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Chapter 34: Bacteria

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Prokaryotes• Bacteria are prokaryotes• Characteristics

– single-celled– semi-rigid wall around plasma membrane– no membrane-bound organelles– genetic material free in cytoplasm

Fig. 34.1: Relative sizes of microbes

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The first cellular life• Bacteria were the earliest forms of life on Earth

– oldest fossils of bacteria are 3.5 billion years old

• Early forms existed under conditions hostile to most modern living organisms– anaerobic atmosphere with H2, NH3, H2S

– high levels of UV radiation

• Descendants of early bacteria now found in hot, hypersaline or anoxic areas that resemble ancient earth

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Early photosynthetic bacteria• Evolution of photosynthesis allowed bacteria to fix

carbon• Early photosynthetic pathways were anoxygenic

(did not produce oxygen)• Subsequent evolution of oxygenic photosynthesis

(2.5 billion years ago) produced enough O2 to change composition of atmosphere

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Classifying and identifying bacteria• Biochemical, physiological and immunological

characteristics are used as a rapid method of identifying and classifying bacteria– staining reactions– cell shape– cell grouping– presence of special structures– growth medium– antibiotic resistance– DNA sequences– immunological tests

Fig. 34.2: Examples of bacterial cells

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Super kingdoms• Prokaryotes are divided into two groups on the

basis of biochemical characteristics– Super kingdom Bacteria, formerly called Eubacteria

(‘true bacteria’)

– Super kingdom Archaea, formerly called Archaeobacteria (‘ancient bacteria’)

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Fig. 34.3: Evolutionary relationships

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Bacteria• Diverse metabolic pathways have allowed bacteria

to use most materials as sources of energy– only some plastics and organochlorine compounds are

resistant to bacteria

• Characteristics– peptidoglycan is major cell wall polymer– membrane lipids are esters– protein synthesis disrupted by streptomycin– some nitrifying and photosynthetic species

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Bacteria (cont.)• Cyanobacteria are also known as ‘blue-green

algae’– blue phycobilins (a water-soluble pigment) gives them the

characteristic blue-green colour, which is obvious when they form dense mats or blooms in shallow waters

• Under poor conditions, endospores form inside bacteria (such as Clostridium and Bacillus)– endospores are resistant to high temperatures, radiation

and chemicals– many species of endospore-forming bacteria are

important pathogenic agents

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Archaea• Many Archaea occur in extreme environments,

including deep-sea volcanic vents and thermal pools – halophiles (hypersaline)– acidophiles (low pH)– thermophiles (high temperatures)

• Characteristics– peptidoglycan is not the major cell wall polymer– membrane lipids are ethers– protein synthesis disrupted by diphtheria toxin– no nitrifying or photosynthetic species

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Bacterial populations• Very large and dense• Human skin harbours about 100 000 cells/cm-1

– clustered distribution in moist, bacteria-friendly areas– suite of species varies from person to person

• Human faecal material contains about100 000 000 000 cells/gm-1

– high diversity of bacteria in colon

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Nutritional and metabolic diversity of bacteria• Energy source

– phototrophs use radiant (light) energy– chemotrophs use chemical energy

• Carbon source– autotrophs synthesise organic compounds from inorganic

carbon– heterotrophs use organic compounds as energy source

• Four nutritional types– chemoautotrophs– chemoheterotrophs– photoautotrophs – photoheterotrophs

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Autotrophs• Photoautotrophs

– photosynthetic bacteria: cyanobacteria, purple bacteria and green bacteria

– use light energy to reduce CO2

– reductant may be H2O, H2S, H2

• Chemoautotrophs– nitrifying bacteria, methanogenic bacteria, iron-oxidising

bacteria and others

– use chemical energy (NH4+, NO2

-, H2S, S, Fe3+) to reduce

CO2

– reductant may be H2O, H2

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Fig. 34.7 b + c: Cellular metabolic categories

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Heterotrophs• Photoheterotrophs

– anaerobically-growing purple bacteria and green bacteria

– use light energy to reduce CH2O

– reductant may be CH2O, H2S, S, H2

• Chemoheterotrophs– many bacteria (also animals and fungi)

