What is an antibiotic?
Antibiotics are drugs that either kill bacteria or stop them multiplying.
There are many synonyms used for drugs that fulfil these functions, such as antibacterials and antimicrobials. Strictly speaking, each term refers to drugs with slightly different activities and structures.
However, for the purpose of this discussion, the term antibiotic will be used to refer to any drug that has activity against bacteria.
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These distinctions can become more important when one remembers that there are also drugs that are able to kill parasites (antiparasitic agents), drugs that stop viruses multiplying (antivirals), and drugs used to kill fungi (antifungals). It is important to realise that most antibiotics are only active against bacteria (although some are also active against certain parasites). Antibiotics have no effect against viruses.
How does an antibiotic work?
Before answering this question, it is worth spending a little time looking at some of the basic structures of bacteria, as well as how they divide.
Bacteria are fairly simple single-celled organisms. They consist of:
A cell wall (some bacteria don’t have cell walls, but the majority do. The detailed structure of the cell wall does vary between bacteria)
A cell membrane
DNA in the form of a circular chromosome. This contains all the bacterium’s genetic information
Cytoplasm, in which various components can be found, including ribosomes (where proteins are made), proteins and enzymes
When bacteria divide, they firstly synthesise the new structural components of the cell wall and membrane, and the DNA replicates. Once this occurs, the cell splits in two, with each new cell containing a complete copy of the “parent’s” DNA.
Antibiotics interfere with certain aspects of bacterial cell growth or replication. Different types of antibiotic are effective against different parts of the bacterium. Generally speaking, there are three areas of the bacterium that are targeted by antibiotics:
Cell wall synthesis
Protein synthesis
DNA synthesis or replication
These will be discussed in more detail when individual antibiotics are discussed.
By either killing the bacteria, or reducing their ability to multiply, the antibiotics give the body’s own immune system a chance to fight more effectively against the organisms. Unfortunately, even with the best antibiotics, the infection may be so severe, or the patient’s immune system so weak, that bacteria are able to carry on multiplying and the patient may die.
Bacteria are commonly divided into two groups – Gram positive and Gram negative. This is nothing to do with weight, it actually refers to the structure of the cell wall, which causes Gram positive organisms to appear purple, while Gram negative organisms appear pink when using a particular staining technique. This technique is Gram’s stain (named after the scientist Gram).
Gram negative bacteria include organisms such as Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Proteus vulgaris, Salmonella, Shigella and Pseudomonas aeruginosa. Some of these organisms are often found as normal flora of the gastro-intestinal tract, and often cause urinary tract infections, gastro-intestinal infections (such as dysentery and typhoid fever) and intra-abdominal infections (such as peritonitis). Other Gram negative bacteria are Neiserria meningitides (causing meningitis), Neisseria gonorrhoeae (unsurprisingly causes gonorrhoea) and Haemophilus influenzae (ear infections, pneumonia, meningitis).
Gram positive bacteria include organisms such as Staphylococcus aureus (as well as all the other staphylococci), Streptococcus pneumoniae, Streptococcus pyogenes, all the other streptococci and Corynebacterium diphtheriae. These organisms cause a variety of infections, including pneumoniae, abscesses, infections of bone (osteitis), throat infections, diphtheria and meningitis.
The above list of organisms and diseases is not exhaustive, and one should remember that some organisms can cause more than one type of infection, and conversely some infections can be caused by more than one specific organism.
When should or shouldn’t I take an antibiotic?
Antibiotics should be used if you have a bacterial infection. Unfortunately, many viral infections can give similar symptoms and signs of bacterial infections, and antibiotics are often prescribed unnecessarily.
Some common bacterial infections that are usually treated with antibiotics are:
Otitis media (infection of the middle ear)
Streptococcal tonsillitis (infection of the tonsils with the bacterium Streptococcus pyogenes)
Pneumonia (infection of the lungs)
Meningitis (infection of the membrane lining the brain)
The common cold, and influenza can mimic conditions like pneumonia, otitis and tonsillitis (with symptoms like cough, mild fever, earache and sore throat). Some clues that you may have a viral infection are a runny nose and either a mild fever or no fever at all. Remember, however, that this is a generalisation, and if you are in doubt, it is best to consult your doctor.
When shouldn’t I take an antibiotic? As above – if you don’t have a bacterial infection, there is no need to take an antibiotic.
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