II. THE PROKARYOTIC CELL: BACTERIA
A. SIZES, SHAPES, AND ARRANGEMENTS OF BACTERIA
The overall purpose of this Learning Object is to recognize common bacterial shapes and arrangements, and appreciate the extremely small size of bacteria.
LEARNING OBJECTIVES FOR THIS SECTION
Bacteria are:
a. prokaryotic.
b. single-celled, microscopic organisms (Exceptions have been discovered that can reach sizes just visible to the naked eye. They include Epulopiscium fishelsoni, a bacillus-shaped bacterium that is typically 80 micrometers (µm) in diameter and 200-600 µm long, and Thiomargarita namibiensis, a sperical bacterium between 100 and 750 µm in diameter.)
c. generally much smaller than eukaryotic cells.
d. very complex despite their small size.
Most bacteria come in one of three basic shapes: coccus, rod or bacillus, and spiral.
1. The coccus (def)
The cocci are spherical or oval bacteria having one of several distinct arrangements based on their planes of division.a. Division in one plane produces either a diplococcus or streptococcus arrangement.
diplococcus: cocci arranged in pairs
- photomicrograph of a diplococcus
- scanning electron micrograph of a Streptococcus pneumoniae, a diplococcus; courtesy of CDC
- scanning electron micrograph of a Neisseria, a diplococcus; courtesy of Dennis Kunkel's Microscopy
streptococcus: cocci arranged in chains
- photomicrograph of a streptococcus
- scanning electron micrograph of a Streptococcus pyogenes, a streptococcus; courtesy of Dennis Kunkel's Microscopy
- transmission electron micrograph of Streptococcus from the Rockefeller University web page.
b. Division in two planes produces a tetrad arrangement.
a tetrad: cocci arranged in squares of 4
- photomicrograph of a tetrad
c. Division in three planes produces a sarcina arrangement.
sarcina: cocci in arranged cubes of 8
- photomicrograph of a sarcina
d. Division in random planes produces a staphylococcus arrangement.
staphylococcus: cocci arranged in irregular, often grape-like clusters
- photomicrograph of a staphylococcus
- negative image of Staphylococcus aureus
- scanning electron micrograph of Staphylococcus aureus, a staphylococcus; courtesy of Dennis Kunkel's MicroscopyAn average coccus is about 0.5-1.0 micrometer (µm) in diameter. (A micrometer equals 1/1,000,000 of a meter.)
2. The rod or bacillus (def)
Bacilli are rod-shaped bacteria. Bacilli all divide in one plane producing a bacillus, streptobacillus, or coccobacillus arrangement.
a. bacillus: single bacilli
- photomicrograph of a bacillus
- scanning electron micrograph of a bacillus; courtesy of CDC
b. streptobacillus: bacilli arranged in chains
- photomicrograph of a streptobacillusc. a coccobacillus: oval and similar to a coccus
An average bacillus is 0.5-1.0 µm wide by 1.0-4.0 µm long.
3. The spiral (def)
Spirals come in one of three forms, a vibrio, a spirillum, or a spirochete.
a. vibrio: a curved or comma-shaped rod
- photomicrograph of a vibrio
- scanning electron micrograph of a Vibrio cholerae, a vibrio; courtesy of Dennis Kunkel's Microscopy
b. spirillum: a thick, rigid spiral
- photomicrograph of a spirillumc. spirochete: a thin, flexible spiral
- photomicrograph of a spirochete
- scanning electron micrograph of a spirochete; courtesy of CDCSpirals range in size from 1 µm to over 100 µm in length.
4. Exceptions to the above shapes
Trichome-forming, sheathed, stalked, filamentous, square, star-shaped, spindle-shaped, lobed, and pleomorphic (def).
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Updated: June 28, 2006
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