.

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β-Lactam Resistance Mechanisms

1) Transformation (Horizontal Gene Transfer) Leading to Altered PBPs

Transformation:

• DNA carrying resistance genes can be released into the environment when a resistant bacterium dies.
• A nearby sufficiently similar bacterium can uptake naked DNA.
• Homologous transformation transfers the resistance gene into the chromosome.

Over time:

• Remodeling can occur so that genes encoding PBPs are altered.
• Altered PBPs still cross-link peptidoglycan but have reduced affinity for β-lactams → resistance.

Example:

• Penicillin-resistant Streptococcus pneumoniae via gene acquisition from naturally penicillin-resistant Streptococcus species.

2) Enzymatic Inactivation / Modification of β-Lactam Drugs

Bacteria can produce enzymes that destroy/modify β-lactams before they act.

• The resistance gene may be on:
  • Chromosomal DNA
  • or plasmids (small self-replicating genetic elements)

Conjugation spreads resistance plasmids:

1. Bacteria form close contact → a channel forms
2. One bacterium transfers a plasmid copy to the other
3. Expression can produce resistance enzymes

Drug-destroying enzymes:

• β-lactamases

Expression patterns by Gram type:

• Gram-positive: β-lactamase is inducible (more produced in presence of drug)
• Gram-negative: β-lactamase is constitutive (produced even without drug)

Location and efficiency:

• Gram-positive: enzyme released extracellularly, inactivates drug before it enters the cell
• Gram-negative: enzyme retained in the paraplasmic space, enabling more efficient inactivation

Ultimately:

• Destruction of the β-lactam ring prevents binding to PBPs, conferring resistance.

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Researchers or Sources Featured

• No specific researchers, institutions, or named scientific papers were mentioned.