Antibiotics Classification | Their Definition, function & mechanism

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

  1. Sulfonamides
  2. β-Lactams (penicillins and cephalosporins)
  3. Tetracyclines
  4. Aminoglycosides
  5. Macrolide antibiotics
  6. Chloramphenicol
  7. Quinolones and fluoroquinolones
  8. Anti-TB and Anti-leprosy drugs

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

This specificity is possible due to their action on typical bacterial structures that are absent in human and animal cells.

In the past, the human population encountered deadly bacterial infections like the plague, cholera, and other epidemics. The antibiotics came to the rescue.

Antibiotics classification


  • These drugs act by inhibiting folic acid biosynthesis in the bacteria.
  • Unlike human cells that receive Folic acid from nutrition, bacteria 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 nuclear bases, their availability depletes leading to the hindrance of DNA replication and bacterial growth.
  • Thus sulfonamides inhibit bacterial growth and are termed bacteriostatic antibiotics.

β-Lactam antibiotics

  • These are the first discovered natural antibiotics by Alexander Fleming
  • They are produced by the fungus penicillin 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 enzyme peptidase which is essential for cell wall synthesis of bacteria.
  • 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 in exposed bacteria leads to osmosis of bacterial cytoplasm leading to swelling and destruction.


  • 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 hinder the translation process required for protein synthesis.
  • The human cells have a 40s ribosomal subunit instead and are not susceptible.
  • Lack of protein synthesis inhibits the growth and multiplication of bacteria.

Macrolide antibiotics

  • These drugs bind to the 50S ribosomes and inhibit protein synthesis.
  • The ribosome in a human cell is of 80S type, which means it has 40S and 60S ribosomes.
  • So, these drugs cannot bind to the 40S 0r 60S ribosome selectively.

Hence by both means, human cells are not affected.

Quinolones and fluoroquinolones

  • 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, there is selectivity for bacteria alone.

Anti-TB, Anti-leprosy

  • These drugs are specifically used to kill tuberculosis and leprosy bacteria.
  • They act by preventing protein synthesis and also cell wall synthesis by inhibiting mycolic acid.

Function & mechanism of action of these antibiotics

  • The bacterial cell has different physiology and anatomy than human and animal cells.
  • Antibiotics especially target this feature and kill bacteria by different methods.
  • This difference in mechanism creates their selectivity.
antibiotics mechanism of action

Attacking and damage to cell wall:

  • Human cells don’t have cell walls; hence these are not affected.
  • These antibiotics cause pores in the cell wall, which can lead to osmosis due to an excess influx of water.
  • With this water influx so the cell swells or causes a loss of water, so the cell shrinks, and thus cell is burst & 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 is different from the bacterial ribosome, and 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, ofloxacin, etc., attack DNA-related enzymes of bacteria.
  • These enzymes are pretty different from those of humans and hence kill them without harm to our bodies.

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

Antibiotics vs. Antibacterial

Chemical structureComplexQuite simple and defined
Spectrum of activityNarrow spectrum. Acts against a few sets of bacteria.Narrow spectrum. It acts against a few sets of bacteria.
ExamplesAmoxicillinSulfadiazine, levofloxacin.

Anti-infective vs. Antibiotic

TargetDestroy the overall cell structuresKills only bacteria
Mode of actionDestroy the over all cell structuresInhibit the growth or damage the cell.
UsesFor sterilization, cleaning, sanitation, etc.To treat bacteria infections
ExamplesEthanol, benzalkonium chloridePenicillin, cefixime, doxycycline


  • Antibiotic use has become inevitable, and in most cases, medicos prescribe them for even simple diseases like cough and cold.
  • Due to this, irrational use of antibiotics has become an issue as there is a development of resistance by bacteria.
  • Yet, this is avoided or overcome by the use of multiple antibiotics.

But World health organization is worried that the bacteria could develop resistance to most antibiotics soon, which is alarming to the medical world.

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