This chapter presents the diverse group of eubacteria known as the proteobacteria. The distinguishing characteristics of these gram-negative bacteria: morphology, physiology, metabolism, and ecology are presented. The phylogenetic relationships are discussed and representative species are examined

After reading this chapter you should be able to:

• discuss the importance of this diverse group of organisms, either a disease agents or because of their effects on the habitat
• discuss the use of Escherichia coli as a major experimental organism that is studied in many laboratories
• describe the diverse life styles and metabolism of members of this group of organisms
• discuss the complex structures (prosthecae, stalks, buds, sheaths, or complex fruiting bodies produced by some members of this group
• discuss the ecological impact by chemolithotrophic bacteria
• discuss the dependence of parasitic eubacteria , such as Bdellovibrio and the rickettsia on their hosts for energy and/or cell constituents

1. The Alpha Proteobacteria

1. Include most of the oligotrophic proteobacteria
2. The Purple Nonsulfur Bacteria

1. Like all purple bacteria, the purple nonsulfur bacteria use anoxygenic photosynthesis, possess bacteriochlorophyls a and b and have their photosystems in membranes that are continuous with the plasma membrane
2. Flexible in their choice of an energy source; normally they are grown anaerobically as photoorganoheterotrophs, but can grow aerobically
3. They do not oxidize elemental sulfur to sulfate
4. Most have not been cultured
5. They are found in the mud and water of lakes and ponds with abundant organic matter and low sulfide levels

1. Rickettsia and Coxiella

1. It lacks the glycolytic pathway, and does not use glucose as an energy source;
2. Oxidize glutamate and tricarboxylic acid cycle intermediates (e.g., succinate)

1. Order contains many important pathogens

1. R. prowazekii and R. typhi - typhus fever
2. R. ricketsii - Rocky Mountain Spotted Fever
3. Coxiella burnetii - Q fever

1. They also are important pathogens in dogs, horses, sheep, and cattle

1. The Caulobacteraceae and Hyphomicrobiaceae

1. Have one of three distinguishing features

1. Prostheca - an extension of the cell, including the plasma membrane, that is

narrower than the mature cell

2. Stalk - a nonliving appendage produced by the cells and extending from it
3. Reproduction by budding - parental cell retains its identity and progeny are much smaller than the parental cell

1. Hyphomicrobium - chemoheterotrophic , aerobic, budding bacteria that frequently

1. Mature cell produces a hypha or prostheca that elongates
2. The nucleoid divides and a copy moves into the hypha while a bud forms at its end
3. The bud matures, produces one to three flagella, and a septum divides the bud from the hypha
4. The bud is released as an oval- to pear-shaped swarmer cell
5. Distinctive in its nutrition and physiology - grows on ethanol, acetate and one-carbon molecules such as methanol, formate, and formaldehyde
6. May be as much as 25% of the total bacteria population in oligotrophic or nutrient-poor freshwater habitats

1. May be polarly flagellated rods or may possess prostheca and holdfast, by which they attach to solid substrata

1. Usually found in low-nutrient freshwater and marine habitats, but are also present in

soil

2. Often adhere to bacteria, algae, and other microorganisms and may absorb nutrients

released by their hosts

3. Prostheca differs from that of Hyphomicrobium in that it lacks cytoplasmic

components and is composed almost totally of plasma membrane and cell wall

4. Reproduction involves formation of a single flagellum at the end opposite the prostheca; asymmetric transverse fission forms a swarmer cell that swims off; when the swarmer comes to rest, it forms a new prostheca at the flagellar end and loses the flagellum; whole cycle takes only 2 hours

1. Family Rhizobiaceae

1. Contains the genera Rhizobium and Agrobacterium
2. Rhizobium

1. Agrobacterium

1. Not capable of nitrogen fixation
2. Transforms infected plant cells into autonomously proliferating tumors
3. A. tumefaciens (best studied) causes crown gall disease by means of tumor including (Ti) plasmid

1. Nitrifying Bacteria

1. Bradyrhizobiaceae - Nitrobacter and Nitrococcus
2. Nitrosomonadaceae - Nitrosomonas, Nitrosococcus, and Nitrosospira
3. All are aerobic, gram-negative organisms without endospores and able to oxidize either ammonia or nitrate
4. May be rod-shaped, ellipsoidal, spherical, spirillar or lobate with either polar or peritrichous flagella
5. Nitrobacter and Nitrococcus oxidize nitrite to nitrate
6. Nitrosomonas, Nitrosospira and Nitrosococcus oxidize ammonia to nitrate
7. If two genera such as Nitrobacter and Nitrosomonas grow together in a niche, ammonia is converted to nitrate (nitrification)
8. Nitrate is readily used by plants but also is easily leached from the soil or denitrified to nitrogen gas

