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Microbes, Germs and Antibiotics

11 November 2011 | posted in: Infections

This article gives a brief overview of the different types of microbes that can cause infections, and a brief overview of the use of antibiotics.

What are microbes?

Doctors classify microbes or ‘germs’ into different groups. Four common groups of microbes that cause illness are described below.


There are many different types of bacteria. Some are helpful and protective to humans. Some flourish naturally in our bodies – particularly in the bowel and vagina – and help to protect the body. However, infection with some bacteria can cause serious illnesses such as meningitis, pneumonia, tuberculosis, etc. A bacterial infection may be treated with a course of antibiotics.


These are smaller and different to bacteria. Many different types exist. Most of the common ‘minor’ illnesses are caused by viruses. For example, colds, coughs, ‘flu, sore throats, chickenpox, and some other rashes. It is a standing joke that doctors often diagnose ‘a virus’, but in fact most common infections are due to a viral infection.

For many viral infections there are no effective anti-viral drugs (unlike antibiotics for bacteria). Fortunately, the immune system in the body usually fights off most viral infections within a few days. Taking ‘symptomatic’ treatments for fever or catarrh such as paracetamol is all that needs to be done.

However, some viruses are not fought off and can be very serious. The HIV virus is a good example. There are some anti-viral drugs that are used for certain infections – such as antiretroviral drugs used to treat HIV. Another example is aciclovir and related drugs which are used to treat certain herpes virus infections. As a rule, anti-viral drugs do not clear the virus from the body. They usually work by stopping the virus from multiplying and so ‘control’ the virus and the infection that it causes.


Many types of fungi exist and cause problems in humans, animals and plants. Fungal infections commonly affect the skin and nails in humans. They cause ringworm, athlete’s foot, other localised skin rashes, and infections in and around nails. Modern creams usually work well to clear a local fungal rash quickly. However, nail infections can be rather stubborn and may need long term treatment of anti-fungal medicines taken by mouth.


The most common yeast infection is thrush. This is due to a yeast called Candida which thrives in moist airless, warm areas of the body. It causes infections in the vagina, some cases of nappy rash in babies, and sometimes infects other areas of the body. Treatment usually works well with anti-yeast creams and medicines.


Antibiotics are no ‘cure all’ for infections. Antibiotics will only clear infections caused by bacteria. They do not work when an infection is caused by viruses, fungi or yeasts. As mentioned, most common infections are caused by viruses when an antibiotic will not be of use. Even if you have a bacterial infection, the immune system can clear most bacterial infections. For example, antibiotics usually do little to speed up recovery of bronchitis, or most ear, nose, and throat infections that are caused by bacteria.

However, you do need antibiotics if you have certain serious infections caused by bacteria such as meningitis or pneumonia. In these situations, antibiotics are often life-saving. When you are ill, doctors are skilled at checking you over to rule out serious illness, and to advise if an antibiotic is needed.

How do antibiotics work?

Some work by killing the bacteria. This is often done by interfering with the structure of the cell wall of the bacteria. Some work by stopping the bacteria from multiplying.

Some possible problems with antibiotics

Antibiotics are not without problems. This is why it is not usually good practice to take antibiotics ‘just in case’ an infection is bacterial, but to only take them when they are really needed. For example:

  • Antibiotics can cause side effects such as allergies, diarrhoea, rashes and nausea. Side effects are quite common. Most side effects are not serious, but some people have died from a severe allergic reaction to an antibiotic.
  • Antibiotics can kill off normal ‘defence’ bacteria which live in the bowel and vagina. This may then allow thrush to develop.
  • Overuse of antibiotics has led to some bacteria mutating and becoming resistant to some antibiotics which may then not work when really needed. For example, MRSA is a bacterium that has become resistant to many different antibiotics and is difficult to treat.
  • Some antibiotics may interact with other medicines that you might take. This may cause reactions, or reduce the effectiveness of one or other of the treatments. One well known example is that certain antibiotics can reduce the effectiveness of ‘the pill’ (the oral contraceptive pill). So, when you are prescribed an antibiotic you should tell a doctor if you are on ‘the pill’ or if you take other medicines.
  • Food and drink affect the absorption of some antibiotics – so follow the instructions on how to take a course of antibiotics.

In summary

Doctors are skilled in diagnosing which conditions are in need of antibiotics. So do not be surprised if a doctor does not recommend an antibiotic for conditions caused by viruses or non-bacterial infections, or even for a mild bacterial infection.

Occasionally, a virus infection or minor bacterial infection develops into a more serious ‘secondary’ bacterial infection. See a doctor to review the situation if an illness appears to change, becomes worse, does not go after a few days, or if you are worried about any new symptom that develops.