Antibiotics is one of those technical terms in biology which actually describes its object. ANTI = against, BIOTIC = life. So antibiotics are weapons of mass destruction… sort of.
They are substances which occur both naturally, as well as artificially as made by humans. The reason they are so widespread and important is because they solve a problem humanity has had for a very long time (i.e. forever). They are used to treat bacterial infections. Today that might seem like a small thing, yet around the globe millions of people still die all the time due to bacterial infections (e.g. pneumonia). It’s not a small thing, it is one of the greatest medical discoveries.
A one-week course of antibiotics taken orally, for example, can easily treat bacterial infections and the associated disease. This is an amazing achievement. Antibiotics are substances which kill prokaryotic cells, such as bacteria, while leaving eukaryotic cells (in humans and others) untouched.
Each type of antibiotic targets different things in bacteria. One of the main differences between bacteria and human cells is that the former have a cell wall, while the latter don’t. Some antibiotics prevent the formation of cell walls. This renders the bacteria vulnerable to water flooding inside and bursting them. Bursted bacteria can’t replicate (really?), and hence the infection ceases. This is called osmotic lysis. Lysis means breaking or disintegration, while osmotic refers to the osmotic effect which results in water flooding into the bacteria, from higher water potential (outside the bacteria) to lower water potential (inside the bacteria).
Penicillin is actually a cell wall-inhibiting antibiotic, naturally produced, and hence discovered from, certain species of the Penicillium fungus. It is of the beta-lactam antibiotic variety, all of which act in the same way to kill bacteria. Antibiotics that actually kill bacteria are called bactericidal. Antibiotics that inhibit bacterial reproduction without killing them directly are called bacteriostatic.
Other ways in which antibiotics target and kill bacteria include interfering with their DNA replication, so they can’t replicate further, and interfering with their protein synthesis. This essentially blocks the normal running of their metabolic functions, rendering them dead or unable to replicate.
Tetracycline works this way by preventing protein synthesis. This makes it a bacteriostatic antibiotic. It works by attaching to the small (30S) subunit of the ribosome and hence preventing the tRNA carrying an amino acid from binding. Tetracycline has a binding affinity to both prokaryotic and eukaryotic 30S ribosome subunit, but unlike in humans, the tetracycline in bacteria actually gets pumped into the cell.