Microbiology, 2021

4 • Summary 171
SUMMARY
4.1 Prokaryote Habitats,
Relationships, and Microbiomes
• Prokaryotes are unicellular microorganisms
whose cells have no nucleus.
• Prokaryotes can be found everywhere on our
planet, even in the most extreme environments.
• Prokaryotes are very flexible metabolically, so
they are able to adjust their feeding to the
available natural resources.
• Prokaryotes live in communities that interact
among themselves and with large organisms
that they use as hosts (including humans).
• The totality of forms of prokaryotes (particularly
bacteria) living on the human body is called the
human microbiome, which varies between
regions of the body and individuals, and
changes over time.
• The totality of forms of prokaryotes (particularly
bacteria) living in a certain region of the human
body (e.g., mouth, throat, gut, eye, vagina) is
called the microbiota of this region.
• Prokaryotes are classified into domains Archaea
and Bacteria.
• In recent years, the traditional approaches to
classification of prokaryotes have been
supplemented by approaches based on
molecular genetics.
4.2 Proteobacteria
• Proteobacteria is a phylum of gram-negative
bacteria discovered by Carl Woese in the 1980s
based on nucleotide sequence homology.
• Proteobacteria are further classified into the
classes alpha-, beta-, gamma-, delta- and
epsilonproteobacteria, each class having
separate orders, families, genera, and species.
• Alphaproteobacteria include several obligate
and facultative intracellular pathogens,
including the rickettsias. Some
Alphaproteobacteria can convert atmospheric
nitrogen to nitrites, making nitrogen usable for
other forms of life.
• Betaproteobacteria are a diverse group of
bacteria that include human pathogens of the
genus Neisseria and the species Bordetella
pertussis.
• Gammaproteobacteria are the largest and the
most diverse group of Proteobacteria. Many are
human pathogens that are aerobes or facultative
anaerobes. Some Gammaproteobacteria are
enteric bacteria that may be coliform or
noncoliform. Escherichia coli, a member of
Gammaproteobacteria, is perhaps the most
studied bacterium.
• Deltaproteobacteria make up a small group
able to reduce sulfate or elemental sulfur. Some
are scavengers and form myxospores, with
multicellular fruiting bodies.
• Epsilonproteobacteria make up the smallest
group of Proteobacteria. The genera
Campylobacter and Helicobacter are human
pathogens.
4.3 Nonproteobacteria Gram-
Negative Bacteria and Phototrophic
Bacteria
• Gram-negative nonproteobacteria include the
taxa spirochetes; the Chlamydia, Cytophaga,
Fusobacterium, Bacteroides group;
Planctomycetes; and many representatives of
phototrophic bacteria.
• Spirochetes are motile, spiral bacteria with a
long, narrow body; they are difficult or
impossible to culture.
• Several genera of spirochetes contain human
pathogens that cause such diseases as syphilis
and Lyme disease.
• Cytophaga, Fusobacterium, and Bacteroides are
classified together as a phylum called the CFB
group. They are rod-shaped anaerobic
organoheterotrophs and avid fermenters.
Cytophaga are aquatic bacteria with the gliding
motility. Fusobacteria inhabit the human mouth
and may cause severe infectious diseases.
Bacteroides are present in vast numbers in the
human gut, most of them being mutualistic but
some are pathogenic.
• Planctomycetes are aquatic bacteria that
reproduce by budding; they may form large
colonies, and develop a holdfast.
• Phototrophic bacteria are not a taxon but,
rather, a group categorized by their ability to use
the energy of sunlight. They include
Proteobacteria and nonproteobacteria, as well
as sulfur and nonsulfur bacteria colored purple
or green.
• Sulfur bacteria perform anoxygenic
photosynthesis, using sulfur compounds as
donors of electrons, whereas nonsulfur bacteria
use organic compounds (succinate, malate) as
donors of electrons.
• Some phototrophic bacteria are able to fix