Mastering Concepts
17.1
1. What are two domains that contain prokaryotes?
Bacteria and Archaea are the two domains that include prokaryotes.
2. List several ways that prokaryotes have influenced evolution.
Prokaryotes have influenced evolution by being the first cells on Earth, contributing O2 to the
atmosphere, helping to create the ozone layer, and becoming incorporated into eukaryotic cells
as mitochondria and chloroplasts.
3. In what habitats do bacteria and archaea live?
Prokaryotes live in nearly every conceivable habitat; few areas on or near Earth’s surface are free
of microbes.
4. Why are most species of prokaryotes little understood?
The best-understood microbes are the ones that can be cultivated in laboratories. However, many
types of microbes cannot grow in the laboratory. We know these organisms exist because their
DNA sequences have been discovered, but we know little about their abundance, distribution,
metabolic activities, or roles in ecosystems.
17.2
1. What are the three most common cell shapes of microbes?
The three most common shapes are spheres (coccus), rods (bacillus), and spirals (spirillum).
2. What are plasmids, and how are they important?
Plasmids are small loops of DNA that are not part of the prokaryotic chromosome. They are
important because they are easily transmitted from one cell to another, so that prokaryotes can
rapidly acquire new traits from their neighbors. They are also important because they help
biologists ferry genes from one prokaryotic cell to another.
3. What does the Gram stain reveal about a cell?
The basis of Gram staining is differences in the structure of the cell wall. In particular, the cell
walls of Gram positive bacteria are thicker than those of Gram negative bacteria.
4. What are the functions of a glycocalyx, pili, flagella, and endospores?
The glycocalyx helps attach the cell to a surface, keeps the cell from drying out, and protects it
from immune system cells. Attachment pili attach bacteria to surfaces, whereas sex pili allow the
transfer of DNA from cell to cell. Flagella help the cell to move toward or away from a stimulus.
Thick-walled, dormant endospores allow bacteria to survive exposure to harsh conditions.
5. What terms do microbiologists use to describe carbon sources, energy sources, and oxygen
requirements?
Autotrophs are “self-feeders” that acquire carbon from inorganic sources; heterotrophs are
“other-eaters)” that get carbon by consuming other organisms. Phototrophs acquire energy from
the sun; chemotrophs acquire energy from chemical sources. Obligate aerobes require oxygen;
obligate anaerobes are unable to exist in the presence of oxygen; and facultative aerobes can
survive with or without oxygen.
6. How are molecular data changing microbial taxonomy?
Molecular data led to the creation of the three-domain system and have revealed many new
microbial species. However, much work remains to be done before a phylogenetic tree can be
constructed with confidence.
7. Create a Venn diagram depicting the characteristics of the three types of horizontal gene
transfer.
The three types of horizontal gene transfer are transformation, transduction, and conjugation. In
transformation, a cell absorbs naked DNA from the environment; in transduction, a virus
transfers DNA from one cell to another; and in conjugation, a sex pilus transfers DNA (typically
a plasmid) from one cell to another. The Venn diagram should show that DNA is transferred
from one cell to another in all three types of horizontal gene transfer. Both transformation and
transduction typically transfer fragments of DNA (not whole plasmids); also, the source cell dies
in both transformation and transduction.
17.3
1. In what ways are bacteria and archaea similar and different?
The cells of bacteria and archaea are prokaryotic and roughly the same size. Both are
predominantly unicellular and have a circular chromosome; in addition, some members of each
domain can fix nitrogen or grow at temperatures above 80oC. Differences include cell wall and
membrane composition, the presence of chlorophyll in photosynthesis, the ability to generate
methane, sensitivity to streptomycin, and the presence of introns in genes.
2. What are some examples of phyla within domain Bacteria?
Examples of phyla within domain Bacteria include Proteobacteria, Cyanobacteria, Spirochaetes,
Firmicutes, Actinobacteria, and Chlamydiae.
3. What are the three phyla in domain Archaea?
The three phyla in domain Archaea are Euryarchaeota, Crenarchaeota, and Korarchaeota.
17.4
1. In what ways are bacteria and archaea essential to eukaryotic life?
Microbes are decomposers, photosynthetic organisms, food sources, and nitrogen fixers.
2. How are the microbes that colonize your body beneficial?
The microbes that colonize the human body help prevent attack by pathogenic bacteria, help
train the immune system to ignore harmless substances, and produce vitamins.
3. What adaptations enable pathogenic bacteria to enter the body and cause disease?
Pathogenic bacteria enter the body with insect or tick bites, during sexual activity, in food, air, or
water, or by direct contact with wounded skin surfaces. Once inside the body, pili help the
bacteria to attach to host cells, and bacterial enzymes attack host tissues. Bacterial toxins can
disable the host’s circulatory, digestive, or nervous system.
4. What are some practical uses of bacteria and archaea?
Bacteria and archaea are used in the production of food, vitamins, and other useful substances.
Transgenic bacteria produce human proteins for use in pharmaceutical drugs. Bacteria and
archaea also participate in water and waste treatment.
