Cell Structure and Function in the Bacteria and Archaea 4

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4
Chapter Preview and Key Concepts
Cell Structure
14..11 TDhivee Brseitgyi nanmionnggs tohfe M Biaccrtoebriiao laongdy Archaea
11. .(cid:129) T he BacteTrihae a dreis ccolavsesriyfi eodf minitcoro soervgearnails ms
wmasa jdoerp pehnydlean.t on observations made with
and Function 2. thTeh em Aicrcrohsaceoap aere currently classified into two
2. (cid:129)m ajor phyTlha.e emergence of experimental
4.2 Celslc Siehnacpe epsr oavnidde dA rar maneganesm teo ntetsst long held
beliefs and resolve controversies
3. Many bacterial cells have a rod, spherical, or
in the Bacteria 3. MspicirraolI nshqaupirey a1n: dE xapree roimrgeanntiazteido nin atnod a specific
Scientifi c Inquiry
cellular arrangement.
41..32 MAnic Orovoerrgvaienwis mtos Baancdt eDriisael aasned T Ararcnhsameiasls ion
4C.e l(cid:129)l StructurEearly epidemiology studies
and Archaea suggested how diseases could be spread and
4. Bacterial and archaeal cells are organized at
be controlled
the cellular and molecular levels.
5. (cid:129) Resistance to a disease can come
4.4 External Cell Structures
from exposure to and recovery from a mild
5. foPrimli aollfo (wo rc eal lsv etory a stitmacihla tro) sduisrfeaacsees or other
cells.
1.3 The Classical Golden Age of Microbiology
Our planet has always been in the “Age of Bacteria,” ever since the first 6. Flagella provide motility.
6. (1854-1914)
fossils—bacteria of course—were entombed in rocks more than 3 billion 77. .(cid:129) A glycocaTlyhxe pgreortmec tths eaograyi nwsats d beassicecda otino nth, e
attaches cells to surfaces, and helps
years ago. On any possible, reasonable criterion, bacteria are—and always observations that different microorganisms
pathogens evade the immune system.
have distinctive and specifi c roles in nature
have been—the dominant forms of life on Earth. 4.5 The Cell Envelope
8. (cid:129) Antisepsis and identifi cation of the
—Paleontologist Stephen J. Gould (1941–2002) 8. caBuascete orfia al nciemlla wl dalilsse ahseelps rmeianinfotraciend c ethlle s ghearpme
thaenodr yprotect the cell membrane from rupture.
99. . (cid:129)A rchaeal cKeolcl hw'sa lplso shtauvlaet cersy pstraolvliindee dl aay ewrasy. to
“Double, double toil and trouble; Fire burn, and cauldron bubble” is the 10. idMeonlteicfyu lae ss paencdi fii ocn ms iccrroososr gthane icsmel la ms ecmaubsriannge a
spbeyc fiafi cci liintafetectdi oduifsf udsisioena soer active transport.
refrain repeated several times by the chanting witches in Shakespeare’s
1101. . (cid:129)A rchaeal mLaebmobraratonreys sacriee nsctreu acntudr atellaym uwnioqruke .
Macbeth (Act IV, Scene 1). This image of a hot, boiling cauldron actu- 4.6 Thes tCimelull aCtyetdo tphlea dsimsc oavnedry I onft aedrndiatlio nal
ally describes the environment in which many bacterial, and especially Striuncfteuctrieosus disease agents
archaeal, species happily grow! For example, some species can be iso- 1112. . (cid:129)T he nucleVoiirdu sceosn taalsinos c tahne c caeulsl’es desisseeansteial
lated from hot springs or the hot, acidic mud pits of volcanic vents 12. (cid:129)g enetic inMfoarnmy abteionne.fi cial bacteria recycle
13. nPultarsiemnitdss icno tnhtea ienn nviornoensmseennttial genetic
(FIGURE 4.1). information.
When the eminent evolutionary biologist and geologist Stephen J. Gould
14. Ribosomes, microcompartments, and
wrote the opening quote of this chapter, he as well as most microbiologists inclusions carry out specific intracellular
had no idea that embedded in these “bacteria” was another whole domain functions.
