Exersise 12.2 - Examination of Bacterial Colonies

LEVEL I

Materials

• Petri plate cultures of various bacteria

Procedure

1. Obtain an established culture. Note that the bacteria grow in clearly defined groups, known as colonies. In most cases, each colony is the outgrowth from an individual cell, although they may overlap if excessive numbers of cells were plated.

2. Visually examine the individual colonies of bacteria and describe them according to the following characteristics:
   • Size. Pinpoint, small, medium, or large, based on the relative differences between the largest and smallest colonies seen.
     

   • Shape and Margins. Round, regular or irregular.
     

   • Elevation. Flat, convex or rounded, umbonate (flat on margins and raised in the center - like a fried egg), craterlike (with depressed center).
     

   • Consistency. Shiny or rough.
     

   • Color. Describe the color as accurately as possible, distinguishing between different types of gray or white, yellows, and red. If the pigment appears to diffuse into the surrounding medium, rather than coloring only the colony, it is a water-soluble pigment.
     

3. Determine and record the identity of your colonies.

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