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Agar plate

An agar plate is a sterile petri dish that contains agar, and is used to culture bacteria and fungi.

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Innoculation techniques

Streaking

The most common method of innoculating an agar plate is streaking.
  1. With this method, a small amount of sample is placed on the side of the agar plate (either with a swab, or a drop if it's a liquid).
  2. A sterile loop is then used to spread the bacteria out one direction from this. This is done by moving the loop from side to side, entering the initial site of innoculation.
  3. The loop is then sterilised (by flaming) and the first streaks are then spread out themselves.
  4. This is repeated 3-4 times, moving around the agar plate.

What should happen is that single bacteria get isolated by the streaking, and when the plate is incubated, the resulting colony will have started from just one bacterium.

Christmas tree

This is used for culture of urine. A small loop is dipped in the urine, and a single streak is made down the middle of the agar plate. Then the loop is swayed in and out going at through the streak multiple times at right-angles to the first streak.

Preparing a lawn

A lawn is often used for antibiotic sensitivity[?] testing.

Incubation of agar plates Most plates are incubated at 37°C in 5% CO2, which is the temperature and conditions that most of the body's bacteria will grow. Special incubators can maintain these conditions.

Some bacteria must be incubated anaerobically (without an oxygen). These can be placed in containers, along with a substance that removes oxygen, and the tightly sealed container placed in the regular incubator.

Fungi, and some bacteria (e.g. Yersinia sp.) should be incubated slightly cooler. This is usually 30°C, and room air often is used.

Campylobacter is a difficult bacteria to grow. It needs special agar plates, plus its own microaerophilic environment.

Types of agar plates

  • Blood agar - contains blood cells from an animal (e.g. a sheep). It will grow most bacteria.
  • Chocolate agar - the contains lysed blood cells, and is used for growing fastidious (fussy) respiratory bacteria.
  • Neomycin agar - contains the antibiotic neomycin.
  • Sabouraud agar - used for fungi. It contains gentamicin and has a low pH that will kill most bacteria.
  • Thayer-Martin agar - chocolate agar designed to isolate Neisseria gonorrhoeae.
  • XLD agar - Xylose Lysine Deoxycholate agar. It is used for the culture of stool samples, and contains two indicators. It is formulated to inhibit gram-positive bacteria. The growth of gram-negative bacilli is encouraged. The colonies of lactose fermenters appear yellow.



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