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Chapter 22

Bacteria: The Proteobacteria

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The phylum Proteobacteria

• The largest phylogenetically coherent bacterial group with over 2,000 species assigned to more than 500 genera

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Phylogenetic Relationships Among the Procaryotes

Figure 22.1

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Phylogenetic Relationships Among Major Groups Within the -Proteobacteria

Figure 22.2

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The Purple Nonsulfur Bacteria

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Purple nonsulfur bacteria…• Metabolically flexible

– normally grow anaerobically as anoxygenic photoorganoheterotrophs• possess bacteriochlorophylls a or b in• some can use low levels of H2S as electron

source– in absence of light, most grow aerobically as

chemoorganoheterotrophs– in absence of light, some carry out

fermentations and grow anaerobically

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Purple nonsulfur bacteria…

• Found in mud and water of lakes and ponds with abundant organic matter and low sulfide levels; some marine species

• Many genera can produce cellular cysts– resting cells– resistant to desiccation but less tolerant of

heat and UV than bacterial endspores– made in response to nutrient limitation– have thick outer coat and store

polyhydroxybutyrate

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Figure 22.3

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Rickettsia and Coxiella

• Genus Rickettsia– class Alphaproteobacteria; order

Rickettsiales; family Rickettsiaceae

• Genus Coxiella– class Gammaproteobacteria; order

Legionellales; family CoxiellaceaeThese are considered together because of

important similarities

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Common features• Rod-shaped, coccoid, or pleomorphic

– typical gram-negative cell walls– no flagella– very small

• Rickettsia – 0.3 to 0.5 by 0.8 to 2.0 m• Coxiella – 0.2 to 0.4 by 0.4 to 1.0 m

• Parasitic or mutualistic– parasitic species grow in vertebrate

erythrocytes, macrophages, and vascular endothelial cells

• also live in blood-sucking arthropods, which serve as vectors or primary hosts

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Parasitic life stylesRickettsia

enters host by phagocytosis

escapes phagosome

reproduces in

cytoplasm

host cell bursts

Coxiellaenters host by

phagocytosis

remains in phagosome

reproduces in phagolysosome

host cell bursts

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humanfibroblastfilled withRickettsia prowazekii

Figure 22.4 (a)

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Figure 22.4 (c)

Coxiellaburnettigrowingwithinfibroblastvacuole

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Rickettsia metabolism

• Lack glycolytic pathway– do not use glucose as energy source

• Oxidize glutamate and TCA cycle intermediates (e.g., succinate)

• Take up and use ATP and other materials from host cell

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Important pathogens

• Rickettsia prowazekii and Rickettsia typhi – typhus fever

• Rickettsia rickettsii – Rocky Mountain Spotted Fever

• Coxiella burnetti – Q fever

• many are important pathogens in dogs, horses, sheep, and cattle

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Genus Rhizobium

• Gram-negative, motile rods– often contain poly--hydroxybutyrate

granules– become pleomorphic under adverse

conditions

• Grow symbiotically as nitrogen-fixing bacteroids within root nodule cells of legumes

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Figure 22.9

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Genus Agrobacterium

• Do not stimulate nodule formation or fix nitrogen

• Invade crown, roots, and stems of many plants– transform infected plant cells into

autonomously proliferating tumors

• e.g., Agrobacterium tumefaciens– causes crown gall disease by means of

tumor-inducing (Ti) plasmid

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Figure 22.10

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Nitrifying Bacteria• Divided into several taxa

– class Alphaproteobacteria; family Bradyrhizobiaceae – e.g., genus Nitrobacter

– class Betaproteobacteria; family Nitrosomonadaceae – e.g., genera Nitrosomonas and Nitrosospira

– class Gammaproteobacteria• family Ectothiorhodospiraceae – e.g., genus

Nitrococcus

• family Chromatiaceae – e.g., genus Nitrosococcus

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Table 22.2

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Figure 22.11

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Nitrification

• ammonianitritenitrate

• conversion of ammonia to nitrate by action of two genera– e.g., Nitrosomonas – ammonia to nitrite– e.g., Nitrobacter – nitrite to nitrate

• Fate of nitrate– easily used by plants– lost from soil through leaching or

denitrification

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Phylogenetic Relationships Among Major Groups Within the -Proteobacteria

Figure 22.12

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Table 22.3

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Order Neisseriales

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Genus Neisseria

• Inhabitants of mucous membranes of mammals– some human pathogens

• Neisseria gonorrhoeae – gonorrhea• Neisseria meningitidis - meningitis

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Order Burkholderiales

• Contains four families, three with well-known genera– genus Burkholderia in family

Burkholderiaceae– genus Bordetella in family

Alcaliginaceae– genera Sphaerotilus and Leptothrix in

family Comamonadaceae

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Genus Burkholderia• Gram-negative, non–spore-forming,

straight rods– most motile with single flagellum or tuft of

polar flagella

• aerobic and mesophilic

• nonfermentative chemoorganotrophs– catalase positive; often oxidase positive– most use poly--hydroxybutyrate as carbon

reserve

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e.g., Burkholderia cepacia

• degrades > 100 organic molecules– very active in recycling organic

material

• plant pathogen• has become a major nosocomial

pathogen– particular problem for cystic fibrosis

patients

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Nitrogen Fixation by Burkholderia and Ralstonia

• Both genera form symbiotic associations with legumes similar to that formed by rhizobia

• Both genera have nodulation genes (nod) similar to rhizobia suggesting a common genetic origin– genetic information may have been

obtained through lateral gene transfer

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Order Nitrosomonadales

• Contains a number of chemolithotrophs– two genera of nitrifying bacteria

• Nitrosomonas and Nitrosospira

– genus Gallionella• stalked bacterium

– genus Spirillum (in family Spirillaceae)

