Penicillin Resistance

Introduction to Penicillin Resistance and Antibiotic Pressure

  • Selective Pressure: Antibiotics do not create resistance but rather exert selective pressure. This environment selects for mutations that provide bacteria with a survival advantage against the drug.

  • Mechanism of Action: Penicillins function by binding to and inactivating Penicillin Binding Proteins (PBPsPBPs). These proteins are essential for peptidoglycan synthesis; their inactivation weakens the bacterial cell wall, leading to cell death.

Principal Mechanisms of Resistance

Bacteria utilize four primary strategies to resist the effects of penicillins:

  • Restricting Access: Changes in permeability (e.g., alterations in outer membrane porins) prevent the drug from entering the cell.

  • Efflux Pumps: Active transport of the antibiotic out of the bacterial cell before it can reach its target.

  • Production of β-lactamases\beta\text{-lactamases}: Enzymes that degrade and inactivate the antibiotic before it can bind to its target.

  • Target Modification (PBPPBP Modification):

    • Mutational Change: Alterations in existing PBPPBP genes (e.g., Streptococcus pneumoniae).

    • Acquisition: Gaining a novel, resistant PBPPBP via horizontal gene transfer (e.g., Staphylococcus aureus).

Drug Inactivation: β-lactamases\beta\text{-lactamases}

  • The β-lactam\beta\text{-lactam} Ring: This structure is essential for the function of penicillin. β-lactamases\beta\text{-lactamases} work by hydrolyzing the amide bond of the β-lactam\beta\text{-lactam} ring, thereby inactivating the antibiotic.

  • Efficiency: β-lactamase\beta\text{-lactamase} molecules are highly active enzymes; a single molecule can hydrolyze 10310^3 penicillin molecules per second.

Molecular Classes of β-lactamases\beta\text{-lactamases}

There are four molecular classes (A-D) based on catalytic mechanisms:

  • Serine-β-lactamases\beta\text{-lactamases} (Classes A, C, and D): Use a serine residue in the active site for nucleophilic attack. They have a smaller, less flexible active site groove resulting in a narrower spectrum of activity.

  • Metallo-β-lactamases\beta\text{-lactamases} (Class B): These are metal-dependent, using a Zinc (ZnZn) ion to coordinate water molecules for the attack on the β-lactam\beta\text{-lactam} ring. They feature a wide, flexible active site groove, providing a broad spectrum of activity against most β-lactam\beta\text{-lactam} antibiotics (excluding monobactams).

Comparison by Gram Stain

  • Gram-Negative Bacteria:

    • Example: E. coli.

    • Location: β-lactamase\beta\text{-lactamase} is secreted and stored in the periplasmic space between the inner and outer membranes.

    • Expression: Often constitutively expressed (present even without the antibiotic).

    • Selection: Constant presence leads to the selection of increasingly potent variants.

  • Gram-Positive Bacteria:

    • Example: Staphylococcus aureus.

    • Expression: Typically inducible production. Large amounts are produced only when β-lactams\beta\text{-lactams} are detected in the environment.

    • Location: Lacks a periplasmic space; therefore, the enzyme is secreted into the extracellular environment to inactivate antibiotics before they reach the cell.

Summary of Chemical Reaction Mechanism

  1. Non-covalent association: β-lactamase\beta\text{-lactamase} binds to penicillin.

  2. Nucleophilic Attack: The hydroxyl group of a serine residue (in serine-β-lactamases\beta\text{-lactamases}) attacks the β-lactam\beta\text{-lactam} carbonyl.

  3. Acyl Intermediate: A covalent bond forms between the enzyme and the antibiotic.

  4. Hydrolysis: A water molecule (held in the active site) hydrolyzes the bond, releasing inactive penicilloic acid.

  5. Regeneration: The enzyme is rejuvenated and ready to inactivate more antibiotic molecules.

Extended Spectrum β-lactamases\beta\text{-lactamases} (ESBLs)

  • Definition: Enzymes that can hydrolyze most β-lactams\beta\text{-lactams}, including older penicillins and many newer derivatives (e.g., third-generation cephalosporins).

  • Epidemiology: ESBLs emerged in the 1980s due to the widespread use of broad-spectrum antibiotics. They are typically plasmid-encoded or mobile, commonly found in Enterobacteriaceae like E. coli.

Target Site Modification: Low Affinity PBPs

Bacteria can modify their targets so that penicillins can no longer bind effectively, while the PBPPBP retains its ability to synthesize the cell wall.

  • Mechanism: Efficient acylation of the PBPPBP leads to cell death; inefficient acylation (due to low affinity) allow the bacteria to survive.

  • Mosaicism: The creation of resistant PBPPBP genes through homologous recombination of DNA fragments from different strains or species. This is often facilitated by horizontal gene transfer mechanisms like transformation.

Case Study: Streptococcus pneumoniae PBP1a and PBP2x

Resistance in S. pneumoniae is exclusively due to PBPPBP changes via mosaic genes. Resistance develops in a stepwise manner.

  • PBP2x Mutations:

    • T338AT338A combined with M339FM339F results in a 1,0001,000-fold reduction in acylation efficiency.

    • Q552EQ552E introduces a negative charge at the active site entrance, repelling negatively charged β-lactams\beta\text{-lactams}.

Case Study: Methicillin-Resistant Staphylococcus aureus (MRSA)

  • The mecA Gene: MRSA acquired the mecAmecA gene (likely from S. fleuretti) via horizontal gene transfer. This gene is located on a mobile element called the Staphylococcal cassette chromosome mec (SCCmecSCCmec).

  • PBP2a: The mecAmecA gene encodes a novel, 78 kDa78\,kDa protein called PBP2aPBP2a. It has transpeptidase activity but an extremely low affinity for almost all β-lactams\beta\text{-lactams}.

  • Allosteric Control: PBP2aPBP2a is controlled allosterically. The active site is closed until a substrate (nascent peptidoglycan) binds to the allosteric site. This binding triggers a conformational change mediated by salt bridge interactions (77 identified by crystallography, potentially 1717 total) that opens the active site.

  • Acylation Resistance: The acylation rate of PBP2aPBP2a is negligible (15 M−1s−115\,M^{-1}s^{-1}). Resistance is caused by a distorted active site where Ser403Ser403 is in a poor position for nucleophilic attack, and steric clashes occur with Gly599Gly599 and Ser598Ser598.

Impermeability and Efflux

  • Gram-Negative Specificity: These bacteria use the combination of β-lactamases\beta\text{-lactamases} and altered membrane permeability.

  • Porins: β-lactams\beta\text{-lactams} enter via porin channels (e.g., OmpFOmpF) in the outer membrane. Resistance occurs through porin loss or structural changes that block antibiotic passage.

  • Efflux Pumps: Active removal of the drug from the periplasm or cytoplasm.