Divided into five classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, and Epsilonproteobacteria
The observation that purple photosynthetic bacteria are scattered throughout this phylum has led to the proposal that the proteobacteria arose from a photosynthetic ancestor
Class Alphaproteobacteria
Includes most of the oligotrophic proteobacteria
The Purple Nonsulfur Bacteria
Like all purple bacteria, the purple nonsulfur bacteria use anoxygenic photosynthesis, possess bacteriochlorophylls a or b, have their photosystems in membranes that are continuous with the plasma membrane, and are usually motile by polar flagella; with one exception, all purple nonsulfur bacteria are alphproteobacteria
Flexible in their choice of an energy source; normally they are grown anaerobically as photoorganoheterotophs, but can grow aerobically as chemoorganotrophs, and some can carry out fermentations
May be spirals, rods; some form prosthecae and buds
They are found in the mud and water of lakes and ponds with abundant organic matter and low sulfide levels; some marine species are known
Genera Rickettsia and Coxiella
Members of the genus Rickettsia are in the alphaproteobacteria and members of the genus Coxiella are in the gammaproteobacteria; however they are discussed together because of similar life styles.
These bacteria are rod-shaped, coccoid or pleomorphic, with typical gram-negative walls and no flagella; size varies but tends to be small (0.3 - 2.0 um)
All are parasitic or mutualistic
Rickettsias enter the host by phagocytosis, escape the phagosome, and then reproduce in the cytoplasm by binary fission
Coxiella remains in the phagosome after fusion with a lysosome and reproduces within the resulting phagolysosome.
For both genera, the host cell eventually bursts releasing new organisms
Metabolism of the rickettsias is unusual
Lack the glycolytic pathway and do not use glucose as an energy source; instead oxidize glutamate and TCA cycle intermediates
Take up nutrients, coenzymes, and ATP from host cell
These two genera contain many important pathogens
R. prowazekii and R. typhi-typhus fever
R. ricketsii-Rocky Mountain Spotted Fever
Coxiella burnetii-Q fever
They are also important pathogens in dogs, horses, sheep and cattle
The Caulobacteraceae and Hyphomicrobiaceae
Have one of three distinguishing features
Prostheca-an extension of the cell, including the plasma membrane, that is narrower than the mature cell
Stalk-a nonliving appendage produced by cells and extending from it
Reproduction by budding-parental cell retains its identity and progeny are much smaller than the parental cell
Genus Hyphomicrobium-chemoheterotrophic, aerobic, budding bacteria that frequently attach to solid objects in freshwater, marine and terrestrial environments
During budding process, mature cell produces a hypha or prostheca that elongates; the nucleoid divides and a copy moves into the hypha while a bud forms at its end; the bud matures, produces one to three flagella, and a septum divides the bud from the hypha; the bud is released as an oval- to pear-shaped swarmer cell
Has distinctive nutrition and physiology-grows on ethanol, acetate and one-carbon molecules such as methanol, formate, and formaldehyde (i.e., it is a facultative methylotroph)
May be as much as 25% of the total bacterial population in oligotrophic or nutrient-poor freshwater habitats
Genus Caulobacter
May be polarly flagellated rods or may possess prostheca and holdfast, by which they attach to solid substrata
Usually found in low-nutrient freshwater and marine habitats, but also present in soil; often adhere to bacteria, algae, and other microorganisms and may absorb nutrients released by their hosts
Prostheca differs from that of Hyphomicrobium in that it lacks cytoplasmic components and is composed almost totally of plasma membrane and cell wall
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, and begins to form swarmers; whole cycle takes only 2 hours
Family Rhizobiaceae
Contains the gram-negative genera Rhizobium and Agrobacterium
Genus Rhizobium
Motile rods (often containing poly-b-hydroxybutyrate granules) that become pleomorphic under adverse conditions
Grow symbiotically within root nodules cells of legumes as nitrogen-fixing bacteroids
Genus Agrobacterium
Not capable of nitrogen fixation
Invades crown, roots, and stems of many plants and transforms infected plant cells into autonomously proliferating tumors
