Diagnosis of an Infection: Microbial Cultures

Growing Microbes in a Lab

Infectious diseases are often diagnosed following cultures of samples isolated from the infection site. What exactly is a culture, though, and how does it help in diagnosing an infection?

What is a culture?

A culture is a way of growing a microbe in a laboratory setting. Many bacteria, fungi, parasites and viruses can be grown in a lab when appropriate conditions are used. The precise characteristics of the growing culture can be used to identify the specific microbe.

Use of a “selective agent” can be used to determine features of the microbe. For example, growth of Staph aureus in a culture that contains methicillin (the selective agent) would be indicative of methicillin-resistant Staph aureus (MRSA).

What kinds of infections can be diagnosed using cultures?

Diagnostic cultures are commonly used to identify infectious microbes from samples isolated from urine (urinary tract infections), stool (diarrheal and food-borne diseases), genital tract (STDs), throat (strep throat) and skin (skin infections). Samples isolated from other body parts, such as blood and the spinal cord, may also be cultured; these kinds of infections tend to be more serious and require hospitalization.

What kinds of cultures are used for identifying microbes?

There are three main types of cultures:

  1. Solid culture. Bacteria and fungi can grow on a solid surface made of a mix of nutrients, salts and agar (a gelling agent isolated from seaweed). A single microbe placed on the solid surface can grow into colonies or individual groups comprised of thousands of cells. Colonies are made up of clones, in which all cells are identical to each other. This feature is what makes solid cultures so useful for microbial identification. Different kinds of colonies from various species will have distinct traits and characteristics (e.g., color, size, shape and growth rate of the colony), which help microbiologists identify the microbe.
  1. Liquid culture. A liquid culture is grown in “media” or a “broth” of nutrients. Microbial growth is observed for how quickly the broth becomes cloudy. A cloudier broth typically means a greater number of microbes. Liquid cultures can often contain multiple microbial species, so they tend to be less useful than solid cultures for diagnosis of bacteria and fungi. Liquid cultures, though, are more useful for diagnosis of parasites, which do not form normal colonies in solid cultures.
  1. Cell culture. Some microbes, such as Chlamydia or Rickettsia, and viruses cannot be grown in solid or liquid cultures, but can be grown in human or animal cells. Cultures of human or animal cells are used by “infecting” the cell culture with the microbe and observing the effect on the cells. For example, many viruses have detrimental or “cytopathic” effects on the cells that can be observed by microbiologists. Since cell culture methods tend to be more specialized and require more work and longer periods for diagnosis, though, cell culture is usually used secondarily to other diagnostic methods.

What are the ingredients used in cultures?

Depending on the particular type of culture, the ingredients will vary. In general, most cultures will require a combination of the following:

  • Amino-nitrogen source (digested proteins)
  • Growth factors (blood, serum or yeast extract)
  • Energy source (sugars, carbohydrates)
  • Salts for buffering pH (phosphate, citrate)
  • Minerals (calcium, magnesium or iron)
  • Selective agents (antibiotics or chemicals)
  • Indicators or dyes (for determining acidity levels)
  • Gelling agent for solid cultures (agar)


Mims CA, Playfair, JH, Roitt IM, Wakelin D, and Williams, R. Medical Microbiology. London: Mosby-Year Book

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