15. Cytoskeletal proteins regulate cell division
of organisms. Thanks to the pioneering studies of Carl Woese and his col-
and help determine cell shape.
leagues, it now is quite evident there are two distinctly different groups of
4.7 The Bacteria/Eukaryote Paradigm—
“prokaryotes”—the Bacteria and the Archaea (see Chapter 3). Many of the Revisited
organisms Woese and others studied are organisms that would live a happy 16. Cellular processes in bacterial cells can be
life in a witch’s cauldron because they can grow at high temperatures, produce similar to those in eukaryatic cells.
methane gas, or survive in extremely acidic and hot environments—a real MICROINQUIRY 4: The Prokaryote/Eukaryote
Model
cauldron! Termed extremophiles, these members of the domains Bacteria
97
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98 CHAPTER 4 Cell Structure and Function in the Bacteria and Archaea
(A) (B)
FIGURE 4.1 Life at the Edge. Bacterial and archaeal extremophiles have been isolated from the edges of natural cauldrons, including (A) the Grand
Prismatic Spring in Yellowstone National Park, Wyoming, where the water of the hot spring is over 70ºC, or (B) the mud pools surrounding sulfurous
steam vents of the Solfatara Crater in Pozzuoli, Italy, where the mud has a very low pH and a temperature above 90º. »» How do extremophiles survive
under these extreme conditions?
and Archaea have a unique genetic makeup and As more microbes have had their complete genomes
have adapted to extreme environmental conditions. sequenced, it now is clear that there are unique as
In fact, Gould’s “first fossils” may have been well as shared characteristics between species in the
archaeal species. Many microbiologists believe the domains Bacteria and Archaea.
ancestors of today’s archaeal species might represent In this chapter, we examine briefly some of the
a type of organism that first inhabited planet Earth organisms in the domains Bacteria and Archaea.
when it was a young, hot place (see MicroFocus 2.1). However, because almost all known “prokaryotic”
These unique characteristics led Woese to propose pathog ens of humans are in the domain Bacteria,
these organisms be lumped together and called the we emphasize structure within this domain. As we
Archaebacteria (archae = “ancient”). see in this chapter, a study of the structural features
Since then, the domain name has been of bacterial cells provides a window to their activi-
changed to Archaea because (1) not all members ties and illustrates how the Bacteria relate to other
are extremophiles or related to these possible living organisms.
ancient ancestors and (2) they are not As we examine bacterial and archaeal cell
Bacteria—they are Archaea. Some might also structure, we can assess the dogmatic statement
debate using the term prokaryotes when refer- that these cells are characterized by a lack of a
ring to both domains, as organisms in the two cell nucleus and internal membrane-bound organ-
domains are as different from each other as they elles. Before you finish this chapter, you will be
are from the Eukarya. equipped to revise this view.
4.1 Diversity among the Bacteria and Archaea
In this section, we discuss bacterial and archaeal bacterial and archaeal species and a suspected
diversity using the current classification scheme, 10 million species. In this section, we will high-
which is based in large part on nucleotide light a few phyla and groups using the phyloge-
sequence data. There are some 7,000 known netic tree in FIGURE 4.2 .
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4.1 Diversity among the Bacteria and Archaea 99
ARCHAEA
BACTERIA
(2 major phyla)
(>70 major phyla)
Chlamydia
Euryarchaeota
Spirochaetes Extreme
Methanogens
halophiles
Proteobacteria
Crenarchaeota
Gram-positive Eukarya
bacteria
Hyperthermophiles
UNIVERSAL
ANCESTOR
FIGURE 4.2 The Phylogenetic Tree of Bacteria and Archaea. The tree shows several of the bacterial and archaeal phyla
discussed in this chapter. »» What is common to the branch base of both the Bacteria and Archaea?
