Why Are Agar Plates Kept Inverted Whenever Possible?
Agar plates are used in cell and molecular biology by scientists to carry out research experiments. They are plastic petri dishes filled with agar mixed with nutrients that support the growth of a range of microorganisms, such as E.coli bacteria. They are considered a routine laboratory consumable, because they are used up rapidly and normally made in-house, in large batches, and subsequently stored.
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Bacterial Agar Plates
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These plates are usually produced by scientific technicians by incorporating laboratory-grade agar with a bacterial broth such as Luria-Bertani, SOC, Terrific or Eosin-Methylene Blue. The broth-agar preparation is supplemented with other factors that may be required for the bacteria to survive or a specific antibiotic (for example, Ampicillin or Kanamycin) to prevent the growth of unwanted bacteria.
Pouring Agar Plates
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Agar plates are prepared by simply mixing the nutrient-agar mix with a pH-balanced salt solution and heating this up until it melts. Once it has cooled slightly, heat-sensitive supplements are added. This is poured while still warm (and therefore liquefied) into the bottom of each petri dish, allowed to dry and solidify into a surface on which bacterial suspensions can be placed. After the plates have cooled completely they are labeled and stored until use. Proper storage of agar plates is important to ensure that the correct bacteria are cultured up and no unwanted contamination occurs, which can interfere with experimental results.
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Storing Agar Plates
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There are several ways to store agar plates, depending on what supplements they contain. If light-sensitive supplements are present, then the plates are stored in the dark, either by wrapping them in aluminum foil or keeping them in a light-proof container. Plates are always stored in a cold-room or a laboratory refrigerator at 4 degrees C or 39.2 degrees F. Cold storage can prevent or minimize the growth of unwanted microbiological contaminants.
Inverting Agar Plates During Storage
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Plates that are unused are always inverted or flipped upside down onto their lids during storage. This is to prevent the plate from undergoing evaporation if the plate is stored for extended periods, which can affect the efficiency of bacterial growth, or allow the growth of undesirable microorganisms such as molds.
Inverting Agar Plates During Inoculation and Incubation
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Once the plates have been streaked with a wet suspension of bacteria, this must be allowed to evaporate slightly before incubation overnight. However, a moderate amount of wetness will still be present. If the plate is not inverted during incubation, the bacteria will not be able to settle onto the agar properly, which will either prevent them from growing and forming colonies properly, or encourage the growth of undesirable microorganisms. Either one of these results will invalidate the experiment. In addition, any condensation or wetness can cause streaking, which will make picking and analyzing separate colonies much more difficult. Once the bacteria have formed colonies, the plate is also stored upside down to maintain moisture levels.
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References
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