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Microbiology, Fifth Edition
Microbiology, 5/e
Lansing M Prescott, Augustana College
Donald A Klein, Colorado State University
John P Harley, Eastern Kentucky University
Bacteria: The Proteobacteria

Study Outline

  1. Introduction
    1. Largest and most diverse group of bacteria
    2. Divided into five classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, and Epsilonproteobacteria
    3. The observation that purple photosynthetic bacteria are scattered throughout this phylum has led to the proposal that the proteobacteria arose from a photosynthetic ancestor
  2. Class Alphaproteobacteria
    1. Includes most of the oligotrophic proteobacteria
    2. The Purple Nonsulfur Bacteria
      1. 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
      2. 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
      3. May be spirals, rods; some form prosthecae and buds
      4. 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
    3. Genera Rickettsia and Coxiella
      1. 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.
      2. 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)
      3. All are parasitic or mutualistic
        1. Rickettsias enter the host by phagocytosis, escape the phagosome, and then reproduce in the cytoplasm by binary fission
        2. Coxiella remains in the phagosome after fusion with a lysosome and reproduces within the resulting phagolysosome.
        3. For both genera, the host cell eventually bursts releasing new organisms
      4. Metabolism of the rickettsias is unusual
        1. Lack the glycolytic pathway and do not use glucose as an energy source; instead oxidize glutamate and TCA cycle intermediates
        2. Take up nutrients, coenzymes, and ATP from host cell
      5. These two genera contain many important pathogens
        1. R. prowazekii and R. typhi-typhus fever
        2. R. ricketsii-Rocky Mountain Spotted Fever
        3. Coxiella burnetii-Q fever
        4. They are also important pathogens in dogs, horses, sheep and cattle
    4. 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 cells and extending from it
        3. Reproduction by budding-parental cell retains its identity and progeny are much smaller than the parental cell
      2. Genus Hyphomicrobium-chemoheterotrophic, aerobic, budding bacteria that frequently attach to solid objects in freshwater, marine and terrestrial environments
        1. 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
        2. 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)
        3. May be as much as 25% of the total bacterial population in oligotrophic or nutrient-poor freshwater habitats
      3. Genus Caulobacter
        1. May be polarly flagellated rods or may possess prostheca and holdfast, by which they attach to solid substrata
        2. 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
        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, and begins to form swarmers; whole cycle takes only 2 hours
    5. Family Rhizobiaceae
      1. Contains the gram-negative genera Rhizobium and Agrobacterium
      2. Genus Rhizobium
        1. Motile rods (often containing poly-b-hydroxybutyrate granules) that become pleomorphic under adverse conditions
        2. Grow symbiotically within root nodules cells of legumes as nitrogen-fixing bacteroids
      3. Genus Agrobacterium
        1. Not capable of nitrogen fixation
        2. Invades crown, roots, and stems of many plants and transforms infected plant cells into autonomously proliferating tumors
        3. A. tumefaciens (best studied) causes crown gall disease by means of a tumor inducing (Ti) plasmid
    6. Nitrifying Bacteria
      1. Nitrifying bacteria fall into alpha-, beta-, and gammaproteobacteria, but are considered together here
        1. All are aerobic, gram-negative organisms without endospores; able to oxidize either ammonia or nitrite
        2. May be rod-shaped, ellipsoidal, spherical, spirillar or lobate with either polar or peritrichous flagella
        3. Often have extensive membrane complexes in cytoplasm
      2. Are ecologically important
        1. Nitrobacter and Nitrococcus oxidize nitrite to nitrate whereas Nitrosomonas, Nitrosospira and Nitrosococcus oxidize ammonia to nitrite
        2. If two genera such as Nitrobacter and Nitrosomonas grow together in a habitat, ammonia is converted to nitrate (nitrification)
        3. Nitrate is readily used by plants but is also easily leached from the soil or denitrified to nitrogen gas
  3. Class Betaproteobacteria
    1. Order Neisseriales-contains one family with 14 genera, including the genus Neisseria (described below)
      1. 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
      2. Chemoorganotrophic, oxidase-positive, and almost always catalase-positive
      3. 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)
    2. Order Burkholderiales-contains five families, three of them with well-known genera
