Antibiotics Classification | Their Definition, function & mechanism

Antibiotics are some of the most valuable and indispensable drug molecules in the present world.

Many people survive from deadly bacterial infections and communicable diseases like tetanus, septicemia, typhoid, tuberculosis,  etc. due to these antibiotics.

The use of antibiotics greatly decreases the untimely mortality of many infected patients.

There are many types and are selected based on prior experience or by antibiotic sensitivity test for the said infection.

There use has become inevitable and in most cases, medicos prescribe them for even simple diseases like cough and cold.

Irrational use of antibiotics has become an issue as there is a development of resistance by bacteria.

Yet, this is avoided or overcome by use of multiple antibiotics.

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

Antibiotics Definition:

Antibiotics are natural or semi-synthetic molecules which in small concentrations destroy bacteria and other microbes in the body.

Their specialty is they kill the microbes without affecting the body into which they were administered. This is again due to specificity or selectivity for the bacteria and not the body of administration.

Antibiotics classification

1. Sulfonamides
   
2.  β-Lactam antibiotics
3. Tetracycline
4. Macrolide antibiotics
5. Quinolones and fluoroquinolones
6. Anti-TB, Anti-leprosy

Function & mechanism of action of these antibiotics:

Antibiotics especially kill bacteria and hence this creates their selectivity. The bacterial cell has different physiology and anatomy than human and animal cells.

1. Attacking and damage to cell wall: Antibiotics like penicillin, cephalexin damage the cell wall of bacteria. 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 excess influx of water so that cell swells or loss of water so that cell shrinks and thus cell is burst & damaged.
2. Attack on ribosomes & inhibit their growth and multiplication: Tetracycline, streptomycin, etc antibiotics especially attack bacterial ribosomes and inhibit protein formation. Thus bacteria don’t grow and multiply and are killed. In human cells ribosome is different from the bacterial ribosome and hence they are not affected. (Bacteria ribosomes have 70’S ribosome while animals and plants have 80’S ribosomes).
3. Attack DNA synthesis; Antibiotics like ciprofloxacin, ofloxacin, etc. attack DNA related enzymes of bacteria which are quite different from those of humans and hence kill them without harm to our body.

Sulfonamides: 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. Since the synthesis is inhibited, bacterial growth is inhibited.

β-Lactam antibiotics: These are the first discovered antibiotics from fungus penicillin notatum by Alexander Fleming. They inhibit cell wall synthesis by preventing peptide bond formation.

Tetracycline: 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.

In both cases, humans are not affected. 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.

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.