The Domain Bacteria Contains Some springs, the fringes of space, and the tissues of ani-
of the Most Studied Microbial mals. Bacterial species, along with their archaeal
Organisms relatives, have so completely colonized every part
KEY CONCEPT of the Earth that their mass is estimated to out-
1. The Bacteria are classified into several major phyla. weigh the mass of all plants and animals com-
bined. Let’s look briefly at some of the major phyla
There are about 18 phyla of Bacteria identified from and other groups.
culturing or nucleotide sequencing. It should come Proteobacteria. The Proteobacteria (proteo =
as no shock to you by now to read that the vast “first”) contains the largest and most diverse group
majority of these phyla play a positive role in nature of species and includes many familiar gram-negative
(MICROFOCUS 4.1). Although not unique to just genera, such as Escherichia (FIGURE 4.3A). The phylum
the bacterial phyla, they digest sewage into simple also includes some of the most recognized human
chemicals; they extract nitrogen from the air and pathogens, including species of Shigella, Salmonella,
make it available to plants for protein production; Neisseria (responsible for gonorrhea), Yersinia
they break down the remains of all that die and (responsible for plague), and Vibrio (responsible
recycle the carbon and other elements; and they pro- for cholera). It is likely that the mitochondria
duce oxygen gas that we and other animals breathe. of the Eukarya were derived through endosym-
Of course, we know from Chapter 1 and per- biosis from an ancestor of the Proteobacteria (see
sonal experience that some bacterial organisms are MicroInquiry 3).
harmful—many human pathogens are members The group also includes the rickettsiae (sing.,
of the domain Bacteria. Certain species multiply rickettsia), which were first described by Howard
within the human body, where they disrupt tissues Taylor Ricketts in 1909. These tiny bacterial cells
or produce toxins that result in disease. can barely be seen with the most powerful light
The Bacteria have adapted to the diverse envi- microscope. They are transmitted among humans
ronments on Earth, inhabiting the air, soil, and primarily by arthropods, and are cultivated only Arthropods:
water, and they exist in enormous numbers on in living tissues such as chick embryos. Different Animals having
jointed appendages and
the surfaces of virtually all plants and animals. species cause a number of important diseases,
segmented body (e.g., ticks,
They can be isolated from Arctic ice, thermal hot including Rocky Mountain spotted fever and lice, fleas, mosquitoes).
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100 CHAPTER 4 Cell Structure and Function in the Bacteria and Archaea
4.1
Bacteria in Eight Easy Lessons1
Mélanie Hamon, an assistante de recherché at the Institut Pasteur in Paris, says that when she intro-
duces herself as a bacteriologist, she often is asked, “Just what does that mean?” To help explain her
discipline, she gives us, in eight letters, what she calls “some demystifying facts about bacteria.”
Basic principles: Their average size is 1/25,000th of an inch. In other words, hundreds of thousands
of bacteria fit into the period at the end of this sentence. In comparison, human cells are 10 to 100
times larger with a more complex inner structure. While human cells have copious amounts of membrane-
contained subcompartments, bacteria more closely resemble pocketless sacs. Despite their simplicity, they
are self-contained living beings, unlike viruses, which depend on a host cell to carry out their life cycle.
Astonishing: Bacteria are the root of the evolutionary tree of life, the source of all living organisms.
Quite successful evolutionarily speaking, they are ubiquitously distributed in soil, water, and extreme
environments such as ice, acidic hot springs or radioactive waste. In the human body, bacteria account
for 10% of dry weight, populating mucosal surfaces of the oral cavity, gastrointestinal tract, urogenital
tract and surface of the skin. In fact, bacteria are so numerous on earth that scientists estimate their
biomass to far surpass that of the rest of all life combined.
Crucial: It is a little known fact that most bacteria in our bodies are harmless and even essential for
our survival. Inoffensive skin settlers form a protective barrier against any troublesome invader while
approximately 1,000 species of gut colonizers work for our benefit, synthesizing vitamins, breaking down
complex nutrients and contributing to gut immunity. Unfortunately for babies (and parents!), we are
born with a sterile gut and “colic” our way through bacterial colonization.
Tools: Besides the profitable relationship they maintain with us, bacteria have many other practical
and exploitable properties, most notably, perhaps, in the production of cream, yogurt and cheese.
Less widely known are their industrial applications as antibiotic factories, insecticides, sewage proces-
sors, oil spill degraders, and so forth.