      1. Family Burkholderiaceae-contains the genus Burkholderia (described below)
        1. 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
        2. Catalase-positive and often oxidase-positive; use poly-b-hydroxybutyrate as their carbon reserve
        3. 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
      2. Family Alcaliginaceae-contains the genus Bordetella (described below)
        1. Gram-negative, aerobic coccobacilli
        2. Chemoorganotrophs with respiratory metabolism; require organic sulfur and nitrogen (in the form of amino acids) for growth
        3. Mammalian parasites that multiply in respiratory epithelial cells
        4. B. pertussis-nonmotile, encapsulated species that is the causative agent for whooping cough
      3. Family Comamonadaceae-contains 15 genera with diverse characteristics
        1. 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
        2. 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
        3. 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
    3. Order Nitrosomonadales
      1. Includes the nitrifying bacteria Nitrosomonas, Nitrosococcus, and Nitrosospira, discussed earlier
      2. Also includes the genera Gallionella (a stalked chemolithotroph) and Spirillum
    4. Order Hydrogenophilales-contains genus Thiobacillus, a prominent member of the colorless sulfur bacteria (use sulfur as electron source, but are not photosynthetic)
      1. Thiobacillus-gram-negative rods, lacking extensive internal membranes
      2. 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
      3. Sulfur-oxidizing bacteria have wide distribution and great practical importance
        1. Found in soil and aquatic habitats, especially those acidified by sulfuric acid
        2. Their production of sulfuric acid and ferric iron allows them to corrode concrete and pipe structures
        3. May also be beneficial by increasing soil fertility and processing low grade ores (leaching)
  4. Class Gammaproteobacteria
    1. Largest class of proteobacteria; divided into 13 orders, 20 families, and around 160 genera
    2. The Purple Sulfur Bacteria (order Chromatiales)
      1. Divided into two families: Chromatiaceae and Ectothiorhodospiraceae
        1. 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
        2. The typical purple sulfur bacteria are in the family Chromatiaceae, which contains 22 genera
        3. 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
      2. Genera Thiospirillum, Thiocapsa, and Chromatium are typical purple sulfur bacteria; they are usually found in anaerobic, sulfide-rich zones of lakes
    3. Order Thiotrichales-contains three families, the largest of which is the Thiotrichiaceae; this family contains some of the colorless sulfur bacteria
      1. Genus Beggiatoa
        1. Microaerophilic; grow in sulfide-rich habitats
        2. Filamentous; 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; some may incorporate CO2 autotrophically
      2. Genus Leucothrix
        1. Aerobic chemoorganotrophs that form long filaments (trichomes); are marine bacteria that attach to solid substrates by a holdfast
        2. Have 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; requires organic compounds for growth (mixotroph); found in sulfide-rich flowing water and activated sludge sewage systems
    4. Order Methylococcales
      1. 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)
        1. When oxidizing methane have complex arrays of intracellular membranes
        2. Found in anaerobic habitats, where methane is often abundant
      2. Use methane as source of both energy and carbon
        1. Methane is oxidized to methanol and then to formaldehyde
        2. Formaldehyde is then assimilated into cell material
    5. Order Pseudomonadales
      1. 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
        1. Aerobic respiratory chemoheterotrophs, though sometimes carry out anaerobic respiration using nitrate as the final electron acceptor
        2. Have functional TCA cycle and use Entner-Dourderoff pathway rather than glycolytic pathway
      2. Members of this order have great impact
        1. Mineralization of a wide variety of organic compounds; useful in sewage treatment
        2. Important experimental subjects
        3. Some are major animal and plant pathogens
        4. Some involved in the spoilage of refrigerated food because they can grow at 4oC and degrade lipids and proteins
      3. 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
    6. Order Vibrionales
      1. Contains only one family, Vibrionaceae
        1. Are gram-negative, straight or curved rods with polar flagella
        2. Are oxidase-positive and use D-glucose as their sole or primary carbon and energy source
        3. Are aquatic with widespread distribution in freshwater and marine habitats
        4. Contains six genera: Vibrio, Photobacterium, Enhydrobacter, Salinivibrio, Listonella, and Allomonas