1. The Beta Proteobacteria

1. Overlap alpha-proteobacteria metabolically but tend to use substances that diffuse from

organic decomposition in the anaerobic zone of habitats

2. Considerable metabolic diversity
3. Order Neisseriales

1. Members of the genus Neisseria are nonmotile, aerobic, gram-negative cocci that most

often occur in pairs with adjacent sides flattened

2. May have capsules and fimbriae
3. Chemoorganotrophic, oxidase-positive, and almost always catalase-positive
4. Neisseria gonorrhoeae - causative agent of gonorrhea
5. Neisseria meningitidis - one of the causative agents of bacterial meningitis

1. The order contains four families, three of them with well-known genera
2. Burkholderiaceae - contains seven genera, formed from the genus Pseudomonas based on rRNA data, including the genus Burkholderia

1. Alcaliginaceae - contains the genus Bordetella

1. Gram-negative, aerobic coccobacilli
2. Chemoorganotroph, with respiratory metabolism, that requires organic sulfur and nitrogen (in the form of amino acids) for growth
3. Mammalian parasite that multiplies in respiratory epithelial cells
4. B. pertussis - nonmotile, encapsulated species that is the causative agent for whooping cough

1. Contains twelve genera with diverse characteristics
2. Some have a sheath, a hollow tubelike structure surrounding a chain of cells

1. Help bacteria attach to surfaces
2. Help to obtain nutrients from slowly running water as it flows past
3. Help protect against predators

1. Sphaerotilus forms long chains and prefers slowly running freshwater polluted with

sewage or industrial waste; can form tangled masses that interfere with activated

sludge tanks used in sewage treatment

2. Leptothrix deposits large amounts of iron and manganese oxides in its sheath; this

1. Order Nitrosomonadaceae

1. Includes the nitrifying bacteria, Nitrosomonas, Nitrosococcus, and Nitrosospira,

discussed earlier

2. Gallionella - a stalked chemolithotroph
3. Spirillaceae - Spirillum and Thiobacillus; the latter is one of the best studied chemolithotrophs and the most prominent of the colorless sulfur bacteria

1. The Gamma Proteobacteria

1. Largest subgroup of proteobacteria, including several deeply branching groups
2. The 2^nd edition of Bergey's Manual divides this group into 13 orders, 19 families, and around 130 genera
3. The Purple Sulfur Bacteria

1. Divided into two families - Chromatiaceae and Ectothiorhodospiraceae in the order

1. Ectothiorhodospira has red, polarly flagellated, spiral-shaped cells that deposit sulfur

globules externally; internal photosynthetic membranes are organized as lamellar

stacks

2. The family Chromatiaceae contain twelve genera

1. Strict anaerobes and usually photolithoautotrophs; deposit sulfur granules internally
2. Hydrogen may serve as an electron donor
3. Thiospirillum, Thiocapsa, and Chromatium are typical and are usually found in anaerobic, sulfide-rich zones of lakes

1. Order Thiotrichales

1. Contains some of the colorless sulfur bacteria that are nonphotosynthetic, nonfruiting,

gliding bacteria

2. Two of the best studied gliding genera are Beggiatoa and Leucothrix

1. Microaerophilic and grows in sulfide-rich habitats
2. They lack a sheath
3. Metabolically versatile; can oxidize hydrogen sulfide to sulfur (deposited internally) and can oxidize sulfur to sulfate; can also grow heterotrophically with acetate as a carbon source and some may incorporate CO₂ autotrophically

1. Aerobic chemoorganotroph, marine, attached to solid substrates by a holdfast
2. Complex lifestyle in which dispersal is by formation of gonidia
3. Thiotrix is a related genus that forms sheathed filaments and is chemolithotrophic; oxidizes hydrogen sulfide and deposits sulfur granules internally

1. Order Methylococcales

1. Contains rods, vibrios, and cocci that use methane and methanol as their sole carbon and

energy source (methylotrophs)

2. Contains five genera including Methylococcus (spherical, nonmotile) and Methylomonas

(straight, curved, or branched rods with a single polar flagellum)

3. Complex arrays of intracellular membranes
4. Methane is oxidized to methanol and then to formaldehyde which can then be assimilated into cell material

1. Order Pseudomonadales

1. The genus Pseudomonas is the most important in this order and contains straight or

slightly curved rods; motile by polar flagella, lack a sheath or prosthecae

2. Aerobic chemoheterotrophs
3. Over 70 species in the genus Pseudomonas
4. Around 25 species have recently been removed and at least seven new genera have been formed
5. Great impact

1. The genus Azotobacter are large ovoid bacteria that are motile by peritrichous flagella; aerobic, catalase positive; fix nitrogen nonsymbiotically; widespread in soil and water