A. tumefaciens (best studied) causes crown gall disease by means of a tumor inducing (Ti) plasmid
Nitrifying Bacteria
Nitrifying bacteria fall into alpha-, beta-, and gammaproteobacteria, but are considered together here
All are aerobic, gram-negative organisms without endospores; able to oxidize either ammonia or nitrite
May be rod-shaped, ellipsoidal, spherical, spirillar or lobate with either polar or peritrichous flagella
Often have extensive membrane complexes in cytoplasm
Are ecologically important
Nitrobacter and Nitrococcus oxidize nitrite to nitrate whereas Nitrosomonas, Nitrosospira and Nitrosococcus oxidize ammonia to nitrite
If two genera such as Nitrobacter and Nitrosomonas grow together in a habitat, ammonia is converted to nitrate (nitrification)
Nitrate is readily used by plants but is also easily leached from the soil or denitrified to nitrogen gas
Class Betaproteobacteria
Order Neisseriales-contains one family with 14 genera, including the genus Neisseria (described below)
Members of the genus Neisseria are nonmotile, aerobic, gram-negative cocci that most often occur in pairs with adjacent sides flattened; may have capsules and fimbriae
Chemoorganotrophic, oxidase-positive, and almost always catalase-positive
Inhabitants of the mucous membranes of animals; some are human pathogens (e.g., Neisseria gonorrhoeae-causative agent of gonorrhea and Neisseria meningitidis-one of the causative agents of bacterial meningitis)
Order Burkholderiales-contains five families, three of them with well-known genera
Family Burkholderiaceae-contains the genus Burkholderia (described below)
Gram-negative, aerobic, nonfermentative, non-spore-forming, mesophilic, straight rods; all but one species are motile with a single flagellum or a tuft of polar flagella
Catalase-positive and often oxidase-positive; use poly-b-hydroxybutyrate as their carbon reserve
B. cepacia is very active in recycling organic materials; is a plant pathogen; can cause disease in hospital patients due to contaminated equipment and medications; a particular problem with cystic fibrosis patients
Family Alcaliginaceae-contains the genus Bordetella (described below)
Gram-negative, aerobic coccobacilli
Chemoorganotrophs with respiratory metabolism; require organic sulfur and nitrogen (in the form of amino acids) for growth
Mammalian parasites that multiply in respiratory epithelial cells
B. pertussis-nonmotile, encapsulated species that is the causative agent for whooping cough
Family Comamonadaceae-contains 15 genera with diverse characteristics
Some (e.g., Sphaerotilus and Leptothrix) have a sheath, a hollow tubelike structure surrounding a chain of cells, which helps bacteria attach to surfaces and obtain nutrients from slowly running water as it flows past; sheath also provides protection against predators
Members of the genus Sphaerotilus form long sheathed chains of rods, often attach to solid surfaces by a holdfast, reproduce and spread via swarmer cells, prefer slowly running freshwater polluted with sewage or industrial waste, and can form tangled masses that interfere with activated sludge tanks used in sewage treatment
Members of the genus Leptothrix deposit large amounts of iron and manganese oxides in the sheath; this provides protection and allows growth in the presence of high concentrations of soluble iron compounds
Order Nitrosomonadales
Includes the nitrifying bacteria Nitrosomonas, Nitrosococcus, and Nitrosospira, discussed earlier
Also includes the genera Gallionella (a stalked chemolithotroph) and Spirillum
Order Hydrogenophilales-contains genus Thiobacillus, a prominent member of the colorless sulfur bacteria (use sulfur as electron source, but are not photosynthetic)
Thiobacillus-grow aerobically by oaxidizing inorganic sulfur compounds; supply carbon needs with carbon dioxide (chemolithoautotrophs); some are heterotrophs; some grow anaerobically, using nitrate as an electorn acceptor
Sulfur-oxidizing bacteria have wide distribution and great practical importance
Found in soil and aquatic habitats, especially those acidified by sulfuric acid
Their production of sulfuric acid and ferric iron allows them to corrode concrete and pipe structures
May also be beneficial by increasing soil fertility and processing low grade ores (leaching)
Class Gammaproteobacteria
Largest class of proteobacteria; divided into 13 orders, 20 families, and around 160 genera
The Purple Sulfur Bacteria (order Chromatiales)
Divided into two families: Chromatiaceae and Ectothiorhodospiraceae
