Prokaryotic Module1
This module will help you become more familiar with prokaryotes, the smallest cellular
microorganisms. We’ll concentrate mostly on Bacteria, with only a little information
about Archaea. By way of explanation, we use “Bacteria” (in italics and with the “B”
capitalized) when referring to a group of organisms in the Domain Bacteria and
“bacteria” when referring to prokaryotic organisms we are familiar with that live in and
on us and in our environment. For a discussion of Domains, refer to the Classification
module.
In the figure below (a) is a diagram of a prokaryotic cell and (b) is a diagram of a
eukaryotic cell.
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The reference for this module was Brock Biology of Microorganisms, 10th ed., by
Michael Madigan, John Martinko, and Jack Parker. The publisher is Prentice Hall, Upper
Saddle River, NJ, 2003.
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Figure from Brock Biology of Microorganisms, 10th ed., by Michael Madigan, John
Martinko, and Jack Parker. The publisher is Prentice Hall, Upper Saddle River, NJ, 2003.
Used with permission.
As shown in the illustration, prokaryotic cells are typically smaller than eukaryotic cells.
Notice that from a structural standpoint, the prokaryotic cell looks much simpler. And in
most ways it is. A prokaryotic cell doesn’t have a lot of membranes and compartments
like a eukaryotic cell does; however, prokaryotic cells have all the basics needed to be a
living cell. They can take in nutrients and use them to make the molecules and structures
they need and excrete the waste products. They can replicate their genetic material and
reproduce. They can respond to certain stimuli from outside the cell. Some prokaryotic
cells are nonmotile, and some are motile, usually because they have flagella.
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Micrograph of Bordatella bronchiseptica flagella stain.
Michael Miller, author. ASM Division C Image Library (linked to
http://www.asmusa.org/division/c/library.htm)
Some are photosynthetic; some aren’t. Some require oxygen, just as most eukaryotes do;
some can use it if it is present but don’t require it; and some can grow only if oxygen is
absent. A few are obligate intracellular parasites, able to live only inside another cell.
Bacteria
Advertisements for antibacterial products can lead us to believe that all bacteria are
harmful. Contrary to popular impression, relatively few of them cause disease in humans
(or other animals or plants). Most bacteria that live in or on us are harmless, and many
are beneficial. Bacteria in the environment are absolutely necessary to life. Some bacteria
recycle nitrogen; others recycle carbon; and still others recycle sulfur. Some bacteria are
photosynthetic, capturing energy from the sun and putting it into the food chain, and
many of these also produce oxygen.
We use bacteria to make items important in our everyday life, including some foods.
Bacteria are used in the production of cheese, pickles, yogurt, and vinegar and a variety
of other things we eat or drink. Diet soft drinks are sweetened with aspartame made from
two amino acids produced by bacteria, and regular soft drinks are sweetened with high
fructose syrup produced by bacteria. Your laundry detergent contains enzymes made by
bacteria. Among the most important bacterial products are health care items, including
most antibiotics; vitamins C, B12, and riboflavin; and an assortment of specialty proteins
made in bacteria by genetic engineering. This is only a partial list of the ways bacteria
help supply our needs.
In the Classification module guessing game, the players quickly learned that the object
was a member of the Domain Bacteria. The next question was “Is it gram-negative?”
That was a good question because most Bacteria can be divided into two major groups,
gram-positive and gram-negative, according to differences in their cell wall structure that
affect their reactions in the Gram stain procedure. This grouping is a useful one, because
members of each group resemble other organisms in that group in some important ways.
Notice, however, that the next question asked was “Is it gram-positive?” It was necessary
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to be sure of this because there are some unusual Bacteria that do not belong to either the
gram-negative or gram-positive group.
The next question was “Is it a coccus?” When the answer was “No,” the players asked “Is
it rod-shaped?” While multicelled organisms come in a variety of shapes and sizes, the
options for Bacteria are pretty limited. Most are either rod-shaped . . .
Microcolonies growing on a catheter segment Part 1 Figure 2, Mahmoud Yassien and
Nancy Khardon, authors. Licensed for use, ASM MicrobeLibrary (linked to
http://www.microbelibrary.org)
or round (coccus) . . .
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Microcolonies growing on a catheter segment Part 1 Figure 4
Mahmoud Yassien and Nancy Khardon, authors. Licensed for use, ASM MicrobeLibrary
(linked to http://www.microbelibrary.org)
Cocci (plural of coccus) often do stay together after cell division, some in bunches as in
the micrograph above (staphlococci), some in chains as shown below (streptococci), and
a few in other formations such as groups of four or eight. The terms streptococcus and
staphylococcus as used here describe the groupings. This can be a bit confusing because
the same words italicized and with the first letter capitalized are used as a part of the
names of some organisms, for example Streptococcus pyogenes and Staphylococcus
epidermidis. Are there Bacteria that have shapes other than round or rod-shaped? Yes, a
few. We will discuss a group of spiral shaped Bacteria below.
