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2.7: Colony Morphology and Pigmentation

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    122949
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    A colony (see Fig. \(\PageIndex{1}\)) is a visible mass of microorganisms growing on an agar surface and usually originating from a single organism or arrangement of organisms. Different microorganisms will frequently produce colonies which differ in their morphological appearance (form, elevation, margin, surface, optical characteristics, and pigmentation). Single colonies can be described using standard terms, as listed in Appendix A.

    Fig. \(\PageIndex{1A}\): Isolated Colonies of Escherichia coli

    Fig. \(\PageIndex{1B}\): A Microcolony of Escherichia coli

    Photograph of an agar plate showing isolated colonies of <i>Escherichia coli</i>. Scanning electron micrograph of a microcolony of <i>Escherichia coli</i> showing the individual bacilli in the colony. Note the layers of the bacillus-shaped E. coli.
    Escherichia coli on Luria agar. © Kevin Hedetniemi and Min-Ken Liao, authors.
    Licensed for use, ASM MicrobeLibrary.
    Escherichia coli Microcolony. © James Shapiro and Clara Hsu, authors. Licensed for use, ASM MicrobeLibrary.
    Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0

    Probably the most visual characteristic is pigmentation (color). Some microorganisms produce pigment during growth and are said to be chromogenic. Often, however, formation of pigment depends on environmental factors such as temperature, nutrients, pH and moisture. For example, Serratia marcescens produces a deep red pigment at 25°C, but does not produce pigment at 37°C.

    Pigments can be divided into two basic types: water-insoluble and water-soluble. If the pigment is water-insoluble (see Figs. \(\PageIndex{2A}\)-\(\PageIndex{2E}\)), as is the case with most chromogenic bacteria, it does not diffuse out of the organism. As a result, the colonies are pigmented but the agar remains the normal color. If the pigment is water-soluble as in the case of Pseudomonas aeruginosa (see Fig. \(\PageIndex{2F}\) and Fig. \(\PageIndex{2G}\)), it will diffuse out of the organism into the surrounding medium causing the agar to appear pigmented. Note that pigment can generally be detected only on a medium that was originally colorless.

    Fig. \(\PageIndex{2A}\): TSA Plate Culture of the Chromogenic Bacterium Serratia marcescens

    Fig. \(\PageIndex{2B}\): TSA Plate Culture of the Chromogenic Bacterium Micrococcus luteus

    Fig. \(\PageIndex{2C}\): TSA Pate Culture of the Chromogenic Bacterium Mycobacterium phlei

    Photograph of a TSA plate culture of <i>Serratia marcescens</i> showing its red, water insoluble pigment. Note red, water insoluble pigment. Photograph of a TSA plate culture of <i>Micrococcus luteus</i> showing its yellow, water insoluble pigment. Note yellow, water insoluble pigment. Photograph of a TSA plate culture of <i>Mycobacterium phlei</i> showing its orange, water insoluble pigment. Note orange, water insoluble pigment.
    Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0

    Fig. \(\PageIndex{2D}\): TSA Plate Culture of the Chromogenic Bacterium Micrococcus roseus

    Fig. \(\PageIndex{2E}\): TSA Plate Culture of the Chromogenic Bacterium Staphylococcus aureus

    Photograph of a TSA plate culture of <i>Micrococcus roseus</i> showing its pink rose colored, water insoluble pigment. Note pink rose, water insoluble pigment. Photograph of a TSA plate culture of <i>Staphylococcus aureus</i> showing its gold, water insoluble pigment. Note gold, water insoluble pigment.
    Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0

    Fig. \(\PageIndex{2F}\): TSA Plate Culture of the Chromogenic Bacterium
    Pseudomonas aeruginosa

    Fig. \(\PageIndex{2G}\): Pseudomonas aeruginosa Growing on Trypticase Soy Agar

    Photograph of a TSA plate culture of <i>Pseudomonas aeruginosa</i> showing its green, water soluble pigment. Note green, water soluble pigment. Photograph of a TSA plate culture of <i>Pseudomonas aeruginosa</i> showing its green, water soluble pigment. Note green, water soluble pigment.
    Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0
    Note

    A link to some Petri Plate art

    Contributors and Attributions

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


    2.7: Colony Morphology and Pigmentation is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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