– CH2O is reductant and provides energy

Question 1

Chemoautotrophs:

a) Must consume organic molecules for energy and carbon

b) Harness light energy to drive the synthesis of organic compounds from carbon dioxide

c) Use light to generate ATP but obtain their carbon in organic form

d) Need only CO2 as a carbon source, but obtain energy by oxidising inorganic substances

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Fig. 37.7 d + a: Cellular metabolic categories

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Anaerobic metabolism by bacteria

Anaerobic pathways use compounds other than O2 as terminal oxidants

CH2O + NO3- CO2 + N2

or SO42-, HCO3

-, Fe3+ or fumarate

or S, CH4, Fe2+ or succinate

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Nitrogen cycle• Nitrogen-fixing bacteria (cyanobacteria, plant

symbiotes, Clostridium, others) are the only organisms capable of fixing molecular nitrogen

N2 + 8H+ + 6e- 2NH4+

• The reaction is sensitive to molecular oxygen and other oxidants, so it occurs in a highly reducing or anaerobic environment

• Ammonium ion is used to form glutamine and glutamate (amino acids) in bacterial cell

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Nitrogen cycle (cont.)• Nitrifying bacteria oxidise ammonium to nitrite

(Nitrosomonas) and nitrate (Nitrobacter) – transform fixed nitrogen from nitrogen fixers or

decomposing organisms

• Denitrifying bacteria (Pseudomonas, anaerobic bacteria) use nitrite and nitrate as terminal electron receptors– produce gaseous nitrous oxide and molecular nitrogen– nitrogen is no longer available for other organisms,

except nitrogen-fixing organisms

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Fig. 34.8a: Nitrogen cycle

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Commercial applications of bacterial fermentations• Fermentation (anaerobic energy metabolism)

produces a range of end products, many of which are used in agriculture and food and alcohol production, for example:– Lactic acid: Lactobacillus, Lactococcus and other

bacteria are used in the production of yoghurt and milk– Ethanol: bacteria decarboxylate pyruvate to form acetate,

which is then reduced to ethanol

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Methane-producing bacteria• Chemoautotrophic methanogens use hydrogen

and carbon dioxide to produce methane

4H2 + CO2 CH4 + 2H2O

• Methanogens occur in anaerobic environments, such as animal intestines, waterlogged soils and mud

or acetate or formate

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Genetic systems of bacteria• Bacteria reproduce asexually by fission (cell

division)• Genetic variation in bacteria is due to

– mutation– mixing genetic material between different cells

transformation conjugation transduction

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Transformation: gene transfer by free molecules of DNA• Bacteria may take free DNA molecules into their

cells• DNA recognised as foreign may be broken down• DNA similar to the bacterium’s DNA may

– recombine with the chromosomal or plasmid DNA– become a plasmid

• This process of taking up free DNA and making it part of the cell is transformation

Fig. 34.10a: Transformation

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Conjugation: gene transfer by plasmids• DNA may be transferred directly between bacteria

via plasmids in the process of conjugation• A plasmid may pass a copy of itself from one cell

to another• Once in a new cell, a plasmid may

– establish itself as an independent plasmid in the cell– combine with another plasmid– combine with the chromosomal DNA

Fig. 34.10b: Conjugation

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Transduction: gene transfer by bacteriophages• Bacteriophages (viruses that live in bacterial cells)

integrate their DNA into the host’s chromosomal DNA

• Temperate (non-virulent) phages become virulent under certain conditions, rupturing the cell and releasing virions (phage particles)

• A virion may inadvertently carry the original host’s DNA into another cell, where it may recombine or integrate with the new host’s DNA

Fig. 34.10c: Transduction

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Plasmids and phages• Plasmids and phages are abundant in bacterial

populations• Gene transfer often confers new properties on host

bacteria– antibiotic resistance– antibiotic synthesis– toxin synthesis– production of tissue-damaging enzymes– gall production in plants– resistance to phage attack

Fig. B34.4: Life cycle of a tailed bacteriophage

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Summary• Prokaryotes are microscopic and unicellular• Prokaryotes have evolved along two major

evolutionary lineages: bacteria and archaea• Bacteria are the most abundant organisms on

earth, due to their remarkable range of nutritional and metabolic types

• Multiple genetic mechanisms have enabled remarkable adaptability and diversity

• Bacteria are highly complex entities, capable of performing a myriad of functions

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