Transcript Bacteria - McGraw Hill Higher Education

Chapter 34: Bacteria
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Prokaryotes
• Bacteria are prokaryotes
• Characteristics
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single-celled
semi-rigid wall around plasma membrane
no membrane-bound organelles
genetic material free in cytoplasm
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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
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staining reactions
cell shape
cell grouping
presence of special structures
growth medium
antibiotic resistance
DNA sequences
immunological tests
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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
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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
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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 about
100 000 000 000 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
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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
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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
or acetate
or formate
• Methanogens occur in anaerobic environments,
such as animal intestines, waterlogged soils and
mud
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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
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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
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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
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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
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antibiotic resistance
antibiotic synthesis
toxin synthesis
production of tissue-damaging enzymes
gall production in plants
resistance to phage attack
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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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