CHAPTER
11
Characterizing and
Classifying Prokaryotes
Chapter Outline
General Characteristics of Prokaryotic Organisms (pp. 321–326)
Morphology of Prokaryotic Cells
Endospores
Reproduction of Prokaryotic Cells
Arrangements of Prokaryotic Cells
Modern Prokaryotic Classification (p. 326)
Survey of Archaea (pp. 326–329)
Extremophiles
Methanogens
Survey of Bacteria (pp. 329–345)
Deeply Branching and Phototrophic Bacteria
Low G + C Gram-Positive Bacteria
Chapter Summary
General Characteristics of Prokaryotic Organisms (pp. 321–326)
Prokaryotes are by far the most diverse group of cellular microbes. Only a small number of
Morphology of Prokaryotic Cells
There are three basic shapes of prokaryotic cells: coccus (plural cocci), which is roughly
spherical; bacillus (plural bacilli), which is rod-shaped; and spirals. Spiral-shaped prokaryotes
are either spirilla, which are stiff, or spirochetes, which are flexible. Slightly curved rods are
vibrios, and the term coccobacillus is used to describe cells that are intermediate in shape
between cocci and bacilli. In addition, there are star-shaped, triangular, and rectangular
prokaryotes as well as pleomorphic prokaryotes, which vary in shape and size.
Endospores
The Gram-positive bacteria Bacillus and Clostridium can produce endospores (not to be
confused with reproductive spores). Endospores are stable resting stages that barely metabolize.
Reproduction of Prokaryotic Cells
All prokaryotes reproduce asexually. The most common method of reproduction is binary
fission, in which the parental cell disappears with the formation of progeny. A variation of
binary fission called snapping division occurs in some Gram-positive bacilli. In snapping
division, the parent cell’s outer wall tears apart with a snapping movement to create the daughter
Arrangements of Prokaryotic Cells
Arrangements of prokaryotic cells result from two features of binary fission: the plane of
division and whether daughter cells remain attached. Cocci are often found arranged in groups.
Video Tutor: Arrangements of Prokaryotic Cells
Modern Prokaryotic Classification (p. 326)
Taxonomists categorize living things into groups. Living things are currently classified into
three domains—Archaea, Bacteria, and Eukarya—based largely on genetic relatedness. One
Chapter 11 Characterizing and Classifying Prokaryotes
Survey of Archaea (pp. 326–329)
The Archaea have unique rRNA sequences and share other common characteristics that
distinguish them from bacteria:
They lack true peptidoglycan in their cell walls.
Cytoplasmic membrane lipids have branched or ring-form hydrocarbons.
The initial amino acid in their polypeptide chains is methionine rather than N-
formylmethionine used by bacteria.
Extremophiles
Extremophiles are microbes that require extreme conditions of temperature, pH, and/or salinity to
survive. (Bacterial extremophiles also exist.) Prominent among them are thermophiles and halophiles.
Thermophiles thrive at temperatures over 45ºC. Hyperthermophiles require temperatures
above 80ºC. They are found in both of the major phyla. Their DNA, membranes, and proteins
Methanogens
Methanogens are obligate anaerobes that convert CO2, H2, and organic acids into methane gas
(CH4). These microbes constitute the largest known group in the phylum Euryarchaeota.
Survey of Bacteria (pp. 329–345)
Bacteria are currently largely classified according to differences in their 16S rRNA sequences,
resulting in a scheme that includes 24 phyla.
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
Deeply Branching and Phototrophic Bacteria
Deeply branching bacteria are so named because their rRNA sequences and growth
characteristics lead scientists to conclude that these organisms are similar to the earliest bacteria;
that is, they branched off the “tree of life” at an early stage. They are autotrophic and live in hot,
acidic, and anaerobic environments, often with intense exposure to sun. For example, Aquifex is
chemoautotrophic, thermophilic, and microaerophilic. Deinococcus has a variety of unusual
characteristics that make it resistant to high levels of radiation. For example, when exposed to
high doses of radiation that fragment its chromosome, Deinococcus utilizes several enzymes to
repair the damage.