17.5
1. How did Musser’s team use their data set to help answer two different questions?
Musser’s team assembled their genetic data into a tree and looked at the pattern to see if all the
MRSA strains were contained within one branch or scattered across the branches. They used the
same approach to learn about the origin of bacteria associated with the toxic shock syndrome
epidemic.
2. Propose a change to figure 17.15 that would support the hypothesis that all strains associated
with toxic shock syndrome arose from one “superpathogen.”
If all of the toxic shock syndrome strains came from one superpathogen, they should be identical
or at least occupy the same branch.
Write It Out
1. Explain why the antibiotics penicillin and polymyxin are not effective against archaea.
(Review these antibiotics in the Apply It Now box on page 000.)
Penicillin attacks peptidoglycan, which is present in the cell walls of bacteria but not archaea.
Polymyxin’s target is the cell membrane, which differs between the cells of bacteria and archaea.
2. Why do some microbiologists disagree with classifying bacteria and archaea as
“prokaryotes”?
The term prokaryote is controversial because it encompasses both bacteria and archaea, implying
that they share a close evolutionary relationship. Instead, the evidence shows a closer
relationship between the archaea and the eukaryotes.
3. Give five examples that illustrate how bacteria and archaea are important to other types of
organisms.
Prokaryotes play a vital role in global nutrient cycles by decomposing organic matter in soil and
water; they carry out photosynthesis, providing food and O2 for countless organisms; they cause
diseases in many other organisms; they fix nitrogen, which makes this element available to other
organisms in ecosystems; and they produce a variety of commercial products.
4. If you were developing a new “broad-spectrum” antibiotic to a kill wide variety of bacteria,
which cell structures and pathways would you target? Which of those targets also occur in
eukaryotic cells, and why is that important? How would your strategy change if you were
designing a new “narrow-spectrum” antibiotic active against only a few types of bacteria?
A broad-spectrum antibiotic would target structures that are similar in most or all types of
bacteria, including ribosomes, peptidoglycan in the cell wall, the chemicals of the cell
membrane, or certain metabolic enzymes that are unique to bacteria. New drugs that target
peptidoglycan or bacterial ribosomes would not harm eukaryotic cells, but drugs that target
membrane proteins and enzymes that are present in bacteria and eukaryotic cells could have
harmful or deadly side effects. If you were designing a narrow-spectrum antibiotic, you would
focus on specific pathways and molecules that are unique to a few types of bacteria.
5. Describe your own metabolic classification. Are you a photoautotroph, photoheterotroph,
chemoautotroph, or chemoheterotroph? Are you an obligate aerobe, an obligate anaerobe, or a
facultative anaerobe? Are you a nitrogen fixer?
You are a chemoheterotroph and an obligate aerobe, and you are not a nitrogen fixer.
6. How do prokaryotes reproduce, and what are three ways they can acquire genes other than by
vertical gene transfer?
Prokaryotes reproduce by binary fission, an asexual process that replicates DNA and distributes
it and other cell parts into two daughter cells. Through horizontal gene transfer, a cell receives
DNA from another cell that is not its ancestor. Horizontal gene transfer has three forms:
transformation, transduction and conjugation.
7. Ernst Mayr defined a biological species as a population whose members can exchange genetic
material during reproduction (see chapter 14). How does horizontal gene transfer complicate this
definition of species?
The biological species concept assumes that members of one species can exchange genetic
information only with other members of the same species. In horizontal gene transfer, however, a
cell can acquire genes from multiple species.
8. Why did the discovery of archaea generate interest in searching for cells on other planets?
Because many archaea have been discovered in habitats with extremely harsh conditions, raising
expectations that life could exist on some inhospitable planet in space.
9. Why is it incorrect to say that bacteria produce toxins to harm their host?
The statement suggests that bacteria produce toxins with the intention of harming the host.
Instead, the toxins contribute to the reproductive success of the bacteria by helping them
colonize the host or spread to a new host.
10. How have humans harnessed the metabolic diversity of bacteria and archaea for industrial
purposes?
Disease-causing bacteria can be used as weapons. Bacteria that use fermentation are used in the
commercial-scale production of wine, cheese, olives, yogurt, and other products. Enzymes from
bacteria that live in extremely warm environments are used in PCR. Heterotrophic bacteria are
important in water and waste treatment facilities and in bioremediation.
11. Ecosystems rely on nitrogen fixing bacteria, which convert atmospheric nitrogen (N2) into
ammonia (NH3). Into what types of organic molecules do plants incorporate the nitrogen in
ammonia?
The organic molecules that contain nitrogen include proteins, DNA, RNA, ATP, phospholipids,
and chlorophyll (see their chemical structures in chapters 2, 3, and 5).
12. Stomach ulcers, once thought to be entirely a product of spicy food or high stress, are now
known to be caused by bacteria (Helicobacter pylori). How has ulcer treatment changed because
of this new knowledge?
