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Bacteria
Jessica Harwood
Douglas Wilkin, Ph.D.
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AUTHORS
Jessica Harwood
Douglas Wilkin, Ph.D.
EDITOR
Douglas Wilkin, Ph.D.
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Printed: July 1, 2015
CONTRIBUTORS
Doris Kraus, Ph.D.
Niamh Gray-Wilson
Jean Brainard, Ph.D.
Sarah Johnson
Jane Willan
Corliss Karasov
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C HAPTER
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Chapter 1. Bacteria
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Bacteria
Describe common bacterial shapes.
Summarize the similarities between bacterial cells and eukaryotic cells.
Summarize the differences between bacterial cells and eukaryotic cells.
Explain the roles of the cell wall and the flagella in bacteria.
Are bacteria living things?
Bacteria are individual living cells. Bacteria cells are similar to your cells in many ways; yet, they also have distinct
differences. Bacteria have many unique adaptations allowing them to live in many different environments.
Characteristics of Bacteria
Bacteria are the most successful organisms on the planet. They lived on this planet for two billion years before the
first eukaryotes and, during that time, evolved into millions of different species.
Size and Shape
Bacteria are so small that they can only be seen with a microscope. When viewed under the microscope, they have
three distinct shapes ( Figure 1.1). Bacteria can be identified and classified by their shape:
1. Bacilli are rod-shaped.
2. Cocci are sphere-shaped.
3. Spirilli are spiral-shaped.
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FIGURE 1.1
Bacteria come in many different shapes.
Some of the most common shapes are
bacilli (rods), cocci (spheres), and spirilli
(spirals). Bacteria can be identified and
classified by their shape.
Similarities to Eukaryotes
Like eukaryotic cells, bacterial cells have:
1.
2.
3.
4.
Cytoplasm, the fluid inside the cell.
A plasma or cell membrane, which acts as a barrier around the cell.
Ribosomes, in which proteins are put together.
DNA. By contrast though, bacterial DNA is contained in a large, circular strand. This single chromosome is
located in a region of the cell called the nucleoid. Many bacteria also have additional small rings of DNA
known as plasmids.
See bacterial cell pictured below ( Figure 1.2).
FIGURE 1.2
The structure of a bacterial cell is distinctive from a eukaryotic cell because of
features such as an outer cell wall, the
circular DNA of the nucleoid, and the lack
of membrane-bound organelles.
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Chapter 1. Bacteria
Unique Features
Bacteria lack many of the structures that eukaryotic cells contain. For example, they don’t have a nucleus. They
also lack membrane-bound organelles, such as mitochondria or chloroplasts. The DNA of a bacterial cell is also
different from a eukaryotic cell. Bacterial DNA is contained in one circular chromosome, located in the cytoplasm.
Eukaryotes have several linear chromosomes. Bacteria also have two additional unique features: a cell wall and
flagella.
The Cell Wall
Bacteria are surrounded by a cell wall consisting of peptidoglycan. This complex molecule consists of sugars and
amino acids. The cell wall is important for protecting bacteria. The cell wall is so important that some antibiotics,
such as penicillin, kill bacteria by preventing the cell wall from forming.
Some bacteria depend on a host organism for energy and nutrients. These bacteria are known as parasites. If the
host starts attacking the parasitic bacteria, the bacteria release a layer of slime that surrounds the cell wall. This
slime offers an extra layer of protection.
Flagella
Some bacteria also have tail-like structures called flagella ( Figure 1.3). Flagella help bacteria move. As the flagella
rotate, they spin the bacteria and propel them forward. Though some eukaryotic cells do have a flagella, a flagella in
eukaryotes is rare.
FIGURE 1.3
The flagella facilitate movement in bacteria. Bacteria may have one, two, or many
flagella—or none at all.
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Vocabulary
• bacilli: Rod-shaped.
• cell wall: Tough outer layer of bacterial cells that helps support and protect the cell; also found around plant
cells.
• cocci: Sphere-shaped.
• flagella: Tail-like structures that help bacteria move.
• nucleoid: DNA, contained in a large circular strand, forming a single chromosome.
• parasite: Organism that benefits in a symbiotic (parasitism) relationship in which one organism is harmed.
• peptidoglycan: Complex molecule consisting of sugars and amino acids that makes up the bacterial cell wall.
• plasmid: Small ring of additional DNA.
• spirilli: Spiral-shaped.
Summary
• Bacteria can be classified by their shape, including bacilli (rods), cocci (spheres), and spirilli (spirals)
• Bacteria are like eukaryotic cells in that they have cytoplasm, ribosomes, and a plasma membrane.
• Features that distinguish a bacterial cell from a eukaryotic cell include the circular DNA of the nucleoid, the
lack of membrane-bound organelles, the cell wall of peptidoglycan, and flagella.
Explore More
Use the resources below to answer the following questions.
Explore More I
• Prokaryotic Cells at http://www.youtube.com/watch?v=gGlhCWg5iOM (2:23)
1.
2.
3.
4.
What does the word prokaryote mean?
How does bacterial DNA differ from the DNA in eukaryotic cells? What is one explanation for this difference?
What is the bacterial capsule? Are all bacterial capsules made of the same material?
How does the movement of bacterial flagella differ from the movement of eukaryotic flagella?
Explore More II
• Cells Alive at http://www.cellsalive.com/cells/bactcell.htm
1. Describe three features of a bacterial cell?
2. What is the nucleoid?
3. What is a plasmid?
Review
1.
2.
3.
4.
4
How are bacteria classified?
How are bacterial cells like your cells?
How are bacterial cells different from your cells?
Describe the bacterial flagella.
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Chapter 1. Bacteria
References
1. Mariana Ruiz Villarreal (LadyofHats). Drawing of rod, sphere, and spiral shaped bacteria. Public Domain
2. Mariana Ruiz Villarreal (LadyofHats). Structure of a bacterial cell. Public Domain
3. Zappy’s. Flagella help bacteria move. CC BY-NC 3.0
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