How Cultures Are Used to Diagnose a Multitude of Ailments

A culture is a method used to identify the organisms suspected of causing an infection. Cultures are used to identify infectious microbes from urine, stool, genital tract, throat and skin samples. In dermatology, a culture test is used to determine whether a rash is caused by an infection and what organism is responsible.

A culture of E. coli bacteria.

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What Is a Culture?

A culture test identifies the cause of infection and allows the doctor to make a more accurate diagnosis and prescribe medication accordingly. A culture consists of the following:

  • A container. Common containers include Petri dishes and test tubes.
  • Culture medium. This is the "goo" that the organisms grow in. There are more than 100 different types of culture media designed to provide the optimum environment for a particular organism to grow, from salt, to sugar, to minerals, to agar, a gelling agent used to make solid cultures.
  • A sample. A culture must include a tissue or fluid sample suspected of being infected.

When a doctor orders a culture, he or she has to specify the type of organism suspected. Sometimes this is easy: a bacterial culture on an abscess or a viral culture on a genital ulcer that looks like herpes. When it's not as clear what type of organism is involved, the doctor may order several types of cultures, like a tissue culture and a fungal culture for an unusual-looking rash.

If the culture identifies the organism, the organism might be exposed to different medications to see which ones are most effective. This is known as determining the sensitivity of the organism.

Types of Cultures

There are three types of cultures: solid, liquid and cell.

  • Solid culture. Bacteria and fungi grow on a surface comprised of nutrients, salts, and agar, which is derived from algae. A single microbe is enough to grow an entire colony made up of thousands of cells, which is what makes solid cultures particularly useful. Different organisms will exhibit different colors, shapes, sizes and growth rates, helping microbiologists make an accurate diagnosis.
  • Liquid culture. A liquid culture is grown in a liquid mixture of nutrients. The more organisms present in the culture, the more quickly the liquid becomes cloudy. Liquid cultures aren't as useful as solid cultures because there are often several different types of organisms present, making it hard to pinpoint a specific one. They are most commonly used to diagnose parasitic infections.
  • Cell culture. In a cell culture, human and animal cells are used to infect the cell culture with the organisms present. Diagnosis is based on how the cells are affected. Cell culture tests are more involved that solid and liquid cultures, so they are used as a secondary method.
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  • "Culture." Dorland's Illustrated Medical Dictionary, 31st Ed. Philadelphia: Saunders, 2007.
  • "Culture." Stedman's Medical Dictionary, 28th Ed. Baltimore: Lippincott Williams & Wilkins, 2006. 469.