      2. Pathogens in this order include V. cholerae (cholera), V. parahaemolyticus (gastroenteritis after eating contaminated seafood), and V. anguillarum (a fish pathogen)
      3. 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
    7. Order Enterobacteriales
      1. Consists of one family containing over 41 genera; all are gram-negative, peritrichously flagellated or nonmotile, facultatively anaerobic, straight rods with simple nutritional requirements
      2. Their metabolic properties are useful for characterization and identification
        1. Degrade sugars by Embden-Meyerhof pathway and cleave pyruvic acid into formic acid in formic acid fermentations
        2. 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
        3. Usually are identified using rapid commercial identification systems (e.g., Enterotube, API 20-E) that are based on these biochemical characteristics
      3. Are very common, widespread, and important
        1. 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)
        2. Salmonella-typhoid fever and gastroenteritis
        3. Shigella-bacillary dysentery
        4. Klebsiella-pneumonia
        5. Yersinia-plague
        6. Erwinia-plant pathogens
    8. Order Pasteurellales
      1. Small, nonmotile, normally oxidase-positive with complex nutritional requirements
        1. Parasitic in vertebrates
        2. Four genera: Pasteurella, Haemophilus, Actinobacillus and Lonepinella
      2. 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
  5. Class Deltaproteobacteria
    1. Orders Desulfovibrionales, Desulfobacterales, and Desulfuromonadales
      1. Gram-negative, sulfate- or sulfur-reducing bacteria; strict anaerobes; use elemental sulfur or sulfur compounds as electron acceptors during anaerobic respiration
      2. Important in sulfur cycling within ecosystems
        1. Thrive in mud, polluted lake sediments, sewage lagoons and methane digesters, waterlogged soils, and anaerobic marine and estuarine sediments
        2. Can have negative impact on industry because of their primary role in the anaerobic corrosion of iron in pipelines, heating systems, and other structures
    2. Order Bdellovibrionales-has only one family and three genera, including the genus Bdellovibrio (described below)
      1. 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
      2. Have a complex lifestyle
        1. Begins with high-speed collision with its prey
        2. 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
        3. Inhabits the host within the space between cell wall and plasma membrane
        4. Inhibits host DNA, RNA, and protein synthesis
        5. Disrupts host cell's plasma membrane so cell constituents leak out
        6. Grows into long filament and then divides (multiple fission) into many smaller flagellated progeny, which escape when the host cell lyses
    3. Order Myxococcales
      1. Myxobacteria are gram-negative, aerobic soil bacteria with gliding motility, and an interesting life cycle
        1. Form fruiting bodies and dormant myxospores
        2. All are rods that may be either slender with tapered ends or stout with rounded, blunt ends
      2. Micropredators or scavengers that lyse bacteria and yeasts by secretion of an array of digestive enzymes
        1. Many also secrete antibiotics to kill prey
        2. Use the released peptides and amino acids as primary carbon, nitrogen, and energy source
        3. All are chemoheterotrophs with respiratory metabolism
      3. Lifestyle resembles that of cellular slime molds
        1. When food is plentiful, they migrate along solid surfaces, feeding and leaving a slime trail
        2. When their nutrient supply is exhausted, they aggregate and differentiate into fruiting bodies
        3. Some cells in the fruiting body develop into myxospores; these are frequently enclosed in walled structures called sporangioles or sporangia
      4. 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
      5. Found in neutral soils, decaying plant material, and animal dung; are most abundant in warm areas but will grow in the arctic tundra
  6. Class Epsilonproteobacteria
    1. Smallest of the proteobacteria groups
      1. All are slender, gram-negative rods that can be straight, curved, or helical
      2. Contains one order with two families
      3. Campylobacter and Helicobacter are the most important genera; both are microaerophiles, motile, helical or vibroid, gram-negative rods
    2. Genus 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 abortion in sheep and enteritis diarrhea in humans
    3. Genus Helicobacter
      1. Isolated from stomachs and upper intestines of humans, dogs, cats and other mammals
      2. H. pylori-cause of gastritis and peptic ulcer disease; produces large quantities of urease and urea hydrolysis appears to be associated with their virulence.








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