Antibiotics are medications used to fight bacterial infections.
Originally, the term “antibiotics” referred to natural compounds produced by certain microorganisms
for the purpose of fending off others; for example, penicillin is produced by the fungus
Penicillium.
Nowadays, this term includes all antibacterial products, most of which are semi-synthetic,
meaning they are modifications of natural products.
Antibiotics are just one type of antimicrobials.
They target bacteria, and are usually not effective against other types of organisms.
Antibiotics cannot treat viral infections such as common cold or flu.
Antibiotics can be bactericidal, meaning they destroy bacterial cells; or bacteriostatic,
meaning they inhibit bacterial growth.
Some antibiotics are broad-spectrum - they are effective against a wide range of bacteria,
including both Gram-positive and Gram-negative; while others are narrow-spectrum - they are
more specific, affecting a smaller group of bacteria.
Antibiotics can be classified by their mechanisms of action:
- Inhibitors of cell wall synthesis.
Bacterial cells are surrounded by cell walls made of peptidoglycan.
Antibiotics that affect bacterial cell wall act at different stages of peptidoglycan synthesis
and cell wall assembly.
Because mammalian cells do not have cell walls, this class of antibiotics is highly selective
- they target bacteria and have minimal effects on mammalian host cells.
- Disruptors of cell membrane.
Some antibiotics disrupt the integrity of cell membrane by binding to membrane phospholipids.
Because cell membrane is also found in mammalian cells, these antibiotics are also toxic to
host cells if administered systemically.
Their clinical use is therefore limited to topical applications.
- Inhibitors of protein synthesis.
Antibiotics that interfere with bacterial protein synthesis may act at different steps
of this process, including: formation of the 30S initiation complex, assembly of the 50S
ribosome subunit, formation of the 70S ribosome from the 30S and 50S complexes, and elongation
process.
Some of these antibiotics also inhibit the eukaryotic mammalian counterparts, but their
effect on bacterial ribosomes is significantly greater.
- Inhibitors of nucleic acid synthesis.
Some antibiotics interfere with DNA synthesis by binding to bacterial topoisomerase II – the
enzyme that relaxes the supercoil DNA before its replication.
Some others interfere with RNA synthesis by inhibiting RNA polymerase.
Some antibiotics of this class are selective - they do not interact with mammalian counterparts
of these enzymes, while others do affect mammalian host cells.
The latter are used for cancer treatment instead.
Because cancer cells grow faster than normal cells, they are more affected by the action
of these agents.
- Inhibitors of folic acid synthesis.
Bacteria synthesize their own folic acid, unlike humans who get the vitamin from food.
Because of this, antibiotics that inhibit enzymes involved in folic acid synthesis only
harm bacterial cells, and not human cells.