Antibiotic Types and Side Effects

1.0Introduction 

Antibiotics are a class of drugs used to treat and prevent bacterial infections. The term "antibiotic" literally means "against life," as it specifically targets the life processes of bacteria without harming the host's cells. They are a cornerstone of modern medicine, dramatically reducing mortality rates from bacterial diseases. For a JEE student, understanding their classification, mode of action, and impact on health is crucial as they relate to topics like biomolecules, polymers, and everyday chemistry.

2.0History and Discovery of Antibiotics

The first true antibiotic, penicillin, was inadvertently discovered by Alexander Fleming in 1928. Fleming noticed that Staphylococcus bacteria on a petri dish were inhibited in their growth by a mold called Penicillium notatum. The widespread manufacture and therapeutic application of penicillin in the 1940s, however, was due to the work of Howard Florey and Ernst Chain. This discovery started the "antibiotic era", which transformed the treatment of infectious diseases.

3.0Mechanism of Action of Antibiotics

Antibiotics work through exploiting the differences in the biochemistry and structure of bacterial cells and host cells. They are selectively toxic, which means they only hurt certain parts or processes of bacteria that are not present or are very different in human cells. Some antibiotics, for instance, attack the bacterial cell wall, which is not found in animal cells. The main ways it works are:

• Inhibiting cell wall synthesis
• Inhibiting protein synthesis
• Inhibiting nucleic acid synthesis (DNA and RNA)
• Disrupting the plasma membrane
• Interfering with metabolic pathways

4.0Classification of Antibiotics

Antibiotics can be classified in several ways, which is important for understanding their therapeutic applications, based on:

Chemical Structure

• Beta-Lactams: Characterized by a beta-lactam ring. Examples: Penicillins, Cephalosporins.
• Tetracyclines: Contain a four-ring core structure. Example: Tetracycline.
• Macrolides: Characterized by a large macrocyclic lactone ring. Example: Erythromycin.
• Aminoglycosides: Contain amino sugars linked by glycosidic bonds. Example: Streptomycin.
• Quinolones: Synthetic antibiotics with a bicyclic ring structure. Example: Ciprofloxacin.

Spectrum of Action

• Broad-Spectrum Antibiotics: Effective against a wide range of bacteria, including both Gram-positive and Gram-negative bacteria. Examples: Tetracycline, Chloramphenicol.
• Narrow-Spectrum Antibiotics: Effective against a specific type or a small group of bacteria. Examples: Penicillin G (primarily against Gram-positive), Polymyxin (primarily against Gram-negative).

Mechanism of Action

This is the most common and biochemically important way to group things. It tells you how they work on a molecular level.

1. Cell Wall Synthesis Inhibitors

These antibiotics stop bacteria from making a new cell wall, which is necessary for their structural integrity. These drugs are not very harmful to people because animal cells do not have a cell wall.

Penicillin: An antibiotic that belongs to the beta-lactam family. It stops the transpeptidase enzyme from working, which is important for linking the peptidoglycan chains that make up the tough cell wall of bacteria. This causes the bacterial cell to burst, or lyse. Penicillin G works against bacteria that are Gram-positive.

Cephalosporins: Also contain a beta-lactam ring and act similarly to penicillin. They are often used for patients with penicillin allergies.

2. Protein Synthesis Inhibitors

These antibiotics prevent the bacterial ribosomes from making proteins, which is what they do. The structure of bacteria's ribosomes (70S) is different from that of eukaryotes' ribosomes (80S), which makes it possible to target them selectively.

• Tetracycline: Binds to the 30S ribosomal subunit, preventing the binding of aminoacyl-tRNA. This blocks the addition of new amino acids to the growing peptide chain, halting protein synthesis. Tetracyclines are broad-spectrum antibiotics.
• Erythromycin: A macrolide that binds to the 50S ribosomal subunit, inhibiting the translocation of the ribosome and preventing protein synthesis.
• Chloramphenicol: Binds to the 50S ribosomal subunit, inhibiting the formation of a peptide bond. It is a very potent broad-spectrum antibiotic, but it can have serious side effects.

3. Nucleic Acid Synthesis Inhibitors

These antibiotics interfere with the replication and transcription processes in bacteria.

• Ciprofloxacin: A fluoroquinolone that stops the bacterial enzyme DNA gyrase from working. This enzyme is important for loosening and tightening the DNA of bacteria as it copies itself. Ciprofloxacin stops DNA replication and transcription by blocking it, which kills bacteria.

• Rifampin: Inhibits DNA-dependent RNA polymerase, thereby blocking the synthesis of messenger RNA (mRNA) and subsequently protein synthesis. It is a key drug in the treatment of tuberculosis.

5.0Side Effects of Antibiotics

While antibiotics are life-saving, their use is not without risk. Side effects can range from mild and common to severe and life-threatening.

Common Side Effects:

• Gastrointestinal Issues: Nausea, vomiting, diarrhoea, and abdominal pain are very common. Antibiotics can disrupt the natural balance of gut flora, leading to these symptoms.
• Allergic Reactions: Ranging from mild skin rashes and itching to severe, life-threatening anaphylaxis. Penicillin is a well-known allergen.

Less Common Side Effects:

• Photosensitivity: Increased sensitivity to sunlight, leading to easy sunburn. This is often associated with tetracyclines and fluoroquinolones.
• Yeast Infections: The destruction of beneficial bacteria in the body (e.g., in the gut and vagina) can lead to the overgrowth of fungi like Candida albicans, causing oral thrush or vaginal yeast infections.

Severe Side Effects:

• Anaphylaxis: A severe, whole-body allergic reaction that can cause a sudden drop in blood pressure, difficulty breathing, and can be fatal.
• Antibiotic-Associated Colitis: The overgrowth of a toxin-producing bacterium, Clostridium difficile, due to the disruption of normal gut flora.
• Liver and Kidney Damage: Some antibiotics, especially in high doses or with prolonged use, can be toxic to the liver and kidneys.

6.0Antibiotic Resistance: A Growing Concern

Antibiotic resistance is one of the greatest threats to health around the world. Bacteria change and become resistant to antibiotics when this happens. This problem is made much worse by the wrong use and overuse of antibiotics. Bacteria can become resistant by mutating or getting resistance genes from other bacteria. This makes it very hard to treat infections that used to be easy to treat, like tuberculosis and pneumonia.