Penicillins and cephalosporins are among the most widely used antibiotics in the world. They belong to a group called beta-lactam antibiotics, which kill bacteria by disrupting their cell wall. Because human cells do not have cell walls, these medicines act selectively on bacteria, making them safe and effective.
Mechanism of Action
Both penicillins and cephalosporins work by:
- Blocking the enzymes (penicillin-binding proteins) responsible for building the bacterial cell wall
- Weakening the cell wall
- Causing the bacteria to burst and die
They are considered bactericidal, meaning they kill bacteria directly.
PENICILLINS
Penicillins are the oldest beta-lactam antibiotics. They differ in spectrum, stability, and resistance to bacterial enzymes.
Classification of Penicillins
1. Natural Penicillins
- Penicillin G (injection)
- Penicillin V (oral)
Effective mainly against gram-positive bacteria. Penicillin G is acid-sensitive; therefore, given by injection.
2. Penicillinase-Resistant Penicillins
These are designed to resist destruction by beta-lactamase (penicillinase).
- Methicillin
- Oxacillin
- Cloxacillin
Mainly used against penicillin-resistant staphylococci.
3. Aminopenicillins
Broader spectrum; active against more gram-negative organisms.
- Ampicillin
- Amoxicillin
4. Anti-pseudomonal Penicillins
Effective against Pseudomonas and other gram-negative bacteria.
- Piperacillin
- Ticarcillin
5. Beta-Lactamase Inhibitor Combinations
These combinations protect penicillins from bacterial beta-lactamase enzymes:
- Amoxicillin + Clavulanic acid
- Ampicillin + Sulbactam
- Piperacillin + Tazobactam
Uses of Penicillins
- Streptococcal infections
- Pneumonia
- Skin and soft tissue infections
- Syphilis (Penicillin G is the drug of choice)
- Ear, nose, and throat infections (amoxicillin)
- Infections caused by staphylococci (penicillinase-resistant types)
Adverse Effects
- Allergic reactions (rash, fever, anaphylaxis)
- Nausea, diarrhoea
- Superinfections
CEPHALOSPORINS
Cephalosporins are beta-lactam antibiotics similar to penicillins but usually broader in spectrum and more resistant to beta-lactamases.
Classification (Generations)
Cephalosporins are classified into five generations based on improving antibacterial coverage.
1. First Generation
- Cefazolin
- Cephalexin
Mainly gram-positive coverage.
2. Second Generation
- Cefuroxime
- Cefoxitin
Better gram-negative coverage compared to first generation.
3. Third Generation
- Ceftriaxone
- Ceftazidime
Excellent gram-negative activity; ceftriaxone is widely used for meningitis.
4. Fourth Generation
- Cefepime
Broad-spectrum, including resistant gram-negative bacteria.
5. Fifth Generation
- Ceftaroline
Effective against MRSA (Methicillin-Resistant Staphylococcus aureus).
Uses of Cephalosporins
- Respiratory tract infections
- Urinary tract infections
- Skin and soft tissue infections
- Meningitis (ceftriaxone, cefotaxime)
- Gonorrhoea (ceftriaxone is the drug of choice)
- Severe hospital-acquired infections (fourth-generation)
Adverse Effects
- Allergic reactions (similar to penicillins)
- Diarrhoea
- Risk of bleeding (with certain types)
- Superinfections like C. difficile
Cross-Reactivity
Patients allergic to penicillins may also react to cephalosporins, though the risk is low.
Bacterial Resistance
Bacteria resist penicillins and cephalosporins by:
- Producing beta-lactamase enzymes
- Changing penicillin-binding proteins
- Reducing drug entry into the cell
Using beta-lactamase inhibitors and newer cephalosporins helps overcome resistance.
Detailed Notes:
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