Using Cultures for Infectious Disease Diagnosis

Infectious diseases are often diagnosed following cultures of samples isolated from the infection site. Ever wonder how the doctors know what bug you have? It's often not that easy to figure it out. The lab workers in the microbiology lab often need to grow out the bug from a sample, say from your blood or sputum. This sample can then be tested by trying to grow it on many different cultures to see where it grows best. Like plants, microbes have their favorite soils and conditions too. They won't grow where they just can't grow.

So, what exactly is a culture, and how does it help in diagnosing an infection?

Scientist about to view a blood sample under a microscope
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Defining a Growing 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, the growth of Staph aureus in a culture that contains methicillin (the selective agent) would be indicative of methicillin-resistant Staph aureus (MRSA). Methicillin is a type of antibiotic, so a bacteria that grows in a culture containing methicillin is resistant to treatment using that specific antibiotic.

These cultures are usually contained on plates or tubes which contain special food that allows a particular pathogen or group of pathogens to grow. This allows lab works to identify which microbe is growing. Lab workers may need to grow the microbe on multiple different culture plates (or tubes) to pinpoint exactly which microbe it is. This can be like a Sherlock Holmes detective puzzle.

Types of Infections That Can Be Diagnosed

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.

Types of Cultures

There are three main types of cultures:

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). This is often on a plate the size of your palm. Many are small dishes with red gel, some gels are yellow or other colors. 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.

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 the diagnosis of bacteria and fungi. Liquid cultures, though, are more useful for the diagnosis of parasites, which do not form normal colonies in solid cultures.

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. It can be particularly hard to grow out some microbes.

Ingredients Used in Cultures

Depending on the particular type of culture, the ingredients will vary. There are many different ingredients used, as these can be used to deduce where a microbe can and can't grow, hence identifying what the microbe is. It often doesn't tell us much about the organism itself, but instead helps us deduce the name of the organism. Each microbe has its own peculiar taste in culture gels and ingredients. 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
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