Evil: Unfortunately, not all bacteria are “good,” and those that cause disease give them all an often
undeserved and unpleasant reputation. If we consider the multitude of mechanisms these “bad”
bacteria—pathogens—use to assail their host, it is no wonder that they get a lot of bad press. Indeed,
millions of years of coevolution have shaped bacteria into organisms that “know” and “predict” their
hosts’ responses. Therefore, not only do bacterial toxins know their target, which is never missed, but
bacteria can predict their host’s immune response and often avoid it.
Resistant: Even more worrisome than their effectiveness at targeting their host is their faculty to
withstand antibiotic therapy. For close to 50 years, antibiotics have revolutionized public health in
their ability to treat bacterial infections. Unfortunately, overuse and misuse of antibiotics have led to
the alarming fact of resistance, which promises to be disastrous for the treatment of such diseases.
Ingenious: The appearance of antibiotic-resistant bacteria is a reflection of how adaptable they are.
Thanks to their large populations they are able to mutate their genetic makeup, or even exchange it,
to find the appropriate combination that will provide them with resistance. Additionally, bacteria are
able to form “biofilms,” which are cellular aggregates covered in slime that allow them to tolerate anti-
microbial applications that normally eradicate free-floating individual cells.
Al ong tradition: Although “little animalcules” were first observed in the 17th century, it was not
until the 1850s that Louis Pasteur fathered modern microbiology. From this point forward, research
on bacteria has developed into the flourishing field it is today. For many years to come, researchers will
continue to delve into this intricate world, trying to understand how the good ones can help and how to
protect ourselves from the bad ones. It is a great honor to be part of this tradition, working in the very
place where it was born.
1Republished with permission of the author, the Institut Pasteur, and the Pasteur Foundation. The original article
appeared in Pasteur Perspectives Issue 20 (Spring 2007), the newsletter of the Pasteur Foundation, which may be
found at www.pasteurfoundation.org © Pasteur Foundation.
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4.1 Diversity among the Bacteria and Archaea 101
(A) (B)
(C) (D)
(E) (F)
FIGURE 4.3 Members of the Domain Bacteria. (A) Escherichia coli (Bar = 10 µm.), (B) Staphylococcus aureus (Bar =
10 µm.), (C) Mycoplasma species (Bar = 2 µm.), (D) Streptomyces species (Bar = 20 µm.), (E) Anabaena species (Bar =
100 µm.), and (F) Treponema pallidum (Bar = 10 µm.). All images are light micrographs except (C), a false-color scanning
electron micrograph. »» What is the Gram staining result for E. coli and S. aureus?
typhus fever. Chapter 12 contains a more thor- specific species that are responsible for anthrax and
ough description of their properties. botulism, respectively. Species within the genera
Firmicutes. The Firmicutes (firm = “strong”; Staphylococcus and Streptococcus are responsible for
cuti = “skin”) consists of many species that are gram- several mild to life-threatening human illnesses
positive. As we will see in this chapter, they share a (FIGURE 4.3B).
similar thick “skin,” which refers to their cell wall Also within the Firmicutes is the genus
structure. Genera include Bacillus and Clostridium, Mycoplasma, which lacks a cell wall but is o therwise
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102 CHAPTER 4 Cell Structure and Function in the Bacteria and Archaea
phylogenetically related to the gram-positive bac- are found in the human oral cavity; in fact, some
terial species (FIGURE 4.3C). Possibly the smallest of the first animalcules seen by Leeuwenhoek were
free-living bacterial cell, one species causes a form probably spirochetes from his teeth scrapings (see
of pneumonia (Chapter 10) while another myco- Chapter 1). Among the human pathogens are
plasmal illness represents a sexually-transmitted Treponema pallidum, the causative agent of syphilis
disease (Chapter 13). and one of the most common sexually transmitted
Actinobacteria. Another phylum of gram-pos- diseases ( FIGURE 4.3F ; Chapter 13); and specific
itive species is the Actinobacteria. Often called the species of Borrelia, which are transmitted by ticks
actinomycetes, these bacterial organisms form a sys- or lice and are responsible for Lyme disease and
tem of branched filaments that somewhat resemble relapsing fever (Chapter 12).
the growth form of fungi. The genus Streptomyces is Other Phyla. There are many other phyla
the source for important antibiotics (FIGURE 4.3D). within the domain Bacteria. Several lineages
Another medically important genus is Mycobacterium, branch off near the root of the domain. The com-
one species of which is responsible for tuberculosis. mon link between these organisms is that they are
Cyanobacteria. In Chapter 3 we discussed hyperthermophiles; they grow at high tempera-
the cyanobacteria. They are phylogenetically tures. Examples include Aquifex and Thermotoga,
related to the gram-positive species and can exist which typically are found in earthly cauldrons
as unicellular, filamentous, or colonial forms such as hot springs.
