Antibiotics Classification | Their Definition, function & mechanism

By definition, antibiotics are natural molecules that, in small doses, destroy bacteria without affecting the body cells. They are classified as

Antibiotics are drugs meant to kill bacteria without affecting human cells (host cells).

This specificity is possible because of their action on typical bacterial structures that are absent from human and animal cells.

In the past, the human population faced deadly bacterial infections such as the plague and cholera, as well as other epidemics. The antibiotics came to the rescue.

Antibiotics classification

Sulfonamides

• These drugs act by inhibiting folic acid biosynthesis in the bacteria.

• Unlike human cells that receive Folic acid from nutrition, bacterial cells synthesize it on their own.

• This biosynthesis is inhibited by sulfonamides, leading to folic acid deficiency in bacteria.

• Since folic acid is essential for the formation of purine and thymidine nucleotide bases, its depletion hinders DNA replication and bacterial multiplication.

• Thus, sulfonamides inhibit bacterial growth in numbers and are termed bacteriostatic antibiotics.

β-Lactam antibiotics

✅ Inhibitors of Cell Wall Synthesis

Common Examples: Amoxicillin, Penicillin V, Cephalexin (Keflex), Ceftriaxone.

• Bacteria require a cell wall to survive, and it is built during bacterial multiplication.

• These antibiotics act on molecules in the cell wall, weakening it.

• They have a β-lactam ring in their chemical structure. are produced by the fungus Penicillium notatum and have a β-Lactam ring in their chemical structure.

• These antibiotics are quite effective and widely used in healthcare.

• They act by inhibiting the peptidase enzyme, which is essential for bacterial cell wall synthesis.

• This leads to the formation of porous or weak cell walls, which makes the bacteria lose their shape and ability to multiply.

• Sometimes, the porous cell wall of exposed bacteria causes osmotic influx of water into the bacterial cytoplasm, leading to swelling and destruction.

Tetracyclines

✅ Inhibitors of Protein Synthesis (Ribosome Targets)

Common Examples: Doxycycline, Minocycline

• As the name suggests, these compounds have 4 cyclic rings in their structure.

• These drugs act by binding to the 30S ribosome subunit in bacteria and hindering the translation process required for protein synthesis.

• Human cells have a 40s ribosomal subunit instead and are not susceptible.

• A lack of protein synthesis inhibits bacterial growth and multiplication.

Macrolide antibiotics

✅ Inhibitors of Protein Synthesis (Ribosome Targets)

Common Examples:

1. Azithromycin,
2. Erythromycin,
3. Clarithromycin

• These drugs bind to the 50S ribosomes and inhibit protein synthesis.

• The ribosome in a human cell is of the 80S type, which means it consists of 40S and 60S subunits.

• So these drugs cannot selectively bind to the human 40S or 60S ribosomal subunit.

• Hence, by both means, human cells are not affected.

Aminoglycoside Antibiotics

✅ Inhibitors of Protein Synthesis (Ribosome Targets)

Common Examples: Streptomycin, Gentamicin, Amikacin, Neomycin

• These drugs, unlike the above protein synthesis inhibitors, also damage the cell wall by forming pores, leading to bacterial death.

Chloramphenicol

✅ Inhibitors of Protein Synthesis

• Chloramphenicol binds reversibly to the 50S ribosomal subunit and inhibits the peptidyl transferase enzyme, preventing peptide bond formation during protein chain elongation.

• While it selectively targets bacterial 70S ribosomes, its primary toxicities arise from the fact that human mitochondrial ribosomes are also 70S (similar to bacterial ribosomes).

• It is primarily bacteriostatic, but it exhibits bactericidal activity against high-yield specific pathogens
  • Streptococcus pneumoniae
  • Neisseria meningitidis
  • Haemophilus influenzae

It is used for

• Brain Abscesses & Meningitis: Due tos high lipophilicity, it has excellent penetration across the Blood-Brain Barrier (BBB), making it a backup agent for severe central nervous system (CNS) infections.

• Typhoid Fever & Rickettsial Infections: It can be used to treat Salmonella Typhi and Rocky Mountain spotted fever (when first-line options like Doxycycline are contraindicated).

• Topical Ophthalmic Use: Its common use as superficial topical eye drops/ointments for bacterial conjunctivitis due to broad coverage and minimal systemic absorption.

But it is highly toxic and can lead to

• Gray Baby Syndrome
• Aplastic Anemia

👍 🧠 High-Yield Ribosome Mnemonic for board exams

“BUY AT 30, CCELL at 50″

• 30S Inhibitors: Aminoglycosides, Tetracyclines
• 50S Inhibitors: Chloramphenicol, Clindamycin, Erythromycin (Macrolides), Linezolid

Quinolones and fluoroquinolones

✅ Inhibitors of Nucleic Acid Synthesis

Common Examples: Ciprofloxacin, Levofloxacin, Ofloxacin

• These drugs inhibit the DNA transcription in bacteria by inhibiting the DNA gyrase enzyme.

• Since the DNA replication is blocked, the bacteria die.

• Since our body cells have DNA topoisomerase instead of DNA gyrase, this selectivity is for bacteria alone.

Anti-TB, Anti-leprosy

• These drugs are specifically used to kill the bacteria that cause tuberculosis and leprosy.

• They act by inhibiting mycolic acid synthesis, thereby preventing protein and cell wall synthesis.

Function & mechanism of action of these antibiotics

• The bacterial cell has different physiology and anatomy from human and animal cells.

• Antibiotics specifically target this feature and kill bacteria by various mechanisms.

• This difference in mechanism creates their selectivity.

Attacking and damage to the cell wall:

• Antibiotics such as penicillin and cephalexin damage bacterial cell walls.

• Human cells don’t have cell walls; hence, these are not affected.

• These antibiotics create pores in the cell wall, which can lead to osmotic lysis due to excessive water influx.

• With this, the cell either swells (water influx) or shrinks (water loss), and thus is damaged.

Attack on ribosomes & inhibit their growth and multiplication

• Tetracycline, streptomycin, etc. Antibiotics attack bacterial ribosomes and inhibit protein formation.

• Thus, bacteria don’t grow and multiply, and are killed.

• In human cells, the ribosome differs from the bacterial ribosome; hence, they are not affected.

(Bacteria ribosomes have 70’S ribosomes while animals and plants have 80’S ribosomes).

Attack DNA synthesis;

• Antibiotics like ciprofloxacin and ofloxacin target bacterial DNA-related enzymes.

• These enzymes are quite different from human enzymes and thus kill them without harming our bodies.

For clinical use, the specific antibiotic is chosen based on prior experience or antibiotic sensitivity tests.

Antibiotics vs. Antibacterial

Anti-infective vs. Antibiotic

Conclusion

• Antibiotic use has become inevitable, and in most cases, doctors prescribe them for even simple diseases like cough and cold.

• Due to this, the irrational use of antibiotics has become an issue as bacteria develop resistance.

• Yet this is avoided or overcome by using multiple antibiotics.

But the World Health Organization is concerned that the bacteria could soon develop resistance to most antibiotics, which is alarming to the medical community.