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9.2: Introduction

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    In this lab you will begin to build a culture collection. During each lab your group will subculture several bacteria, each onto a plate, a slant, and into a broth. Your group will use these cultures for many of the subsequent labs and should study them for their growth and physiological characteristics. Eventually, the unknowns will be comprised from instructional stock cultures of these organisms.

    Culture Characteristics on plates, slants, and broths.

    Colony Morphology and characteristics on plates

    Characteristics of individual colonies are best observed by looking at a single isolated colony. Each colony arises from one cell or group of cells (a tetrad or pair for example). Once colonies grow together in a larger mass or lawn it can be difficult to correctly characterize the colony morphology, especially the margins and elevation. Using the Colony Counters or a dissecting scope can help when evaluating colony characteristics. Be careful to protect your cultures by opening the plates only briefly.

    1. Overall Size and Shape-
    1. Margin-the bacterium may form discrete colonies, or may be a “spreader” which spreads quickly across a plate and colonies merge together as they expand.

    1. Elevation-
    1. Surface-a colony will exhibit particular surface characteristics. It might be smooth, shiny, dull, wrinkled/rough, or mucoid (very glossy).
    2. Pigmentation/color-bacteria may produce pigmented colonies in a variety of colors and shades of white/beige. In addition, some bacteria produce diffusible pigments that will affect the color of the media. Compare plates to see if there is a difference in the color of the agar surrounding the colonies in order to assess if a diffusible pigment is being produced.
    3. Opacity-
      • Transparent-can see through the colony like looking through a glass window.
      • Translucent-can see through the colony slightly, like looking through frosted glass.
      • Opaque-cannot see through the colony.
    4. Texture-this is evaluated when subculturing a colony from a plate, or when picking a colony for a stain, etc.
      • Moist/wet
      • Butyrous-like butter!
      • Mucoid/viscous-sticky/snotty
      • Dry-doesn’t stick to the loop; powdery
      • Friable/crusty-breaks or clumps; brittle

    Culture Characteristics on slants-

    Similar growth characteristics to those observed on plates can be seen on slants. There are additional slant characteristics that experienced microbiologists use to evaluate growth. Those characteristics have to do with overall patterns of growth since single colonies are not often observed on a slant. For our purposes, when evaluating slants look for growth, that the culture is not contaminated, and that in general it matches your plate characteristics.

    Culture Characteristics in broths-

    Broth tubes should be observed before disrupting or shaking, then assessed again after gently mixing the tube. One should also note any pigmentation.

    • Turbid-many bacteria will form uniform turbidity (cloudiness) throughout the test tube.
    • Sediment-bacteria may settle to the bottom of the test tube. The broth may be very clear or slightly turbid. Upon mixing, one should make note of what happens to the sediment. Some sediments will disperse evenly in the tube, making it appear more turbid. Other sediments can be quite viscous and will not dissolve completely.
    • Pellicle-bacteria may form a ring around the tube or a film across the top.
    • Flocculence-bacteria may appear in clumps that remain after mixing.

    Contributors and Attributions

    This page titled 9.2: Introduction is shared under a CC BY license and was authored, remixed, and/or curated by Kelly C. Burke.

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