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Culture

Depending upon the type of specimen sent to the laboratory and the suspected causative organism, a liquid or solid medium will be selected to enable further identification. Clinical specimens are inoculated onto agar plates or into nutrient broth, then incubated for a certain period of time and observed for growth (Weston 2008). Different species or strains have different growth rates; for example, Pseudomonas and Clostridium multiply in approximately 10 minutes, Escherichia coli has a growth rate of 8 hours whilst Mycobacterium tuberculosis takes 18–24 hours to grow (Weston 2008). Growth of bacteria is seen as colonies on the culture media and the size, colour and shape vary according to the type of bacteria identified (Gould and Brooker 2008).

Fungal organisms grow in the same type of media as those used for bacteria, although they generally require an incubation period of 24–48 hours (Higgins 2007). Pathogenic fungi can also be grown on special mycological media, upon which they grow better with less risk of bacterial overgrowth (Mims 2004). Fungi are identified from their characteristic appearance on the surface of the agar and microscopic appearance (Gould and Brooker 2008). The presence of fungi, such as Candida albicans, is sometime difficult to interpret because it is commonly present in the upper respiratory, alimentary and female genital tract, and on the skin of healthy people. However, in patients who are immunocompromised this fungus can lead to systemic disease (Garber 2001).

Viruses cannot be cultured outside living cells, although cells grown in nutrient material can be used to culture certain viruses. Their identification is indicated by the characteristic way in which they change the shape of the cell (Gould and Brooker 2008).

Antibiotic sensitivity

Once the pathogenic organism has been identified, it is vital to establish which antibiotics it is sensitive to so the appropriate therapy can be prescribed (Gould and Brooker 2008). The bacteria are inoculated onto a solid media plate and paper discs impregnated with different antibiotics are placed over them and incubated. If the bacteria are sensitive to a particular antibiotic, their growth is inhibited, resulting in a clear zone around the disc.

Serology

Serology is useful in identifying viral infections and bacterial infections with difficult culture organisms and is based upon the host’s immunological reaction. Detection of antigens or antibodies in serum, which are activated in response to infection, may suggest that the patient is, or has been, infected (Higgins 2007).

Despite the disadvantage of the test being performed retrospectively, antibody testing is the main method for diagnosing viral infections (Mims 2004). Two serum tests are collected, once at the beginning of the illness and again at 10–14 days, and compared for changes in antibody content. A raised content suggests that the patient’s infection is current (Mims 2004).

Histology

Histology is the study of cells and tissues within the body. It also studies how the tissues are arranged to then form organs. The histological focus is on the individual cells’ structure and how they are arranged to form the individual organs. The types of tissues that are recognized are epithelial, connective, muscular and nervous (Junqueira and Carneiro 2005).

The tissues are examined under a light microscope where light passes though the tissue components after they have been stained. As most tissues are colourless, they are stained with dyes to enable visualization. An alternative is the electron microscope in which the cells and tissue can be viewed at magnifications of about 120,000 times (Junqueira and Carneiro 2005).

Legal and professional issues

Competencies

In accordance with the NMC’s The Code: Standards of Conduct, Performance and Ethics (NMC 2008a), the collection of specimens should be undertaken by professionals who are competent and feel confident that they have the knowledge, skill and understanding to do so, following