Microbiology

Bacterial Shape Cell Arrangement Colony Morphology



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Bacteria are unicellular microorganisms found in every habitat on Earth. Nearly all have cell walls composed of peptidoglycan and reproduce by binary fission (cloning of cells). Although many of these microbes are harmless or beneficial to humans, others are pathogenic, causing infectious diseases.

* Identification of Bacteria *

Some of the first steps in identifying bacteria is to examine:

1. the shape of the individual bacteria.

2. whether the bacteria exist in specific groupings.

3. the colony morphology (the appearance of a "colony"; a group of millions of bacteria that arose from one single parent cell).

* Bacterial Shapes *

Most bacteria are classified according to shape:

~ bacillus (pl. bacilli) = rod-shaped

~ coccus (pl. cocci sounds like cox-eye) = spherical

~ spirillum (pl. spirilla) = spiral

Some bacteria have more unusual shapes:

~ coccobacilli = elongated coccal form

~ filamentous = bacilli that occur in long threads

~ vibrios = short, slightly curved rods

~ fusiform = bacilli with tapered ends

* Prokaryote Arrangement of Cells *

Bacteria sometimes occur in groups, rather than singly, and the single cell's shape influences the cell arrangements that they form as the bacterial cells divide.

Bacilli divide along a single axis, and are sometimes seen in pairs or chains. Since they only divide along one axis, you will not find bacilli in clusters, such as those formed by Staphylococcal bacteria.

Cocci divide on one or more planes, producing cells in:

~ pairs (diplococci)

~ chains (streptococci)

~ packets (sarcinae)

~ clusters (staphylococci)

Size, shape and arrangement of cells are often the first clues in identification of bacteria. However, since there are many "look-alikes", methods other than microscopy must be used to determine the genus and species of an organism.

* Bacterial Colony Morphology *

Bacterial populations grow extremely fast when they supplied with the nutrients and environmental conditions that allow them to thrive. Through this growth, different types of bacteria will sometimes produce colonies that are distinctive in appearance. Some colonies may be colored, some are circular in shape, while others are irregular. The characteristics of a colony (shape, size, color, etc.) are termed the "colony morphology".

Colony morphology is a way scientists can identify bacteria. There are a few basic characteristics of colony morphology that are typically evaluated.

1. Form - What is the basic shape of the colony? For example, circular, filamentous, etc.

2. Elevation - What is the cross-sectional shape of the colony? To see this, turn the Petri dish on end.

3. Margin - What is the magnified shape of the edge of the colony?

4. Surface - How does the surface of the colony appear? For example, smooth, glistening, rough, dull (opposite of glistening), rugose (wrinkled), etc.

5. Opacity - Is the colony transparent (clear), opaque, translucent (almost clear, but distorted vision, like looking through frosted glass), iridescent (changing colors in reflected light), etc.

6. Chromogenesis (pigmentation) - For example, white, buff, red, purple, etc.

* Sources *

Bauman, R. (2005) Microbiology. Pearson Banjamin Cummings.
Park Talaro, K. (2008) Foundations in Microbiology. McGraw-Hill.

 

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