The next question was “Does it form endospores?”
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Scanning electron micrograph of Streptococcus sobrinus on an unerupted tooth.
Lloyd G. Simonson, author. Licensed for use, ASM MicrobeLibrary (linked to
http://www.microbelibrary.org)
Micrograph of Bacillus subtilis, spore stain. Neal Chamberlain, author. Licensed for use,
ASM MicrobeLibrary (linked to http://www.microbelibrary.org)
The player who asked that might have been thinking of Bacillus anthracis because it has
been in the news and would be a good organism for the game. Most Bacteria do not form
endospores, although the two major genera that do—Bacillus and Clostridium— are
gram-positive. Most members of the genus Bacillus are not pathogenic, Bacillus
anthracis being the most important exception. There are a couple of especially notorious
pathogens in the genus Clostridium, Clostridium tetani and Clostridium botulinum. Their
names are a big clue to the diseases they cause—tetanus and botulism respectively.
Endospores are important enough to deserve some mention. Even though not many
species form endospores, we worry about them quite a bit. Endospores are tough
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structures, very resistant to heat, chemicals, and irradiation, which are the principle
means we use to kill unwanted microorganisms. Very high temperatures must be used to
destroy endospores, and most chemical treatments can disinfect (kill or inhibit the growth
of microorganisms) but not sterilize (kill or remove all living organisms and their
viruses). Some species of the genera Bacillus and Clostridium are present in large
numbers in the soil and other places in the environment. Where stray microorganisms are
unwelcome, such as in health care facilities, workers must be knowledgeable about
endospores—how to destroy them and how to keep them out of items that have been
sterilized.
We’ve mentioned a few Bacteria of special interest to us: Bacillus anthracis, Clostridium
tetani, Clostridium botulinum, Streptococcus pyogenes, and Staphylococcus aureus. All
of these are gram-positive, and all are pathogens. Most gram-positive organisms are not
pathogenic, however, and some gram-positive organisms are important industrially.
Lactobacillus acidophilus is used in making yogurt, and Lactobacillus sanfrancisco is the
reason sourdough bread has that distinctive sour taste. Several species of Streptomyces
produce antibiotics, (e.g., erythromycin, tetracycline, and streptomycin).
How about a few interesting gram-negative Bacteria?
The most familiar gram-negative Bacteria are those that occasionally make the news, like
Escherichia coli (abbreviated as E. coli) and species of the genus Salmonella, usually
when contaminated foods have resulted in an outbreak of illness.
Micrograph of E. coli 0157H:7 (phase contrast) David E. Graham, author. Licensed for
use, ASM MicrobeLibrary (linked to http://www.microbelibrary.org)
E. coli is a normal inhabitant of the intestinal tract of animals, but a few strains are
pathogenic, including E. coli O157:H7 shown in the micrograph above. E. coli is also
known because it has been especially well studied and used extensively in microbiology
laboratories for teaching and in research. Many of the genetic engineering procedures
used today are based on work done with E. coli. The genus Salmonella contains species
that are pathogens, the most serious being Salmonella typhi, which causes typhoid fever.
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Micrograph of Salmonella typhi, India ink stain. The organisms were incubated with O,
Vi, and H antibodies prior to staining. (1000X oil) Michael Miller, author. ASM Division
C Image Library (linked to http://www.asmusa.org/division/c/library.htm)
On the other hand, several gram-negative Bacteria are critical environmentally or useful
industrially. Some species carry out crucial steps in the nitrogen cycle. Rhizobium species
can use nitrogen present in air, converting it from N2 to NH3., a process called nitrogen
fixation. Nitrosomonas and Nitrobacter species carry out reactions to transfer the
nitrogen into NO3, which can be used as a source of nitrogen by many other organisms.
Acetobacter species are industrially useful organisms. They can frequently be isolated
from fermented fruit juices and are used in the production of vinegar.
All the Bacteria discussed above are either rods or cocci, but a few Bacteria have
different shapes. An example is the spirochetes, a group of spiral-shaped Bacteria. These
organisms are thin and long and can vary widely in size. Some are so thin (0.2
micrometers in diameter) they can be seen with light microscopy only with special
staining. Other spirochetes may be 0.7 micrometers in width and up to 250 micrometers
in length. For comparison, an E. coli cell is about 1 micrometer x 3 micrometers; a
Streptococcus pneumoniae cell is 0.8 micrometers in diameter; and a Saccharomyces
cerevisiae (yeast) cell is 5-10 micrometers in diameter. Many spirochetes live in aquatic
environments, but some are normal flora in animals and a few of them are important
pathogens. The most significant of these is Treponema pallidum, the causative agent of
syphilis. Another spirochete pathogen of interest is Borrelia burgdorferi, which causes
Lyme disease.
