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. Even though bacteria are single-celled organisms, they are able to communicate with one another through a process called quorum sensing. In this way they can function as a multicellular population rather than as individual bacteria. This will be discussed in greater detail in Unit 2.

To view a nice interactive illustration comparing size of cells and microbes, see the Cell Size and Scale Resource at the University of Utah.

 

Bacterial cell shape is determined primarily by a protein called MreB. MreB forms a spiral band – a simple cytoskeleton – around the interior of the cell just under the cytoplasmic membrane.  It is thought to define shape by recruiting additional proteins that then direct the specific pattern of bacterial cell growth. For example, bacillus-shaped bacteria that have an inactivated MreB gene become coccoid shaped, and coccus-shaped bacteria naturally lack the MreB gene.

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.
- scanning Electron Micrograph of Enterococcus

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
- scanning electron micrograph of Micrococcus luteus showing several tetrads

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 Microscopy
- Scanning electron micrograph of methicillin-resistant Staphylococcus aureus (MRSA); courtesy of CDC

An 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
- Scanning electron micrograph of Escherichia coli O157H7, a bacillus; courtesy of CDC

b. streptobacillus: bacilli arranged in chains
- photomicrograph of a streptobacillus

c. 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 spirillum

c. spirochete: a thin, flexible spiral
- photomicrograph of a spirochete
- scanning electron micrograph of the spirochete Leptospira; courtesy of CDC
- scanning electron micrograph of the spirochete Treponema pallidum; courtesy of CDC

Spirals 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).

 

 

ASSIGNMENT FOR UNIT-1, LECTURE -2

Flipped-class assignment
Read II. The Prokaryotic Cell: Bacteria: B. Prokaryotic CellStructure, 1. The Cytoplasmic Membrane in your E-text and answer the 3 learning objectives for this section.

We will be doing a classroom group activity on this section so it is critical that you come prepared. I will be assuming that you have done this preparatory assignment.

 

 

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