Scientists often culture bacteria or other microorganisms in petri dishes. The petri dish is partially filled with nutrient agar, a gel-like substance that supports the growth of bacteria. The nutrient agar is then inoculated with bacteria, using an inoculating loop or sterile swab. If done correctly, bacteria will flourish, allowing scientists to perform additional experimentation.
Microorganisms are ubiquitous, making it difficult to eliminate contamination when inoculating petri dishes. In the laboratory, scientists use several methods, collectively called aseptic technique, to ensure that contamination is minimal. Doors and windows to the work area should be closed to prevent air currents. Generally, work is performed near an open flame, such as a Bunsen burner or alcohol lamp, to kill bacteria that would otherwise contaminate the petri dish. All work surfaces and glassware should be sterilised with alcohol before use. Petri dish lids should never be placed on a lab table or other surface. They should be lifted as little as possible while the dish is inoculated. Petri dishes should be incubated or stored in the inverted position so that condensation does not drip onto the surface of the agar.
Inoculation may be done in several ways. Scientists often use an inoculating loop, which is a loop of steel wire attached to a metal stick, to introduce bacteria to the petri dish. Often, when taking samples from people or surfaces, a sterile swab is used to inoculate the petri dish.
Swabbing the Petri Dish
Scientists often use sterile swabs to collect samples of microorganisms. Sterile swabs are sealed in a package. When using swabs, open the package, collect a sample, and either inoculate a petri dish or place the swab in a sterile plastic bag to inoculate a petri dish later.
When swabbing the petri dish, lift the lid slightly, then smear the surface of the agar with the swab. Replace the lid and invert the dish for incubation.
If swabs are used to take a sample, smear a quarter of the dish with the swab. Then use an inoculating loop or another sterile swab to drag a portion of the first smear into a second quarter of the dish. Pass the loop into the flame or get another sterile swab, then drag a portion of the second smear into a third quarter of the dish. Again, place the loop in the flame, or get another sterile swab, and drag a portion of the third smear into the fourth smear. The petri dish now has four quadrants, each with a different concentration of microorganisms.
If a scientists is only looking for the presence of microorganisms, it is acceptable to smear the entire dish with the initial swab.
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