Family Ectothiorhodospiraceae contains five genera, including Ectothiorhodospira, which has red, polarly flagellated, spiral-shaped cells that deposit sulfur globules externally and internal photosynthetic membranes that are organized as lamellar stacks
The typical purple sulfur bacteria are in the family Chromatiaceae, which contains 22 genera
Typical purple sulfur bacteria are strict anaerobes and usually photolithoautotrophs; oxidize hydrogen sulfide to sulfur and deposit it internally as sulfur granules; hydrogen may also serve as an electron donor
Genera Thiospirillum, Thiocapsa, and Chromatium are typical purple sulfur bacteria; they are usually found in anaerobic, sulfide-rich zones of lakes
Order Thiotrichales-contains three families, the largest of which is the Thiotrichiaceae; this family contains some of the colorless sulfur bacteria
Genus Beggiatoa
Microaerophilic; grow in sulfide-rich habitats
Filamentous; lack a sheath
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; some may incorporate CO2 autotrophically
Genus Leucothrix
Aerobic chemoorganotrophs that form long filaments (trichomes); are marine bacteria that attach to solid substrates by a holdfast
Have complex lifestyle in which dispersal is by formation of gonidia
Thiotrix is a related genus that forms sheathed filaments and is chemolithotrophic; oxidizes hydrogen sulfide and deposits sulfur granules internally; requires organic compounds for growth (mixotroph); found in sulfide-rich flowing water and activated sludge sewage systems
Order Methylococcales
Contains rods, vibrios, and cocci that use methane and methanol as their sole carbon and energy source (methylotrophs) under aerobic or microaerobic conditions; among the six genera are Methylococcus (spherical, nonmotile) and Methylomonas (straight, curved, or branched rods with a single polar flagellum)
When oxidizing methane have complex arrays of intracellular membranes
Found in anaerobic habitats, where methane is often abundant
Use methane as source of both energy and carbon
Methane is oxidized to methanol and then to formaldehyde
Formaldehyde is then assimilated into cell material
Order Pseudomonadales
The genus Pseudomonas is the most important in this order and contains straight or slightly curved rods; they are motile by polar flagella and lack a sheath or prosthecae
Aerobic respiratory chemoheterotrophs, though sometimes carry out anaerobic respiration using nitrate as the final electron acceptor
Have functional TCA cycle and use Entner-Dourderoff pathway rather than glycolytic pathway
Members of this order have great impact
Mineralization of a wide variety of organic compounds; useful in sewage treatment
Important experimental subjects
Some are major animal and plant pathogens
Some involved in the spoilage of refrigerated food because they can grow at 4oC and degrade lipids and proteins
Also in this order is the genus Azotobacter; are large ovoid bacteria that are motile by peritrichous flagella; are aerobic, catalase positive and fix nitrogen nonsymbiotically; are widespread in soil and water
Order Vibrionales
Contains only one family, Vibrionaceae
Are gram-negative, straight or curved rods with polar flagella
Are oxidase-positive and use D-glucose as their sole or primary carbon and energy source
Are aquatic with widespread distribution in freshwater and marine habitats
Contains six genera: Vibrio, Photobacterium, Enhydrobacter, Salinivibrio, Listonella, and Allomonas
Pathogens in this order include V. cholerae (cholera), V. parahaemolyticus (gastroenteritis after eating contaminated seafood), and V. anguillarum (a fish pathogen)
Some (e.g., 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
Order Enterobacteriales
Consists of one family containing over 41 genera; all are gram-negative, peritrichously flagellated or nonmotile, facultatively anaerobic, straight rods with simple nutritional requirements
Their metabolic properties are useful for characterization and identification
Degrade sugars by Embden-Meyerhof pathway and cleave pyruvic acid into formic acid in formic acid fermentations
The majority (e.g., Escherichia, Proteus, Salmonella and Shigella) carries out mixed acid fermentation, while others (e.g., Enterobacter, Serratia, Erwinia and Klebsiella) carry out butanediol fermentation
Usually are identified using rapid commercial identification systems (e.g., Enterotube, API 20-E) that are based on these biochemical characteristics