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Micrograph of Borrelia burgdorferi. Jeffrey Nelson, author. Licensed for use, ASM
MicrobeLibrary (linked to http://www.microbelibrary.org)
Archaea
The Domain Archaea is composed of especially interesting organisms. They have a
prokaryotic cell structure but resemble organisms classified in the Domain Eukarya in
several other ways. Most striking are the similarities in the mechanisms of protein
synthesis in Archaea and Eukarya.
Many Archaea are found in extreme environments—places or conditions that seem
unlikely to contain anything alive. Two examples are Pyrolobus fumarii, which can grow
in hydrothermal vents in the ocean at temperatures up to 113 oC, and Picrophilus
oshimae, which can grow in very acidic hot springs, down to pH = –0.06. (That is a
hydrogen ion concentration of 100.06 or 1.15 moles per liter.) Other Archaea can grow in
other extremes: in soda lakes (pH = 12, which is a hydrogen ion concentration of 10-12 or
0.000000000001 moles per liter) and in saturated salt solutions. Many of them have a
metabolism radically different from ours, using inorganic chemicals such as hydrogen as
their energy source rather than glucose or other organic compounds. Just as in the
Domain Bacteria, some Archaea require oxygen, and some cannot tolerate it.
Archaea are also found in more routine places, such as soil, lakes, and oceans. While
some Archaea live in the intestinal tracts of warm-blooded animals, none is known to be
pathogenic.
Archaea play a significant role in structuring our environment. For example, methaneproducing Archaea are the most important if not the only biological producers of natural
gas. Organisms in this group also are vital as degraders of organic matter in nature.
Although prokaryotes are microscopic in size, they play a big role in the communities of
living organisms on our planet. They are found almost everywhere and carry out
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activities that are essential for all the rest of us. Without them, life on earth as we know it
would not exist.
Places to Go and Things to See
The ASM website given in the Classification module is a good place to look further for
more images or to find other sites containing images and information about a wide
variety of microorganisms.
http://www.microbelibrary.org/Visual/page1.htm
See Images in the ASM Microbe Library
Near the top of the page, click on Search Visual Resources.
This will take you to a page that explains the rules for using materials from
MicrobeLibrary. Permission is granted for use of the images for educational purposes, but
users are asked to inform and credit the authors and the American Society for
Microbiology (ASM). To see the images on this site, click Accept and you will be
allowed to enter the site and taken to the ASM MicrobeLibrary Search Screen.
To see the entire list of images available, skip all the drop-down menus, and click on
Submit.
See the List of Other Sites
At the bottom of the page, click on Additional Microbial Resources. This will take you
to a page with a list of links to other sites with a wealth of other information.
Questions
1. Because of their simple cellular structure, prokaryotic cells are unable to . . .
a. take in nutrients and excrete waste products.
b. reproduce.
c. respond to stimuli.
d. none of the above; prokaryotic cells can carry out all of these functions.
2. Which of the following is a true statement about the oxygen requirements of
prokaryotes?
a. Some prokaryotes require oxygen for survival.
b. Some prokaryotes do not require oxygen but can use it if it is present.
c. Some prokaryotes cannot grow in the presence of oxygen.
d. a, b, and c
3. Eukaryotic cells are
a. usually larger than
b. usually about the same size as
c. usually smaller than
prokaryotic cells.
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d. none of the above; there is no relationship between size of cells and cell type.
4.
species of bacteria cause disease in humans.
a.
b.
c.
d.
All
Most
Relatively few
No known
5. Most bacterial cells are . . .
a. square or rectangular.
b. round or rod-shaped.
c. spiral shaped.
d. six-sided (hexagons).
6. Bacteria recycle . . .
a. carbon.
b. sulfur.
c. nitrogen.
d. all of the above
7.
are a concern in healthcare facilities because they are difficult to
destroy by heat, chemicals, and irradiation.
a. Archaea
b. Gram-negative Bacteria
c. Bacterial endospores
d. Spirochetes
8. Clostridium botulinum, Clostridium tetani, Treponema pallidum, Salmonella typhi
and Bacillus anthracis all . . .
a. fix nitrogen.
b. are important in food production.
c. are pathogenic for humans.
d. are found in extreme environments, such as low pH and high temperatures.
9. Archaea have a . . .
a. structure similar to that of viruses.
b. prokaryotic cell structure.
c. eukaryotic cell structure.
d. none of the above; the structure of Archaea does not resemble any other known
biological agents.
10. Prokaryotes . . .
a. play a vital role in our environment.
b. make products we want and need.
c. are used in food production.
d. all of the above
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