How Cultures Are Used to Diagnose a Multitude of Ailments

A culture is a method used to identify the organisms that are causing an infection. Cultures are used to identify infectious microbes from urine, stool, genital tract, throat, skin, cerebrospinal fluid (CSF), or any area of the body where there is concern about an infection. In dermatology, for example, a culture test may be used to determine whether a rash is caused by an infection and what organism is responsible. Cultures can be used to grow and identify bacteria, fungi, viruses, and parasites. The results may take days or longer.

What Is a Culture?

A culture test identifies the cause of infection and allows the healthcare provider to make an accurate diagnosis and prescribe medication accordingly. It can be a helpful test, but it isn't right in every situation. Cultures require access to the infection so a sample can be taken, and the results can take longer than some other, less definitive tests.

Often, cultures aren't needed if the cause of an infection is clear from the medical history, exposures, symptoms, or speedier diagnostic tests. And for some infections, antibody tests can confirm past or recent exposure. Additionally, a PCR test can detect the presence of some viruses by identifying the viral genetic material.

Culture Containers

Common containers used for cultures include Petri dishes and test tubes. Blood cultures can be used to grow bacteria inside special vials grown under certain conditions,

The temperature should be optimized for microorganism growth. And some cultures need air, light, or other environmental controls so the infectious microorganism can grow without being contaminated by other microorganisms in the environment.

Culture Medium

This is the material that the organisms grow in. Different pathogens (like bacteria, fungi, and viruses) need different environments in which to grow.

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.

The Sample

A culture must include a tissue or fluid sample suspected of being infected. This may be obtained with a minimally invasive superficial scraping of a skin infection or a throat infection, or with a standard blood sample. But sometimes an invasive procedure, such as a biopsy or a lumbar puncture, is needed to obtain the sample.

When a healthcare provider orders a culture, they have to specify the type of organism suspected. Sometimes this is easy: such as 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 provider may order several types of cultures, like a viral 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 are 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 or animal cells are used. Viruses are grown in a cell culture. Viruses cannot grow in an aceullular (without cells) medium. The culture medium used for common viruses like CMV, adenovirus, and herpes simplex virus contains the cells that are targets of the virus. Then, the sample is examined to see if there's a cytopathic effect (CPE), which means that the virus caused damage to the cells.

Cultures can be useful for identifying an infectious organism, as well as determining the type of treatment that's most likely to eradicate the infection. Sometimes, a culture is obtained, and then treatment is started before the results are ready. If needed, the treatment can be adjusted based on the culture test results.