( FIGURE 4.3E ). Once known as blue-green algae CONCEPT AND REASONING CHECKS
because of their pigmentation, pigments also may 4.1 What three unique events occurred within the
be black, yellow, green, or red. The periodic red- Proteobacteria and Cyanobacteria that contributed
to the evolution of the Eukarya and the oxygen-rich
Blooms: ness of the Red Sea, for example, is due to blooms
atmosphere on Earth?
Sudden increases in of cyanobacteria whose members contain large
the numbers of cells
amounts of red pigment.
of an organism in
an environment. The phylum Cyanobacteria are unique among
bacterial groups because they carry out photo- The Domain Archaea Contains
synthesis similar to unicellular algae (Chapter 6) Many Extremophiles
using the light-trapping pigment chlorophyll. KEY CONCEPT
Their evolution on Earth was responsible for the 2. The Archaea are currently classified into two
major phyla.
“oxygen revolution” that transformed life on the
young planet. In addition, chloroplasts probably
are derived from the endosymbiotic union with a Classification within the domain Archaea
cyanobacterial ancestor. has been more difficult than within the domain
Chlamydiae. Roughly half the size of the Bacteria, in large part because they have not been
rickettsiae, members of the phylum Chlamydiae studied as long as their bacterial counterparts.
are so small that they cannot be seen with the light Archaeal organisms are found throughout
microscope and are cultivated only within living the biosphere. Many genera are extremophiles,
cells. Most species in the phylum are pathogens growing best at environmental extremes, such
and one species causes the gonorrhea-like dis- as very high temperatures, high salt concentra-
ease known as chla mydia. Chlamydial diseases tions, or extremes of pH. However, most species
are described in Chapter 13. exist in very cold environments although there
Spirochaetes. The phylum Spirochaetes are archaeal genera that thrive under more mod-
contains more than 340 gram-negative species est conditions. The archaeal genera can be placed
that possess a unique cell body that coils into a into one of two major phyla.
long helix and moves in a corkscrew pattern. The Euryarchaeota. The Euryarchaeota contain
ecological niches for the spirochetes is diverse: organisms with varying physiologies, many being
from free-living species found in mud and sedi- extremophiles. Some groups, such as the metha-
ments, to symbiotic species present in the digestive nogens (methano = “methane”; gen = “produce”)
tracts of insects, to the pathogens found in the are killed by oxygen gas and therefore are found
urogenital tracts of vertebrates. Many spirochetes in environments devoid of oxygen gas. The pro-
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4.1 Diversity among the Bacteria and Archaea 103
(A)
(B)
FIGURE 4.4 Members of the Domain Archaea. (A) A false-color transmission
electron micrograph of the methanogen Methanospirillum hungatei. (Bar = 0.5 µm.)
(B) An aerial view above Redwood City, California, of the salt ponds whose color
is due to high concentrations of extreme halophiles. (C) A false-color scanning
electron micrograph of Sulfolobus, a hyperthermophile that grows in waters as hot
as 90ºC. (Bar = 0.5 µm.) »» What advantage is afforded these species that grow in
such extreme environments?
(C)
duction of methane (natural) gas is important in Crenarchaeota. The second phylum, the
their energy metabolism ( FIGURE 4.4A ). In fact, Crenarchaeota, are mostly hyperthermophiles
these archaeal species release more than 2 billion growing at temperatures above 80°C. Hot sulfur
tons of methane gas into the atmosphere every springs are one environment where these archaeal
year. About a third comes from the archaeal spe- species also thrive. The temperature is around
cies living in the stomach (rumen) of cows (see 75°C but the springs are extremely acidic (pH of
Chapter 2). 2–3). Volcanic vents are another place where these
Another group is the extreme halophiles organisms can survive quite happily (FIGURE 4.4C).
