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To determine the cause of an infection, it may be necessary to grow fluids and tissue samples in a laboratory. This is done in order to identify and test bacteria and fungal infections.
When using microscopy, it is possible to find and identify micro-organisms and analyse samples to diagnose diseases in tissue samples.
How is a cell and tissue test performed?
Use of the microscope is very important in the study of tissue structure and cells with unusual appearances. It is, for example, possible to diagnose cancer, as cancer cells very often have an unusual shape and form irregular patterns.
When the pathologist receives a biopsy (tissue sample) it is separated into small pieces that are dipped into formalin or a similar fixing chemical.
The fixed tissue is then placed in paraffin before being cut into extremely thin slices, which can be placed under a microscope. The paraffin is then removed, and the tissue is stained to make the microscopic details easier to see. Then it is ready for microscopy.
If certain types of cells are expected to be present, the sample can be stained in colours that are particularly helpful in detecting them.
Unfortunately, it can sometimes take up to a week before the result of a tissue test is ready. Sometimes it takes even longer if the sample needs further staining. In an emergency, the result can be ready within half an hour, as the tissue can be frozen before it is cut into slices. This may be necessary if the patient urgently needs an operation for which the results of the test are crucial.
How is a culture produced?
When the sample arrives at the laboratory, it is spread out on a special gel in a plastic bowl, which is then covered with a lid.
Extracts of live substances such as meat, bread or chocolate may be added to the gel in order to give it more nourishment. If the sample is thought to hold a particular bacterium, the gel can be mixed with specific substances that encourage the growth of this particular bacterium and suppress the growth of others.
The test is then placed in a heating cupboard at approximately 100ºF (about 35ºC) the temperature at which disease-causing bacteria thrive best. Often, several different bacteria will grow, but the one that causes the disease (the pathogen) will be dominant compared to the non-pathogen.
What can it show?
- The micro-organisms that cause sickness (pathogens) are separated into several different groups, according to their type. The most important are viruses, bacteria and chlamydia (virus-like bacteria). Other groups include eggs and larvae from different parasites and protozoa (which are one-celled parasites such as an amoeba).
- The standard way to identify bacteria is through using a culture (bacteria grown in the laboratory). In this process a non-contaminated group, which can be identified, is purified and tested as a comparison.
- Fungi can also be grown and purified in the same way as bacteria, but this procedure is not often used.
- It is more difficult to analyse viruses and chlamydia, which require more advanced laboratory examinations. Some viruses and other micro-organisms are so difficult to culture that it is necessary to examine the person's blood (serum) for antibodies against the micro-organism.
- Worms, larvae, eggs and protozoans are clearly visible under a microscope and can easily be identified by a trained microbiologist without further examination.
How is fungal and protozoan microscopy performed? - Micro-organisms that are only a little bit bigger than bacteria can be identified by using microscopy. Thrush (oral candidiasis) can be identified using a sample from the mouth or the sexual organs.
- Protozoa that may cause amoebic dysentery (infection in the intestines), malaria or trichomonas (a sexually transmitted disease that causes a vaginal infection) can be isolated from stools, blood sample and discharge.
- Parasites such as roundworm and hookworm can often be identified through eggs or worms in the stools.
- Micro-organisms that are smaller than bacteria, such as chlamydia, rickets and viruses can be identified through blood samples that are then analysed for antibodies against the micro-organism.
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