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How They Work

Antibiotics work by being either bactericidal where they kill microorgnisms; or by being bacteristatic where they inhibit the growth of the microorganisms.

An example of a bactericidal antibiotic is Penicillin. This works by preventing the production of a substance that form the cell wall: peptidoglycan. This means the cell will continue to grow without dividing or developing new cell wall. Therefore, the wall gets weaker, and eventually ruptures (lysis).

The action of Penicillin on bacterial cells

Other antibiotics work by inhibiting the protein synthesis or nucleic acid synthesis. Tetracycline is a bacteristatic antibiotic that binds to ribosomes in bacteria, this means the cell cannot make proteins and therefore can't grow.

When choosing antibiotics to treat diseases, it is important to think carefully. For instance, bacteria have a different type of ribosome (70S) to humans (80S), therefore they will only work to target the bacteria and not affect the patient.

Narrow spectrum antibiotics target specific reaction in particular microorganisms; whereas broad spectrum antibiotics will have an effect on more general features so affect a wide range of pathogens.

Antibiotic Resistance

Antibiotic resistance arises as a result of natural selection. Since bacteria reproduce rapidly, resistance can arise quickly. Those antibiotics resistant will remain after treatment and can continue to divide.

An example of antibiotic resistance can be seen with Penicillin, some bacteria can produce an enzyme called Penicillinase which breaks down Penicillin before it can take effect.

Other mechanisms of resistance include the evolution of a capsule that is resistant to antibiotic, and cell membranes becoming less permeable to antibiotic.

Bacteria can spread resistance genes between each other by bacterial conjugation where two cells join by their pilli and exchange plasmids which often contain genes for antibiotic resistance.

diagram of bacteria conjugation and exchange of plasmid via the Pillus

Antibiotic resistance is made much worse by the overuse of antibiotics in medical treatment. Some bacteria are resistant to most antibiotics (MRSA or mycobacterium-tuberculosis) meaning it is increasingly difficult to treat infection unless new antibiotics are developed.