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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. 

Amongst bacteria, the cell wall composition is a key determinant of what type they belong to. This is important in terms of predicting their response to various antibiotics. Based on different bacteria species’ response to crystal violet stain, Gram positive bacteria are able to take up the stain and appear violet under a microscope, while Gram negative bacteria do not take the stain up and will appear pink if a counterstain is added after washing off the crystal violet stain (this will persist in the Gram positive bacteria).


The difference arises because different bacteria have different cell walls. The bacterial cell wall is one of the main targets of antibiotics.


Notice the difference in thickness of the murein layer in gram positive versus gram negative cells. This layer is what absorbs the violet stain. Hence gram positive bacteria turn violet, while gram negative bacteria lose the stain upon washing.


Penicillin is an antibiotic used against gram positive bacteria. It doesn’t work on gram negative bacteria because their outer membrane (cell envelope) protects against it. Penicillin works by interfering with the production of the cell wall component murein, and as gram positive bacteria have so much of it and at the outer surface, losing it kills them off. Gram negative bacteria have much less murein and an outer membrane, so penicillin doesn’t interfere with their function. Antibiotics that only target Gram positive or Gram negative bacteria are called narrow spectrum antibiotics. Penicillin is one of them.


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.
This mechanism of action renders both Gram positive and Gram negative bacteria susceptible to tetracycline action. Therefore, this is a broad spectrum antibiotic.
The use of antibiotics is a common example of how evolutionary arms races are critical in the development and deployment of medicines that target organisms. As long as some individuals in a targeted population are able to survive the antibiotic, or in time can develop resistance, under the selection pressure of antibiotic use an ever increasing resistant population will emerge.
This process can happen many times, as organisms are extremely versatile. Bacteria have already been subjected to many natural antibiotic attacks from other organisms (the original penicillin is produced by the fungus Penicillium) so they already have certain resistance genes or pathways they can develop when required.
The key is to understand the adaptation cycle of different organisms and use antibiotics effectively.
1. Not use antibiotics inappropriately, such as to treat colds (caused by viruses not bacteria)
2. Complete prescribed antibiotics treatments so bacteria are effectively killed and there is little to no chance of remaining bacteria coming back stronger
3. Avoid overuse of the same antibiotic in the same setting such as in hospitals where patients are susceptible to infection and spread can be rapid
4. Keep many different antibiotics archived, especially the strongest ones, so that they can be used against multi-resistant strains if they develop, to avoid a situation where no antibiotics are available that bacteria are susceptible to





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