2. Green sulfur bacteria are placed in phylum Chlorobi.
4. Purple sulfur bacteria are placed in three classes of phylum Proteobacteria.
5. Purple nonsulfur bacteria are also placed in phylum Proteobacteria.
Cyanobacteria use chlorophyll a for photosynthesis and are oxygenic during photosynthesis:
Green and Purple Phototrophic Bacteria. All of the green and purple bacteria differ from
plants, algae, and cyanobacteria in two ways: they use bacteriochlorophylls for photosynthesis
instead of chlorophyll a, and they are anoxygenic; that is, they do not generate oxygen during
photosynthesis.
Chapter 11 Characterizing and Classifying Prokaryotes
Low G + C Gram-Positive Bacteria
The G + C content is the percentage of all base pairs in a genome that are guanine-cytosine base
pairs. Taxonomists use this characteristic in classifying Gram-positive bacteria. Bacteria with
G + C ratios below 50% are considered “low G + C bacteria”; the remainder are considered
“high G + C bacteria.”
Low G + C bacteria are classified within the phylum Firmicutes, which includes the
following groups:
Clostridia are rod-shaped, obligate anaerobes, many of which form endospores. The group is
Mycoplasmas are pleomorphic, facultative anaerobes and obligate anaerobes that lack cell
walls. Though they stain pink when Gram stained, mycoplasmas are classified in the phylum
Firmicutes based on nucleic acid sequences. Because mycoplasmas lack cell walls, they
Other low G + C Gram-positive bacilli and cocci are important in the environment, industry,
and health care. They include both beneficial and harmful microbes.
Bacillus species form endospores and are common in soils. They are aerobes or facultative
anaerobes, and most are motile by means of flagella. Some bacilli are beneficial in
High G + C Gram-Positive Bacteria
Bacteria with a G + C content greater than 50% are classified in the phylum Actinobacteria.
Corynebacterium. Members of the genus Corynebacterium are pleomorphic aerobes and
facultative anaerobes that reproduce by snapping division. They are characterized by their stores
of phosphate within inclusions called metachromatic granules. C. diphtheriae causes
diphtheria.
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
Gram-Negative Proteobacteria
The phylum Proteobacteria constitutes the largest and most diverse group of bacteria. They are
all Gram-negative and share common 16S rRNA nucleotide sequences. The GC content is not
critical when classifying Gram-negative organisms. There are six distinct classes, designated by
the first six letters of the Greek alphabet:
The class alphaproteobacteria are typically aerobes capable of growing at very low nutrient
levels. They vary in cell morphology. Many have unusual attachment extensions of the cell
called prosthecae used for attachment, nutrient absorption, and sometimes budding
The class betaproteobacteria thrive in habitats with low levels of nutrients, and some
genera, such as Nitrosomonas, are nitrifying. Pathogenic betaproteobacteria include
Neisseria, species of which inhabit the mucous membranes of mammals and cause such
The class gammaproteobacteria constitutes the largest and most diverse class. It includes
purple sulfur bacteria, intracellular pathogens such as Legionella and Coxiella, and
methane oxidizers, which digest most of the methane produced by methanogens before it
can adversely affect the world’s climate.
Glycolytic Facultative Anaerobes. The largest group is composed of Gram-negative,
Chapter 11 Characterizing and Classifying Prokaryotes
The class deltaproteobacteria includes Desulfovibrio, a sulfate-reducing microbe important in
the recycling of sulfur corrosion of iron. Bdellovibrio is pathogenic to bacteria. Myxobacteria
Other Gram-Negative Bacteria
Other assorted Gram-negative bacteria are classified by Bergey’s Manual into nine phyla. Only
three are discussed here.
Chlamydias are intracellular cocci typified by the genus Chlamydia, which is responsible for
neonatal blindness, pneumonia, and a sexually transmitted disease. Within a host cell,
New Media Resources
Connecting Concepts: Bacterial Cell Structures and Pathogen Characteristics.
Learning Catalytics: Assess students in real time using open-ended tasks to probe student
understanding.