1. Order Vibrionales

1. Contains one family that are gram-negative, straight or curved rods with polar flagella
2. Oxidase-positive and use D-glucose as their sole or primary carbon and energy source
3. Aquatic with widespread distribution in freshwater and marine habitats
4. Five genera: Vibrio, Photobacterium, Salinivibrio, Listonella, and Allomonas
5. Pathogens include V. cholerae - cholera, V. parahaemolyticus - gastroenteritis after eating contaminated seafood, and V. anguillarum - a fish pathogen
6. Some, V. fischeri and at least two species of Photobacterium are among the few marine bacteria capable of bioluminescence; some bioluminescent species live symbiotically in the luminous organs of fish while others are free-living

1. Order Enterobacteriales

1. One family containing over 35 genera
2. Degrade sugars by Embden-Meyerhof pathway
3. The majority (e.g., Escherichia, Proteus, Salmonella and Shigella) carry out mixed acid fermentation while others (e.g., Enterobacter, Serratia, Erwinia and Klebsiella) carry out butanediol Fermentation
4. Gram-negative, peritrichously flagellated or nonmotile facultatively anaerobic, straight rods with simple nutritional requirements
5. Similarity in appearance among enteric bacteria requires the use of biochemical tests for identification
6. Rapid commercial identification systems (e.g., Enterotube, API 20-E) are based on these tests
7. Escherichia coli is probably the best studied bacterium and experimental organism of choice for many microbiologists; intestinal tract inhabitant; indicator organism for water quality (fecal contamination)
8. Several important human pathogens

1. Salmonella - typhoid fever and gastroenteritis
2. Shigella - bacillary dysentery
3. Klebsiella - pneumonia
4. Yersinia - plague

1. Order Pasteurellales

1. Small, nonmotile, normally oxidase-positive with complex nutritional requirements
2. Parasitic in vertebrates
3. Four genera: Pasteurella, Haemophilus, Actinobacillus and Lonepinella
4. Best known for the diseases they cause

1. P. multilocida - fowl cholera
2. P. haemolytica - pneumonia in cattle, sheep and goats (e.g., "shipping fever" in cattle)
3. H. influenzae - major human pathogen that causes a variety of diseases, including meningitis in children

1. The Delta Proteobacteria

1. Small collection of genera but with diverse morphological and physiological characteristics
2. Chemoorganotrophs placed in two orders and ten families
3. Order Desulfovibrionales

1. Gram-negative, dissimilatory sulfate- or sulfur-reducing bacteria; strict anaerobes; use elemental sulfur or sulfur compounds as electron acceptors during anaerobic respiration
2. Desulfovibrio - sulfate reducing
3. Desulfuromonas - uses only elemental sulfur
4. Important in sulfur cycling
5. Thrive in muds, polluted lake sediments, methane digesters
6. Negative impact on industry because of their primary role in the anaerobic corrosion of iron in pipelines, heating systems, and other structures
7. Bdellovibrio - gram-negative curved rods with polar flagella

1. Preys on other gram-negative bacteria
2. Alternates between a nongrowing predatory phase and an intracellular reproductive phase
3. Complex lifestyle

1. High-speed collision with its prey
2. Entry involves boring a hole through the host cell wall by a combination of mechanical and enzymatic action; flagellum lost during penetration
3. Inhabits space between cell wall and plasma membrane
4. Inhibits host DNA, RNA, and protein synthesis
5. Disrupts plasma membrane so cell constituents leak out
6. Grows into long filament and then divides into many smaller flagellated progeny

1. Order Myxococcales

1. The myxobacteria are gram-negative, aerobic soil bacteria with gliding motility
2. Form fruiting bodies and dormant myxospores
3. Divided into four families
4. Micropredators or scavengers that lyse bacteria and yeasts by secretion of an array of digestive enzymes
5. Use the released peptides and amino acids as primary carbon, nitrogen, and energy source
6. Lifestyle resembles that of cellular slime molds
7. Aggregate and differentiate into fruiting bodies when nutrient supply is exhausted
8. Myxospores are frequently enclosed in walled structures called sporangioles or sporangia

1. dormant
2. desiccation-resistant
3. fruiting bodies aid dispersal of myxospores

1. Colonial lifestyle aids digestion by providing higher enzyme concentration than any

individual bacterium could do

2. Found in neutral soils, decaying plant material, and animal dung
3. Most abundant in warm areas but will grow in the arctic tundra

1. The Epsilon Proteobacteria

1. Smallest of the proteobacteria groups
2. Slender, gram-negative rods that can be straight, curved, or helical
3. One order with two families and two important genera

1. Campylobacter - contains both pathogenic and nonpathogenic species

1. C. fetus - reproductive disease and abortions in cattle and sheep; can cause

septicemia and enteritis in humans

2. C. jejuni - causes abortions in sheep and enteritis diarrhea in humans

1. 2. Helicobacter

a. Isolated from stomachs and upper intestines of humans, dogs, cats and other

2. mammals