Penicillins and macrolides

Penicillins

1. Structure and Mechanism of Action:

• Penicillins are beta-lactam antibiotics characterized by their beta-lactam ring structure.
• They inhibit bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), which are crucial for the cross-linking of peptidoglycan layers in bacterial cell walls. This leads to cell lysis and death, particularly in actively dividing bacteria.

2. Spectrum of Activity:

• Penicillins are primarily effective against gram-positive bacteria (e.g., Streptococcus, Staphylococcus) and some gram-negative bacteria (e.g., Neisseria).
• Some extended-spectrum penicillins (like piperacillin) have activity against certain gram-negative bacteria.

3. Types of Penicillins:

• Natural Penicillins: e.g., Penicillin G and Penicillin V. These are effective against many streptococci and some staphylococci.
• Aminopenicillins: e.g., Amoxicillin and Ampicillin. They have a broader spectrum and can target some gram-negative bacteria.
• Penicillinase-resistant Penicillins: e.g., Methicillin and Nafcillin. These are designed to resist degradation by staphylococcal beta-lactamase.
• Extended-spectrum Penicillins: e.g., Piperacillin and Ticarcillin. They are effective against a wider range of bacteria, including Pseudomonas aeruginosa.

4. Side Effects and Resistance:

• Common side effects include allergic reactions (ranging from rashes to anaphylaxis), gastrointestinal disturbances, and, in some cases, superinfections.
• Resistance can occur through the production of beta-lactamase enzymes by bacteria, which hydrolyze the beta-lactam ring, rendering the antibiotic ineffective.

Macrolides

1. Structure and Mechanism of Action:

• Macrolides are characterized by a large lactone ring and are derived from Streptomyces bacteria.
• They work by inhibiting protein synthesis by binding to the 50S subunit of the bacterial ribosome, blocking peptide elongation during translation.

2. Spectrum of Activity:

• Macrolides are primarily effective against gram-positive bacteria (e.g., Streptococcus pneumoniae) and atypical pathogens (e.g., Mycoplasma, Chlamydia, Legionella).
• They also have some activity against certain gram-negative bacteria, but their spectrum is generally narrower than that of penicillins.

3. Common Macrolides:

• Erythromycin: One of the first macrolides, often used for respiratory tract infections.
• Clarithromycin: An analog of erythromycin with improved pharmacokinetics and activity against some gram-negative bacteria.
• Azithromycin: Known for its long half-life, allowing for once-daily dosing. It’s commonly used for respiratory infections and STIs.

4. Side Effects and Resistance:

• Common side effects include gastrointestinal disturbances, such as nausea and diarrhea, and potential hepatotoxicity.
• Resistance mechanisms include methylation of the ribosomal RNA target site and efflux pumps that reduce intracellular concentrations of the antibiotic.

Summary

• Penicillins are primarily effective against gram-positive bacteria and work by disrupting cell wall synthesis, while macrolides target protein synthesis and are effective against a range of pathogens, including some atypical bacteria.
• Both classes face challenges related to bacterial resistance, necessitating ongoing surveillance and research into new antibiotic strategies.