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Bacterial Colony: Morphology, Characteristics & Definition

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  • 0:02 What Is a Bacterial Colony?
  • 1:31 Colony Morphology
  • 4:00 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Microscopic bacteria grow together in visible colonies. Learn the characteristics and vocabulary used to describe the appearance of those colonies and test your knowledge with practice examples in this lesson.

What Is a Bacterial Colony?

What do you know about bacteria? You probably know that they cause illnesses, that they are too small to see, and that, in the case of 'germs', they can be transmitted through the air via coughs and sneezes or found on surfaces. Maybe you even have some antibacterial soap or spray to get rid of these invisible, unwelcome guests. If you have ever had strep throat, you may have gone to your doctor and had your throat swabbed for a culture. In this lesson, you will learn what a bacterial culture looks like and how to observe and interpret a culture.

As we discussed, a bacterium (plural bacteria) is a single-celled organism too small to be seen without a microscope. In order to grow bacteria, known as culturing, the bacteria are spread onto the surface of agar contained within a petri dish. This agar is gel-like and contains all the food and nutrients that the bacteria need to grow.

As the bacteria consume the nutrients, they begin to grow and multiply. This generates thousands to millions to billions of cells that begin to pile up, becoming visible to the naked eye. This pile of cells originates from one cell and is called a bacterial colony. Each species of bacteria produces a colony that looks different from the colonies produced by other species of bacteria. Examination of the form and structure of bacterial colonies is termed colony morphology and is one of the first steps in characterizing and identifying a bacterial culture.

Colony Morphology

These are the characteristics used to accurately and consistently describe the morphology of a bacterial colony:

  • Size
  • Shape
  • Color (also known as pigmentation)
  • Texture
  • Height (a.k.a. elevation)
  • Edge (a.k.a. margin)

Each of these categories has its own vocabulary, allowing other scientists reading your description to paint an accurate picture of the colony.

The size of the colony can be described in two ways. The more accurate technique would be to measure the diameter of the colony with a ruler and report the size in millimeters. The second technique would simply be to describe the colonies as punctiform (tiny pinpoints), small, medium, or large.

Shape refers to the overall appearance of the colonies. The descriptors here are punctiform, circular, irregular, filamentous (has individual thin projections), or rhizoid (has thin, branching projections).

Some bacteria produce pigments, giving the colony a distinct color. Pigments span the entire color spectrum. Recording the color is the first step. In addition to describing the color, this is also the time to identify the colony as opaque (you can't see through it), translucent (you can see through it), dull, or shiny.

Texture refers to the characteristics of the colony surface. Colonies can be dry, mucoid (thick, stringy, and wet), moist, smooth, rough, rugose (wrinkled), or contain concentric rings.

The colony height, or elevation, is a description of how the colony grows vertically. To see the elevation of the colonies it may be helpful to look through the side of the petri dish. The descriptors here are flat, raised, convex (sloping up from the edges), pulvinate (sloping steeply from the edges and very high in the center), and umbonate (has a raised center).

Edge, or margin, describes the borders of the colony. The edge can be entire (smooth, with no projections), undulate (wavy), lobate (lobed), filamentous, or rhizoid.

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