Chapter 22
Microbial Symbioses
I. Symbioses between Microorganisms
• 22.1 Lichens
• 22.2 "Chlorochromatium aggregatum"
22.1 Lichens
• Lichens
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Leafy or encrusting microbial symbioses
Often found growing on bare rocks, tree trunks, house
roofs, and the surfaces of bare soils (Figure 22.1)
A mutalistic relationship between a fungus and an alga (or
cyanobacterium)
• Alga is photosynthetic and produces organic matter
• The fungus provides a structure within which the phototrophic
partner can grow protected from erosion (Figure 22.2)
22.2 "Chlorochromatium aggregatum"
• In freshwater there are microbial mutualisms called
consortia
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Consist of green sulfur bacteria (called epibionts) and a
flagellated rod-shaped bacterium (Figure 22.3 and 22.4)
• Consortium given a "genus species" name
• Green sulfur bacteria are obligate anaerobic phototrophs
• Flagellated rod allows for movement
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22.2 "Chlorochromatium aggregatum"
• The epibiont of "Chlorochromatium aggregatum" has
been isolated and grown in pure culture
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Electron micrographs of consortium reveal intimate
contact between epibiont and central bacterium (Figure
22.5)
II. Plants as Microbial Habitats
• 22.3 The Legume–Root Nodule Symbiosis
• 22.4 Agrobacterium and Crown Gall Disease
• 22.5 Mycorrhizae
22.3 The Legume–Root Nodule Symbiosis
• The mutalistic relationship between leguminous plants
and nitrogen-fixing bacteria is one of the most important
symbioses known
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Examples of legumes include soybeans, clover, alfalfa,
beans, and peas
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Rhizobia are the best-known nitrogen-fixing bacteria
engaging in these symbioses
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Rhizobia are species of Alphaproteobacteria or
Betaproteobacteria that can grow in soil or infect
leguminous plants (Figure 22.6)
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22.3 The Legume–Root Nodule Symbiosis
• Infection of legume roots by nitrogen-fixing bacteria
leads to the formation of root nodules that fix nitrogen
(Figure 22.7)
• Leads to significant increases in combined nitrogen in soil
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Nodulated legumes grow well in areas where other
plants would not (Figure 22.8)
22.3 The Legume–Root Nodule Symbiosis
• Nitrogen-fixing bacteria need O2 to generate energy for
N2 fixation, but nitrogenases are inactivated by O2
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In the nodule, O2 levels are controlled by the O2-binding
protein leghemoglobin (Figure 22.9)
22.3 The Legume–Root Nodule Symbiosis
• Cross-inoculation group
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Group of related legumes that can be infected by a
particular species of rhizobia
22.3 The Legume–Root Nodule Symbiosis
• Critical steps in root nodule formation (Figure 22.10):
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Step 1: Recognition and attachment of bacterium to root
hairs (Figure 22.11)
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Step 2: Excretion of Nod factors by the bacterium
Step 3: Bacterial invasion of the root hair
Step 4: Travel to the main root via the infection thread
Step 5: Formation of bacteroid state within plant cells
Step 6: Continued plant and bacterial division, forming the
mature root nodule
22.3 The Legume–Root Nodule Symbiosis
• Bacterial nod genes direct the steps in nodulation
• nodABC gene encodes proteins that produce
oligosaccharides called Nod factors (Figure 22.12)
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Nod factors
• Induce root hair curling
• Trigger plant cell division
• Signal legumes to develop root nodules (Figure 22.13)
22.3 The Legume–Root Nodule Symbiosis
• NodD is a positive regulator that is induced by plant
flavonoids (Figure 22.14)
22.3 The Legume–Root Nodule Symbiosis
• The legume–bacteria symbiosis is characterized by
several metabolic reactions and nutrient exchange
(Figure 22.15)
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22.3 The Legume–Root Nodule Symbiosis
• A few legume species form nodules on their stems
(Figure 22.16)
22.3 The Legume–Root Nodule Symbiosis
• Nonlegume nitrogen-fixing symbiosis also occurs
• The water fern Azolla contains a species of
heterocystous nitrogen-fixing cyanobacteria known as
Anabaena (Figure 22.17)
22.3 The Legume–Root Nodule Symbiosis
• The alder tree (genus Alnus) has nitrogen-fixing root
nodules that harbor the nitrogen-fixing bacterium Frankia
(Figure 22.18)
22.4 Agrobacterium and Crown Gall Disease
• Agrobacterium tumefaciens forms a parasitic symbiosis
with plants, causing crown gall disease (Figure 22.19)
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Crown galls are plant tumors induced by A. tumefaciens
cells harboring a large plasmid, the Ti (tumor induction)
plasmid (Figure 22.20)
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22.4 Agrobacterium and Crown Gall Disease
• To initiate tumor formation, A. tumefaciens cells must
attach to the wound site on the plant (Figure 22.21)
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Attached cells synthesize cellulose microfibrils and
transfer a portion of the Ti plasmid to plant cells
