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.