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7.7: Testing for Bacterial Motility

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    123371
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    Semi-solid Motility Test medium may also be used to detect motility. The agar concentration (0.3%) is sufficient to form a soft gel without hindering motility. When a non-motile organism is stabbed into Motility Test medium, growth occurs only along the line of inoculation. Growth along the stab line is very sharp and defined. (See Fig. \(\PageIndex{23A}\).) When motile organisms are stabbed into the soft agar, they swim away from the stab line. Growth occurs throughout the tube rather than being concentrated along the line of inoculation. Growth along the stab line appears much more cloud-like as it moves away from the stab. (See Fig. \(\PageIndex{23B}\).) A tetrazolium salt (TTC) is incorporated into the medium. Bacterial metabolism reduces the TTC producing formazan which is red in color. The more bacteria present at any location, the darker red the growth appears.

    Fig. \(\PageIndex{1A}\): Motility Medium Showing a Non-motile Bacterium

    Fig. \(\PageIndex{1B}\): Motility Medium Showing a Motile Bacterium

    negative motility in motility test medium positive motility in motility test medium
    Non-motile bacteria do not move away from the stab line. Heavy growth appears only along the stab. The dark red color indicates where the bacteria are growing. Motile bacteria swim away from the stab line. The growth around the stab line appears more diffuse or blurry. The dark red color indicates where the bacteria are growing.

    (Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0)

    3. Flagella staining

    If we assume that bacterial flagella confer motility, flagella staining can then be used indirectly to denote bacterial motility. Since flagella are very thin (20-28 nm in diameter), they are below the resolution limits of a normal light microscope and cannot be seen unless one first treats them with special dyes and mordants that build up as layers of precipitate along the length of the flagella, making them microscopically visible. This is a delicate staining procedure and will not be attempted here. We will, however look at several demonstration flagella stains

    Contributors and Attributions

    • Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)


    7.7: Testing for Bacterial Motility is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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