Antibiotics Classification | Their Definition, function & mechanism

Antibiotics are essential medicines to kill bacterial infections.

Many people survived deadly communicable diseases like tetanus, septicemia, typhoid, and tuberculosis due to these antibiotics.

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

Antibiotics Definition

Antibiotics are natural or semi-synthetic molecules that, in small concentrations, destroy bacteria in the body without affecting the body cells.

This specificity is possible due to their action on typical bacterial structures and not the human and animal cells.

Antibiotics classification

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


  • These drugs act by inhibiting folic acid synthesis.
  • Folic acid is taken from nutrition by human cells.
  • While bacteria have to synthesize on their own.

β-Lactam antibiotics

  • These are the first discovered antibiotics from fungus penicillin notatum by Alexander Fleming.
  • They have a β-Lactam ring in their chemical structure.
  • They act by inhibiting the cell wall synthesis of bacteria by preventing peptide bond formation.


  • As the name suggests, these compounds have 4 cyclic rings in their structure.
  • These drugs act by binding to the 30S ribosome and inhibiting protein synthesis.

Macrolide antibiotics

  • These drugs bind to 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 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.


  • Antibiotics 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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