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DNA transfer is mediated by vir-encoded proteins
22.4 Agrobacterium and Crown Gall Disease
• The Ti plasmid has been used in the genetic engineering
of plants
22.5 Mycorrhizae
• Mycorrhizae
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Mutualistic associations of plant roots and fungi
Two classes:
• Ectomycorrhizae
• Endomycorrhizae
22.5 Mycorrhizae
• Ectomycorrhizae
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Fungal cells form an extensive sheath around the outside of
the root with only a little penetration into the root tissue
(Figure 22.22)
Found primarily in forest trees, particularly boreal and
temperate forests
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22.5 Mycorrhizae
• Endomycorrhizae
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Fungal mycelium becomes deeply embedded within the root
tissue (Figure 22.23)
Are more common than ectomycorrhizae
Found in >80% of terrestrial plant species
22.5 Mycorrhizae
• Mycorrhizal fungi assist plants (Figure 22.25)
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Improve nutrient absorption
• This is due to the greater surface area provided by the fungal
mycelium
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Helping to promote plant diversity
III. Mammals as Microbial Habitats
• 22.6 The Mammalian Gut
• 22.7 The Rumen and Ruminant Animals
• 22.8 The Human Microbiome
22.6 The Mammalian Gut
• Herbivores – animals that consume plants
• Carnivores – animals that consume meat
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Omnivores – animals that consume both
Phylogenetics suggests that different lineages evolved a
herbivorous lifestyle (Figure 22.26)
22.6 The Mammalian Gut
• Microbial associations with certain animals led to ability
to catabolize plant fibers
• Plant fibers composed of insoluble polysaccharides
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• Cellulose most abundant component
Two digestive plans have evolved in herbivorous animals
(Figure 22.27)
Foregut fermentation: fermentation chamber precedes the
small intestine
Hindgut fermentation: uses cecum and/or large intestine
22.7 The Rumen and Ruminant Animals
• Microbes form intimate symbiotic relationships with
higher organisms
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Ruminants
• Herbivorous mammals (e.g., cows, sheep, goats)
• Possess a special digestive organ (the rumen)
• Cellulose and other plant polysaccharides are digested with the
help of microbes (Figure 22.28)
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Rumen well studied because of implanted sampling port
22.7 The Rumen and Ruminant Animals
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The rumen contains 1010 to 1011 microbes per gram of
rumen constituents
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Fermentation in the rumen is mediated by cellulolytic
microbes that hydrolyze cellulose to free glucose that is
then fermented, producing volatile fatty acids (e.g.,
acetic, propionic, butyric) and CH4 and CO2 (Figure
22.29)
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Fatty acids pass through rumen wall into bloodstream
and are utilized by the animal as its main energy source
22.7 The Rumen and Ruminant Animals
• Rumen microbes also synthesize amino acids and
vitamins for their animal host
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Rumen microbes themselves can serve as a source of
protein to their host when they are directly digested
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Anaerobic bacteria dominate in the rumen
Rumen contains 300 to 400 bacterial "species" (Figure
22.30)
22.7 The Rumen and Ruminant Animals
• Abrupt changes in an animal's diet can result in changes
in the rumen flora
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Rumen acidification (acidosis) is one consequence of
such a change
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Anaerobic protists and fungi are also abundant in the
rumen
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Many perform metabolisms similar to those of their
prokaryotic counterparts
22.8 The Human Microbiome
• All sites on a human that contains microorganisms are
part of microbiome
• Humans are monogastric and omnivorous
• Microbes in gut affect early development, health, and
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predisposition to disease (Figure 22.32)
Colonization of gut begins at birth
Different individuals are highly variable in gut communities
(Figure 22.33)
22.8 The Human Microbiome
• Human gut microorganisms produce
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Enzymes
Amino acids
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Human gut microorganisms believed to be responsible
for "maturing" of gastrointestinal tract
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Human gut microorganisms may play a role in obesity
(Figure 22.34)
22.8 The Human Microbiome
• Mouth and skin are also heavily colonized by
microorganisms
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Mouth
• At least 750 species of aerobic and anaerobic bacteria
• Methanogenic Archaea and yeast
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Skin
• Nearly 20 bacterial phyla (four main groups)
• Fungi (mainly yeast)
IV. Insects as Microbial Habitats
• 22.9 Heritable Symbionts of Insects
