Culturing microorganisms means maintaining and reproducing them under laboratory conditions. Because bacteria differ in their nutritional needs, microbiologists use many kinds of culture media. This lesson explains how media are classified by their ingredients and by the way they support or distinguish bacterial growth.
General-purpose media support the growth of many kinds of bacteria. Tryptic soy broth (TSB) is a common example. A non-selective medium does not contain specific inhibitors intended to prevent particular groups of microorganisms from growing. Mueller-Hinton agar is a non-selective medium commonly used for antibiotic susceptibility testing, including the Kirby-Bauer disc diffusion test. Its starch helps support growth and absorb substances that could interfere with antibiotic activity. Its relatively loose agar allows antibiotics to diffuse through the medium.
All flashcards
What is a chemically defined medium?
A medium in which the identity and exact amount of every chemical component are known. EZ medium is an example.
What is a complex medium?
A medium containing extracts or digests, such as yeast, meat, or plant materials, whose complete chemical composition and individual component amounts are not precisely known. TSB is an example.
What is the purpose of an enriched medium?
It supplies extra growth factors, vitamins, or other essential nutrients to support organisms with demanding nutritional requirements, including fastidious organisms.
How are selective and enrichment approaches different?
A selective medium inhibits some organisms while permitting others to grow. An enrichment culture uses favorable conditions, often through repeated transfers, to increase the proportion of a desired organism in a mixed population.
How does MacConkey agar differentiate lactose fermenters?
Lactose fermentation produces acid. The pH indicator neutral red responds to the lower pH, turning strong fermenter colonies and nearby medium hot pink.
Quiz questions
What distinguishes a chemically defined medium from a complex medium?Answer: Its chemical ingredients and their exact amounts are specified.
Why are nutrients such as growth factors added to enriched media?Answer: To meet the nutritional requirements of fastidious organisms.
Which ingredients make MacConkey agar selective against many gram-positive bacteria?Answer: Bile salts and crystal violet inhibit many gram-positive bacteria.
Why might a microbiologist repeatedly transfer a mixed sample into medium with crude oil as its only carbon source?Answer: To increase the proportion of bacteria able to use crude oil.
Illness beginning within hours after eating contaminated food can suggest intoxication. What causes this type of illness?Answer: A toxin already produced in the food before it is eaten.
Based on Microbiology by Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.