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Figure 22.15

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Order Hydrogenophilales

• contains genus Thiobacillus– well studied chemolithotroph

– prominent member of colorless sulfur bacteria• chemolithotrophs that oxidize sulfur

compounds• other colorless sulfur bacteria are in class

Gamma proteobacteria

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Genus Thiobacillus

• found in soil and aquatic habitats– production of sulfuric acid can cause corrosion of

concrete and metal structures

– may increase soil fertility by releasing sulfate

– used in leaching metals from low grade metal ores

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Figure 22.16

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Phylogenetic Relationships Among

Proteobacteria

• largest subgroup of proteobacteria

• contains 14 orders, and 25 families

Figure 22.17 (a)

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Figure 22.17 (b)

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The Purple Sulfur Bacteria

• placed in order Chromatiales– divided into two families,

Chromatiaceae and Ectothiorhodospiraceae

– Family Ectothiorhodospiraceae contains eight genera

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Figure 22.18

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Family Chromatiaceae• typical purple sulfur bacteria• strict anaerobes• usually photoautolithotrophs

– use H2S as electron donor• deposit sulfur granules internally• often eventually oxidize sulfur to sulfate

– may also use hydrogen as electron donor

• usually found in anaerobic, sulfide-rich zones of lakes– can cause large blooms in bogs and lagoons

• e.g., genera Thiospirillum, Thiocapsa, and Chromatium

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Figure 22.19

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Figure 22.20 (a)

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Order Methylococcales

• contains family Methylococcaceae; seven genera• morphologically diverse

– e.g., genus Methylococcus – spherical, nonmotile

– e.g., genus Methylomonas – straight, curved, or branched rods with single polar flagella

– almost all form resting stage (cystlike structure)

• methylotrophs– use reduced one-carbon compounds as sole carbon

and energy source

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Methane oxidation

• occurs in complex arrays of intracellular membranes

• oxidized to methanol and then to formaldehyde– electrons donated to electron transport

chain for ATP synthesis– formaldehyde can be assimilated into

cell material

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Order Pseudomonadales• contains family Pseudomonadaceae; 15

genera– Pseudomonas is the most important genus

in the order Pseudomonadales• gram-negative straight or slightly curved rods• 0.5 to 1.0 m by 1.5 to 5.0 m in length • motile by one or several polar flagella• lack prosthecae or sheaths

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Pseudomonas

• chemoheterotrophs with respiratory metabolism– usually use oxygen as electron acceptor– sometimes use nitrate as electron

acceptor– have functional TCA cycle– most hexoses are degraded by Entner-

Doudoroff pathway

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Practical importance of pseudomonads

• metabolically versatile– degrade wide variety of organic molecules– mineralization

• microbial breakdown of organic materials to inorganic substrates

• important experimental subjects• some are major animal and plant

pathogens• some cause spoilage of refrigerated food

– can grow at 4°C

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Figure 22.24

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Figure 22.25

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Order Enterobacteriales• contains one family,

Enterobacteriaceae; over 44 genera– referred to as enterobacteria or enteric

bacteria (enterobacteria)

• facultative anaerobes• chemoorganotrophs that degrade

sugars by glycolytic pathway– can cleave pyruvate to yield formic

acid (formic acid fermentation)

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Figure 22.29

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Table 22.7

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Table 22.7 (continued)

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Escherichia coli

• Probably best studied bacterium

• inhabitant of intestinal tracts of many animals

• Used as indicator organisms for testing water for fecal contamination

• Some strains are pathogenic– gastroenteritis– urinary tract infections

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Important pathogenic enteric bacteria• Salmonella – typhoid fever and

gastroenteritis• Shigella – bacillary dysentery• Klebsiella – pneumonia• Yersinia - plague• Erwinia – blights, wilts, etc., of crop

plants

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Figure 22.30

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Order Pasteurellales

• contains one family, Pasteurellaceae; six genera

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Important pathogens

• Pasteurella multiocida – fowl cholera

• Pasteurella haemolytica – pneumonia in cattle, sheep and goats

• Haemophilus influenzae – variety of diseases, including meningitis in children

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Class Deltaproteobacteria

• contains eight orders and 20 families– divided into two general groups

• aerobic, chemoorganotrophic predators• anaerobic, chemoorganotrophic sulfur-

and sulfate-reducers

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Figure 22.31

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Orders Desulfovibrionales, Desulfobacterales, and Desulfuromonadales

• Strict anaerobes• Sulfur- or sulfate-reducing bacteria

– use sulfur and sulfate as electron acceptors during anaerobic respiration

– electron transport chain used to generate ATP

• Widespread in muds and sediments of aquatic environments, including sewage treatment systems– important in sulfur cycling

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Figure 22.32

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Order Bdellovibrionales

– best studied is Bdellovibrio• predatory bacteria

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Bdellovibrio bactivorus

Figure 22.23

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Figure 22.34 (a)

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Epsilon Proteobacteria

• Smallest of proteobacterial classes

• Consists of one order, Campylobacteriales; three families

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Figure 22.38

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Genus Campylobacter• Contains both pathogenic and

nonpathogenic species– Campylobacter fetus

• reproductive disease and abortions in cattle and sheep

• septicemia and enteritis in humans– septicemia – pathogens or their toxins in blood

– enteritis – inflammation of intestinal tract

– Campylobacter jejuni• abortions in sheep

• enteritis diarrhea in humans

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Genus Helicobacter

• At least 14 species isolated from stomachs and upper intestines of humans, dogs, cats, and other mammals

• e.g., Helicobacter pylori– causes gastritis and peptic ulcer disease– produces large quantities of urease

• urea hydrolysis appears to be associated with virulence