Culture and tests for drug sensitivity
Culture and tests for drug sensitivity are methods of increasing the numbers of microorganisms to aid their identification and test for sensitivity to antibiotics.
Many tests require large numbers of microorganisms and, to obtain these, the organisms are cultured. Culturing encourages the organisms to reproduce. Most double in number every 20-30 minutes. The organisms generally form clusters, known as colonies. Although it usually takes up to 18 hours to obtain a result, some organisms, such as the bacterium that causes tuberculosis, may take up to 6 weeks to grow sufficient numbers to be easily identified. After they have been cultured, bacteria may be tested to determine which antibiotics are most effective against them.
How are culture and tests for drug sensitivity done?
Viruses and some other organisms can be grown only in cultures of living cells, but most microorganisms are grown in a sterile plastic dish containing a nutrient gel on which they feed. The sample to be cultured is spread on to the gel. If a particular organism is thought to be present, the culture is set up specifically to encourage that microorganism while suppressing others. The culture dish is kept at a suitable temperature until colonies of microorganisms have grown, and it is then examined. Some microorganisms have specific features that may identify them. For example, the bacterium Staphylococcus aureus forms small, golden yellow colonies when cultured.
After culturing, a small portion of the colony may be carefully removed from the dish, stained, and examined under a microscope to identify the organisms present. Other chemical tests may be performed to help in identification.
When bacteria have been identified, drug sensitivity tests may be performed. Paper discs containing different types and amounts of antibiotics are added to the bacterial culture. After 24 hours, the culture is examined to see which antibiotics have been the most effective in destroying the bacteria.
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