• 22.10 Termites
22.9 Heritable Symbionts of Insects
• Microbial symbionts can be acquired from:
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Environmental reservoir (horizontal transmission)
Parent (vertical or heritable transmission)
Heritable symbionts of insects are obligate (lack a free-living
replicative stage)
Primary symbionts are required for host reproduction
• Restricted to bacteriome
• Bacterial cells found in bacteriocytes (Figure 22.36)
22.9 Heritable Symbionts of Insects
• Secondary symbionts
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Not required for reproduction
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Not always present in every individual
Can invade different cells and live extracellularly
Must provide a benefit
• Nutritional
• Protection from environment
• Protection from pathogens
Some parasitic symbionts manipulate host's reproductive
tissue
22.9 Heritable Symbionts of Insects
• Some insects exploit the metabolic potential of the
symbiont
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Primary symbionts exhibit extreme gene reduction
• Genome of insect symbiont ~160 to 800 kbp
• Genome of related free-living bacteria ~2 to 8 Mbp
• Retain only genes needed for host fitness
• Lose catabolic genes
• Pathogens normally lose anabolic genes
22.10 Termites
• Termites decompose cellulose and hemicellulose
• Termites classified as higher or lower based on
phylogeny
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Termite gut consists of foregut, midgut, and hindgut
(Figure 22.37)
• Posterior alimentary tract of higher termites (Termitidae)
• Diverse community of anaerobes, including cellulolytic
anaerobes (Figure 22.38)
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Lower termites
• Anaerobic bacteria and cellulolytic protists
IV. Aquatic Invertebrates as Microbial Habitats
• 22.11 Hawaiian Bobtail Squid
• 22.12 Marine Invertebrates at Hydrothermal Vents and
Gas Seeps
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22.13 Leeches
22.14 Reef-Building Corals
22.11 Hawaiian Bobtail Squid
• A mutualistic symbiosis between the marine bacterium
Aliivibrio fischeri and the Hawaiian bobtail squid is a
model for how animal–bacterial symbioses are
established (Figure 22.39)
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The squid harbors large populations of the
bioluminescent A. fischeri in a specialized structure (light
organ)
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Bacteria emit light that resembles moonlight penetrating
marine waters, which camouflages the squid from
predators
22.11 Hawaiian Bobtail Squid
• The symbiotic relationship is highly specific and benefits
both partners
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Transmission of bacterial cells is horizontal
• The light organ is colonized by A. fischeri shortly after
juvenile squid hatch
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Bioluminescence is controlled by quorum sensing
A. fischeri is supplied with nutrients by the squid
22.12 Marine Invertebrates at Hydrothermal
Vents and Gas Seeps
• Deep-sea hot springs (hydrothermal vents) support
thriving animal communities that are fueled by
chemolithotrophic microbes
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Diverse invertebrate communities develop near
hydrothermal vents, including large tube worms, clams,
mussels (Figure 22.40)
22.12 Marine Invertebrates at Hydrothermal
Vents and Gas Seeps
• Chemolithotrophic prokaryotes that utilize reduced
inorganic materials emitting from the vents form
endosymbiotic relationships with vent invertebrates
(Figure 22.41)
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Vent tube worms harbor several features that facilitate
the growth of their endosymbionts (e.g., trophosome,
specialized hemoglobins, high blood CO2 content)
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Most symbionts of marine invertebrates have small
genomes
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22.13 Leeches
• Leeches (Figure 22.42) are parasitic annelids
(segmented worms)
• Feed on vertebrate blood and secrete anticoagulants and
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vasodilators
Two compartments that have microbial communities
Digestive tract
• Crop has Aeromonas and a Rikenella-like bacterium
• Intestinum has various Alphaproteobacteria,
Gammaproteobacteria, Bacteriodetes, and Firmicutes
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Bladder
• Packed with Ochrobactrum
22.14 Reef-Building Corals
• Coral skeleton is a very efficient light-gathering structure
• Phototrophic symbionts include cyanobacteria,
rhodophytes, chlorophytes, dinoflagellates, and diatoms
22.14 Reef-Building Corals
• Most ecologically significant between stony coral and the
dinoflagellate Symbiodinium (Figure 22.44)
• Reef-building corals reproduce sexually by releasing
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gametes into the seawater
• Algal symbionts are typically found in the egg
Developing coral can also ingest dinoflagellates
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22.14 Reef-Building Corals
• Coral bleaching
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Loss of color caused by lysis of symbiont
High temperature and high light impair the photosynthetic
apparatus of dinoflagellate (Figure 22.45)
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