(halo = “salt”; phil = “loving”). They are distinct Other species are dispersed in open oceans, often
from the methanogens in that they require oxygen inhabiting the cold ocean waters (–3°C) of the
gas for energy metabolism and need high concen- deep sea environments and polar seas.
trations of salt (NaCl) to grow and reproduce. The TABLE 4.1 summarizes some of the charac-
fact that they often contain pink pigments makes teristics that are shared or are unique among the
their identification easy (FIGURE 4.4B). In addition, three domains.
some extreme halophiles have been found in lakes
where the pH is greater than 11.
CONCEPT AND REASONING CHECKS
A third group is the hyperthermophiles that 4.2 Compared to the more moderate environments in
grow optimally at high temperatures approaching which some archaeal species grow, why have others
or surpassing 100°C. adapted to such extreme environments?
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104 CHAPTER 4 Cell Structure and Function in the Bacteria and Archaea
TABLE
4.1 Some Major Differences between Bacteria, Archaea, and Eukarya
Characteristic Bacteria Archaea Eukarya
Cell nucleus No No Yes
Chromosome form Single, circular Single, circular Multiple, linear
Histone proteins present No Yes Yes
Peptidoglycan cell wall Yes No No
Membrane lipids Ester-linked Ether-linked Ester-linked
Ribosome sedimentation value 70S 70S 80S
Ribosome sensitivity to diphtheria toxin No Yes Yes
First amino acid in a protein Formylmethionine Methionine Methionine
Chlorophyll-based photosynthesis Yes (cyanobacteria) No Yes (algae)
Growth above 80°C Yes Yes No
Growth above 100°C No Yes No
4.2 Cell Shapes and Arrangements
Bacterial and archaeal cells come in a bewildering denote a rod-shaped bacterial cell, and as a genus
assortment of sizes, shapes, and arrangements, name (Bacillus).
reflecting the diverse environments in which they A spherically shaped bacterial cell is known as a
grow. As described in Chapter 3, these three char- coccus (pl., cocci; kokkos = “berry”) and tends to be
acteristics can be studied by viewing stained cells quite small, being only 0.5 µm to 1.0 µm in diameter.
with the light microscope. Such studies show that Although they are usually round, they also may be
most, including the clinically significant ones, oval, elongated, or indented on one side. Many bacte-
appear in one of three different shapes: the rod, rial species that are cocci stay together after division
the sphere, or the spiral. and take on cellular arrangements characteristic of the
species (FIGURE 4.5B). Cocci remaining in a pair after
Variations in Cell Shape and Cell reproducing represent a diplococcus. The organism
Arrangement Exist that causes gonorrhea, Neisseria gonorrhoeae, and one
KEY CONCEPT type of bacterial meningitis (N. meningitidis) are diplo-
3. Many bacterial cells have a rod, spherical, or spi- cocci. Cocci that remain in a chain are called strepto-
ral shape and are organized into a specific cellular coccus. Certain species of streptococci are involved in
arrangement. strep throat (Streptococcus pyogenes) and tooth decay
(S. mutans), while other species are harmless enough
A bacterial cell with a rod shape is called a bacil- to be used for producing dairy products such as yogurt
lus (pl., bacilli). In various species of rod-shaped (S. lactis). Another arrangement of cocci is the tetrad,
bacteria, the cylindrical cell may be as long as consisting of four cocci forming a square. A cube-like
20 µm or as short as 0.5 µm. Certain bacilli are packet of eight cocci is called a sarcina (sarcina =
slender, such as those of Salmonella typhi that cause “bundle”). Micrococcus luteus, a common inhabitant
typhoid fever; others, such as the agent of anthrax of the skin, is one example. Other cocci may divide
(Bacillus anthracis), are rectangular with squared randomly and form an irregular grape-like cluster of
ends; still others, such as the diphtheria bacilli cells called a staphylococcus (staphylo = “cluster”). A
(Corynebacterium diphtheriae), are club shaped. well-known example, Staphylococcus aureus, is often
Most rods occur singly, in pairs called diplobacil- a cause of food poisoning, toxic shock syndrome,
lus, or arranged into a long chain called strepto- and several skin infections. The latter are known in
bacillus (strepto = “chains”) (FIGURE 4.5A). Realize the modern vernacular as “staph” infections. Notice
there are two ways to use the word “bacillus”: to again that the words “streptococcus” and “staphy-
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4.2 Cell Shapes and Arrangements 105
(A)Bacillus (rod)
Single
Diplobacillus (pair)
Streptobacillus (chain)
(B)Coccus (sphere)
Diplococcus (pair)
Single
Tetrad
(group of 4)
Staphylococcus
(cluster)
Streptococcus (chain)
(C) Spiral
Vibrio
(comma-shaped)
Spirillum
Spirochete
FIGURE 4.5 Variation in Shape and Cell Arrangements. Many bacterial and archaeal cells have a bacillus (A) or coccus
(B) shape. Most spiral shaped-cells (C) are not organized into a specific arrangement. »» In photomicrograph (C), identify
the vibrio and the spirillum forms.