Are very common, widespread, and important
Escherichia coli is probably the best-studied bacterium and experimental organism of choice for many microbiologists; it is an intestinal tract inhabitant and an indicator organism for water quality (fecal contamination)
Salmonella-typhoid fever and gastroenteritis
Shigella-bacillary dysentery
Klebsiella-pneumonia
Yersinia-plague
Erwinia-plant pathogens
Order Pasteurellales
Small, nonmotile, normally oxidase-positive with complex nutritional requirements
Parasitic in vertebrates
Four genera: Pasteurella, Haemophilus, Actinobacillus and Lonepinella
Best known for the diseases they cause
P. multilocida-fowl cholera
P. haemolytica-pneumonia in cattle, sheep and goats (e.g. shipping fever in cattle)
H. influenzae-major human pathogen that causes a variety of diseases, including meningitis in children
Class Deltaproteobacteria
Orders Desulfovibrionales, Desulfobacterales, and Desulfuromonadales
Gram-negative, sulfate- or sulfur-reducing bacteria; strict anaerobes; use elemental sulfur or sulfur compounds as electron acceptors during anaerobic respiration
Important in sulfur cycling within ecosystems
Thrive in mud, polluted lake sediments, sewage lagoons and methane digesters, waterlogged soils, and anaerobic marine and estuarine sediments
Can have negative impact on industry because of their primary role in the anaerobic corrosion of iron in pipelines, heating systems, and other structures
Order Bdellovibrionales-has only one family and three genera, including the genus Bdellovibrio (described below)
Gram-negative curved rods with polar flagella; prey on other gram-negative bacteria and alternate between a nongrowing predatory phase and an intracellular reproductive phase
Have a complex lifestyle
Begins with high-speed collision with its prey
Enters its prey by boring a hole through the host cell wall; this is accomplished by a combination of mechanical and enzymatic action; flagellum lost during penetration
Inhabits the host within the space between cell wall and plasma membrane
Inhibits host DNA, RNA, and protein synthesis
Disrupts host cell's plasma membrane so cell constituents leak out
Grows into long filament and then divides (multiple fission) into many smaller flagellated progeny, which escape when the host cell lyses
Order Myxococcales
Myxobacteria are gram-negative, aerobic soil bacteria with gliding motility, and an interesting life cycle
Form fruiting bodies and dormant myxospores
All are rods that may be either slender with tapered ends or stout with rounded, blunt ends
Micropredators or scavengers that lyse bacteria and yeasts by secretion of an array of digestive enzymes
Many also secrete antibiotics to kill prey
Use the released peptides and amino acids as primary carbon, nitrogen, and energy source
All are chemoheterotrophs with respiratory metabolism
Lifestyle resembles that of cellular slime molds
When food is plentiful, they migrate along solid surfaces, feeding and leaving a slime trail
When their nutrient supply is exhausted, they aggregate and differentiate into fruiting bodies
Some cells in the fruiting body develop into myxospores; these are frequently enclosed in walled structures called sporangioles or sporangia
Myxospores are dormant and desiccation-resistant; fruiting bodies protect and aid dispersal of myxospores; a colony develops automatically when myxospores are released and this aids digestion by providing higher enzyme concentration than any individual bacterium could
Found in neutral soils, decaying plant material, and animal dung; are most abundant in warm areas but will grow in the arctic tundra
Class Epsilonproteobacteria
Smallest of the proteobacteria groups
All are slender, gram-negative rods that can be straight, curved, or helical
Contains one order with two families
Campylobacter and Helicobacter are the most important genera; both are microaerophiles, motile, helical or vibroid, gram-negative rods
Genus Campylobacter-contains both pathogenic and nonpathogenic species
C. fetus-reproductive disease and abortions in cattle and sheep; can cause septicemia and enteritis in humans
C. jejuni-causes abortion in sheep and enteritis diarrhea in humans
Genus Helicobacter
Isolated from stomachs and upper intestines of humans, dogs, cats and other mammals
H. pylori-cause of gastritis and peptic ulcer disease; produces large quantities of urease and urea hydrolysis appears to be associated with their virulence.
