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Bacteria grow tremendously fast when supplied with an abundance of nutrients. Different types of bacteria will produce different-looking colonies, some colonies may be colored, some colonies are circular in shape, and others are irregular. The characteristics of a colony (shape, size, pigmentation, etc.) are termed the colony morphology. Colony morphology is a way scientists can identify bacteria. In fact there is a book called Bergey's Manual of Determinative Bacteriology (commonly termed Bergey's Manual) that describes the majority of bacterial species identified by scientists so far. This manual provides descriptions for the colony morphologies of each bacterial species available at Amazon.com.

Although bacterial and fungi colonies have many characteristics and some can be rare, there are a few basic elements that you can identify for all colonies:(1)

Please note that 3 additional elements of morphology should be examined only in a supervised laboratory setting: consistency, emulsifiability, and odor.

Refer to the diagram below for illustrated examples of form, elevation, and margin:(2)

Diagram outlines possible form, elevation and margins for bacteria colonies

Common physical characteristics of bacteria colonies are listed and separated into 3 categories. The bacterias form describes how they spread in a petri dish and can be: circular (covering the whole dish) irregular (spreading out in a non-uniform pattern), filamentous (spreading out like roots towards the outer edge), and rhizoid (spreading out like branches with main segments splitting into smaller segments). The elevation of bacteria describes how they grow upwards: raised in a shallow dome shape, convex growth in a steep dome shape, flat growth parallel to the ground, umbonate growth with a small raised bump in the center, and crateriform growth has a raised profile with a concave dip in the center. The margin growth of a bacteria describes how the edges of the colony appear under a microscope and can appear: with a smooth rounded edge called entire. Bacteria can grow with a bumpy edge called undulate. A filiform margin has small branch like growth that spreads outward. A curled margin appears as wave-like layers spreading outward. A lobate margin has rounded finger-like growths that spread outward.

What Can Grow on a Nutrient Agar Plate?

Bacteria

Each distinct circular colony should represent an individual bacterial cell or group that has divided repeatedly. Being kept in one place, the resulting cells have accumulated to form a visible patch. Most bacterial colonies appear white, cream, or yellow in color, and fairly circular in shape.

For example:

A visible colony of the bacteria Bacillus subtilis in an agar plate
Bacillus subtilis(3)

A visible colony of the bacteria Proteus vulgaris in an agar plate
Proteus vulgaris(4)

A visible colony of the bacteria Staphylococcus aures in an agar plate
Staphylococcus aures(5)

A visible colony of the bacteria Streptococcus pyogenes in an agar plate
Streptococcus pyogenes(6)

Yeasts

Yeast, a type of fungi (plural for fungus), is found in many places from nature, to research labs and even everyday kitchens for baking. Yeast colonies generally look similar to bacterial colonies. Some species, such as Candida, can grow as white patches with a glossy surface.

For example:

A visible colony of the yeast Candida Albicans in an agar plate
Candida Albicans) is a type of yeast that can grow on the surface of skin(7)

A visible colony of round yeast in an agar plate
Round yeast colonies(8)

A visible colony of pink yeast in an agar plate
Pink yeast colonies(9)

Molds

Molds are actually fungi, and they often appear whitish grey, with fuzzy edges. They usually turn into a different color, from the center outwards. Two examples of molds are shown below:

A visible colony of green mold in an agar plate
Green Mold (Trichoderma harzianum)(10)

A visible colony of black mold in an agar plate
Black Mold (Aspergillus nidulaus)(11)

Other Fungi

Moss green colonies, a white cloud, or a ring of spores can be attributed to the growth of Aspergillus, which is common in such fungal infections as athlete's foot. Here is an example of what Aspergillus looks like:

A visible colony of the fungi Aspergillus in an agar plate
Aspergillus(12)

Finally, whenever a thorough, visual identification is not possible, examples of additional tests are gram stains (http://www.austincc.edu/microbugz/gram_stain.php), growths on selective media, and enzymatic tests.

Endnotes

(1) "Microbiology 101 Laboratory Manual." Washington State University. http://www.rlc.dcccd.edu/mathsci/reynolds/micro/lab_manual/colony_morph.html, accessed January 14, 2005.

(2) "Microbiology 101 Laboratory Manual." Washington State University. http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/101lab4.html, accessed January 14, 2005.

(3) "Bacterial Colony Morphology." Austin Community College. http://www.austin.cc.tx.us/microbugz/03morphology.html, accessed January 14, 2005.

(4) "Bacterial Colony Morphology." Austin Community College. http://www.austin.cc.tx.us/microbugz/03morphology.html, accessed January 14, 2005.

(5) "Bacterial Colony Morphology." Austin Community College. http://www.austin.cc.tx.us/microbugz/03morphology.html, accessed January 14, 2005.

(6) "Bacterial Colony Morphology." Austin Community College. http://www.austin.cc.tx.us/microbugz/03morphology.html, accessed January 14, 2005.

(7) Silvermedicine. http://www.silvermedicine.org/Candidaalbicans.jpg, accessed January 14, 2005.

(8) Biology at the University of Cincinnati Clermont College. http://biology.clc.uc.edu/fankhauser/Labs/Microbiology/Yeast_Plate_Count/07_yeast_0.2mL_plate_P7201181.jpg, accessed January 14, 2005.

(9) Teachers Experiencing Antarctica and the Arctic. http://tea.rice.edu/Images/stoyles/stoyles_pinkJPG.JPG.jpg, accessed January 14, 2005.

(10) The Shroomery. http://www.shroomery.org/images/23418/green5.jpg, accessed January 14, 2005.

(11) The Shroomery. http://www.shroomery.org/images/23418/Aspergillus_nidulaus.jpg, accessed January 14, 2005.

(12) ETH Life International. http://www.ethlife.ethz.ch/images/aspergillus-l.jpg, accessed January 14, 2005.

Credits

Beatrice Leung, Genentech, Inc.
Shijun Liu, Science Buddies

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