lococcus” can be used to describe cell shape and resembles a comma. The cholera-causing organ-
arrangement, or a bacterial genus (Streptococcus ism Vibrio cholerae is typical. Another spiral form
and Staphylococcus). called spirillum (pl., spirilla) has a helical shape
The third common shape of bacterial cells with a thick, rigid cell wall and flagella that assist
is the spiral, which can take one of three forms movement. The spiral-shaped form known as spi-
( FIGURE 4.5C ). The vibrio is a curved rod that rochete has a thin, flexible cell wall but no flagella
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106 CHAPTER 4 Cell Structure and Function in the Bacteria and Archaea
in the traditional sense. Movement in these organ- cies have appendaged bacter ial cells while others
isms occurs by contractions of endoflagella that consist of branching filaments; and some archaeal
run the length of the cell. The organism causing species have square and star shapes.
syphilis, Treponema pallidum, typifies a spirochete. CONCEPT AND REASONING CHECKS
Spiral-shaped bacterial cells can be from 1 µm to 4.3 Propose a reason why bacilli do not form tetrads or
100 µm in length. clusters.
In addition to the bacillus, coccus, and spiral
shapes, other variations exist. Some bacterial spe-
4.3 An Overview to Bacterial and Archaeal Cell Structure
In the last chapter we discovered that bacterial and Recent advances in understanding bacterial and
archaeal cells appear to have little visible struc- archaeal cell biology indicate these organisms exhibit
ture when observed with a light microscope. This, a highly ordered intracellular organization. This
along with their small size, gave the impression organization is centered on three specific processes
they are homogeneous, static structures with an that need to be carried out (FIGURE 4.6). These are:
organization very different from eukaryotic cells.
However, the point was made that bacterial (cid:129) Sensing and responding to the sur-
and archaeal species still have all the complex rounding environment. Because most
processes typical of eukaryotic cells. It is simply a bacterial and archaeal cells are sur-
matter that, in most cases, the structure and some- rounded by a cell wall, some pattern of
times pattern to accomplish these processes is dif- “external structures” is necessary to sense
ferent from the membranous organelles typical of their environment and respond to it or
eukaryotic species. other cells.
(cid:129) Compartmentation of metabolism. As
Cell Structure Organizes Cell Function described in Chapter 3, cell metabolism
KEY CONCEPT must be segregated from the exterior
4. Bacterial and archaeal cells are organized at the cel- environment and yet be able to transport
lular and molecular levels. materials to and from that environment.
Bacterial/archaeal cell
is composed of
External structures Cell envelope
include includes
Cytoplasm Cell wall Cell membrane
Flagella Glycocalyx
includes
Pili Cytosol Internal structures
consists of
Nucleoid include Cytoskeleton
Slime layer Capsule
Ribosomes Inclusions
Plasmids
FIGURE 4.6 A Concept Map for Studying Bacterial and Archaeal Cell Structure. Not all cells have all the structures shown here. »» Why can’t we see
in the TEM image of a bacterial cell all the structures outlined in the concept map?
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