Early treatments for ulcers included a special diet and less stress. The discovery that bacteria
cause ulcers led to antibiotic treatments.
13. Probiotics are dietary supplements consisting of live bacteria that are normally found in the
human digestive tract. Some people claim that consuming probiotics promotes digestive health.
Design an experiment that would help you determine whether (a) the bacteria in a probiotic
supplement survive the trip from the mouth, through the stomach and small intestine, and into
the large intestine and (b) whether probiotics actually do promote digestive health.
(a) Determining whether the bacteria survive the journey would be difficult to measure directly,
since it would mean somehow tagging the bacteria to differentiate them from a person’s resident
bacteria and also developing a harmless method of sampling bacteria from the mouth, stomach,
small intestine, and large intestine. If the bacteria could be tagged so as to stand out from
resident bacteria, however, it might be possible to use lab animals as a substitute for humans in
the experiment. Another approach would be to simulate the pH, temperature, and microbial
community of the human digestive system in the laboratory and then test whether the probiotic
bacteria could survive in those conditions.
(b) An experiment to test whether probiotics are effective would require researchers to develop a
measure of digestive health. For example, it may be possible to measure the incidence or severity
of diarrhea, gas, or abdominal pain in people who do or do not take the probiotic supplement. It
would be important to standardize the diets of all subjects in the experiment, to control for
preexisting digestive problems, and to assign a placebo to people who do not receive the
probiotic.
14. Botox is a toxin produced by the bacterium Clostridium botulinum. When ingested with
tainted food, Botox can kill by paralyzing muscles needed for breathing and heartbeat.
Physicians inject small quantities of diluted Botox into facial muscles to paralyze them and
reduce the appearance of wrinkles. Some people have expressed concern about a trend in which
people come together for “Botox parties” at hair salons and other nonmedical settings. What are
the risks of getting injections in such a setting?
The equipment at Botox parties may not be sterile, and needles may accidentally be reused. The
location may be contaminated with microorganisms and not as clean as the doctor’s office. The
person administering the injection may not be medically trained and may not know what to do if
the person receiving the injection has a serious allergic reaction. The dose may be too strong and
cause widespread muscle paralysis, which could make the person’s heart stop.
15. If you worked for a school confronting an outbreak of S. aureus, how would you determine
whether the strains were MRSA? What measures would you recommend to control the outbreak?
Culturing the bacteria and exposing them to antibiotics would determine whether the strains were
MRSA. Proper personal hygiene, covering open wounds, and not sharing personal items are the
best ways to control the outbreak.
16. Mycobacterium tuberculosis causes most cases of tuberculosis. Recently, strains of this
bacterium that are resistant to all known antibiotic drugs have become increasingly common.
Explain how this change occurred; use the terms mutation, DNA, and natural selection in your
answer.
Random mutations to the Mycobacterium tuberculosis DNA created resistant strains. The
antibiotic-rich environment favored the resistant strains, which had the greatest reproductive
success. This process of natural selection would have repeated over many bacterial generations
and with multiple drugs until some strains were resistant to all known drugs.
Pull it Together
1. Add autotrophs, heterotrophs, phototrophs, and chemotrophs to this concept map.
These terms could branch off of the “Metabolism” box. The autotrophs and heterotrophs refer to
different ways to acquire carbon, and the phototrophs and chemotrophs refer to different ways to
acquire energy.
2. Where do obligate aerobes, obligate anaerobes, and facultative anaerobes fit into this map?
These terms could branch off of the “Metabolism” box. Obligate aerobes require O2, obligate
anaerobes require the absence of O2, and facultative anaerobes can live with or without O2.
3. Where could you place humans on this concept map?
One possibility would be to reflect the role of humans as hosts for a wide variety of prokaryotic
cells. Another would be to say that genetic sequences show how humans are related to all other
species, including prokaryotes.
4. Describe each of the ways prokaryotes acquire new genes.
Binary fission is one way to acquire new genes (vertical gene transfer); in this case, one cell
copies its DNA and divides in two. The three types of horizontal gene transfer are
transformation, transduction, and conjugation. In transformation, a cell absorbs naked DNA from
the environment. In transduction, a virus transfers DNA from one cell to another. In conjugation,
DNA moves from one cell to another via a sex pilus.
5. Create a new concept map that includes the internal and external parts of a prokaryotic cell.
The internal parts are chromosome, cytoplasm, ribosomes, and plasmids. One way to organize
these would be to say that cytoplasm contains the chromosome, ribosomes, and plasmids and that
the chromosome and plasmids are two forms of DNA. The external parts are the cell wall,
glycocalyx, pili, and flagella. One way to organize these would be to say that the cell wall
surrounds the cell and that it may be coated with a glycocalyx; the pili and flagella are two types
of extensions that reach beyond the cell wall.
6. What are the cell features and metabolic criteria by which biologists classify microbes?
Physical features include shape; the presence or absence of a glycocalyx, pili, and flagella; and
whether the cell is gram-positive or gram-negative. Metabolic features include carbon source,
energy source, and the requirement for oxygen.