Types of antibiotics and their action
In a nutshell
Antibiotics are used to treat bacterial infections. They work by either inhibiting growth and multiplication of the bacteria or by directly killing them. Two important antibiotics are tetracycline and penicillin.
Antibiotics
Definition
Chemical substances that, in low concentrations, are able to kill or inhibit the growth of bacteria. Antibiotics can either be broad-spectrum or narrow-spectrum.
Type | Description | Example |
| Broad-spectrum | Effective against a wide range of different bacterial species | Tetracycline |
| Narrow-spectrum | Only effective against certain bacteria | Penicillin |
Antibiotic action
Antibiotics are usually bacteriostatic or bactericidal. Bacteriostatic antibiotics prevent the growth and spread of bacteria, they do not kill them. Whereas bactericidal antibiotics do kill bacteria.
There are three main biochemical mechanisms targeted by antibiotics:
Target | Effect | Example(s) |
| Cell wall synthesis | Interferes with cell wall synthesis | - Penicillin is able to inhibit the transpeptidase enzyme that catalyses the formation of cross-linkages between the peptidoglycan molecules in the cell wall.
|
| Nucleic acid synthesis | Disrupts DNA replication or transcription | - Sulfonamides prevent nucleic acid precursors from forming.
- Nitroimidazoles bind to DNA and break down the double helix.
- Clofazimine binds to DNA and prevents transcription and replication.
- Quinolones inhibits enzymes that catalyse DNA replication and transcription.
|
| Protein synthesis | Inhibits protein synthesis | - Tetracycline and streptomycin inhibit the binding on tRNA to bacterial ribosomes.
- Erythromycin prevents movement of the tRNA-peptide complex.
- Chloramphenicol prevents the formation of peptide bonds.
|
Penicillin
Bacterial cell walls contain peptidoglycan and Gram positive bacteria have a thick peptidoglycan layer. Penicillin irreversibly inhibits transpeptidase: this enzyme is involved in catalysing the formation of cross-linkages between peptidoglycan layers. Consequently, the bacterial cell wall becomes thinner. In fact, Gram positive bacteria lose their entire cell walls and are left as protoplasts. This causes them to die. Gram negative bacteria only have a thin peptidoglycan layer so aren't affected as drastically.
Tetracycline
Bacterial ribosomes are composed of 30S and 50S subunits. During translation, tRNA molecules bind to the 30S subunit of the ribosome. Tetracycline prevents this from happening. As translation has been interrupted, the polypeptide will no longer be made.
Curiosity: Tetracycline only works on prokaryotic cells because eukaryotic cells do not actively transport it into their cytoplasm.
