Agar plate: Encyclopedia II - Agar plate - Inoculation techniques

Agar plate - Inoculation techniques

Before inoculation, important information is written on the bottom of the plates, close to the rim:

1. date of inoculation
2. temperature of incubation
3. duration of incubation
4. microorganism inoculated

Agar plate - Streaking

The most common method of inoculating 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 as a drop from an inoculating loop if the sample is a liquid).
2. A sterile loop (flamed until red hot, then cooled by touching the agar away from the inoculated sample) is then used to spread the bacteria out in one direction from the initial site of inoculation. This is done by moving the loop from side to side, passing through the initial site.
3. The loop is then sterilised (by flaming) again and the first streaks are then spread out themselves.
4. This is repeated 2-3 times, moving around the agar plate.

What should happen is that single bacterial cells get isolated by the streaking, and when the plate is incubated, forming discrete colonies that will have started from just one bacterium each.

Agar plate - Christmas tree

This pattern 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 through the streak multiple times at right-angles to the first streak.

Agar plate - Stab culture

A needle is flamed then immersed in the culture. It is then stabbed into a small sterile jar of nutrient agar. If the bacteria are anaerobic they will grow, otherwise they do not.

Agar plate - Preparing a lawn

A lawn is often used for testing sensitivity to antibiotics, or for work with bacteriophages. What is needed is an even and complete spread of growth all over the agar plate (a "lawn"). Around an antibiotic disc there will be a clear area in which bacterial growth is inhibited; the diameter of this area can be measured to find out whether that bacterial strain is resistant to the antibiotic.

One way to prepare a lawn is to use a 0.5 McFarland suspension of bacteria in saline (this means the saline is made just slightly turbid.) A sterile swab is dipped into this suspension, then it is moved from side to side down the whole agar plate so all the area is covered. The plate is rotated 90 ° and the swab moved side to side perpedicularly to the first time. This is done once more with the swab rotated 45 °.

Once a lawn has been prepared, a small disk of sterile filter paper is soaked in antibiotic and placed on the plate. After incubation there will be a ring of zero growth visible around the filter paper if the lawn bacteria are sensitive to that antibiotic. A collection of small disks each soaked in a different antibiotic, and attached to a larger ring, can be purchased commercially. They are known as antibiotic sensitivity discs or by the name "Mastrings" (a trademark of Mast Group Ltd) and can be used to test the sensitivity of an organism to a range of antibiotics all at once.

Adapted from the Wikipedia article "Inoculation techniques", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki