Download 2/6/12 Bacterial Growth

I. Bacterial Cell Division
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5.1 Cell Growth and Binary Fission
5.2 Fts Proteins and Cell Division
5.3 MreB and Determinants of Cell Morphology
5.4 Peptidoglycan Synthesis and Cell Division
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5.1 Cell Growth and Binary Fission
• Binary fission: cell division following enlargement
of a cell to twice its minimum size (Figure 5.1)
• Generation time: time required for microbial cells
to double in number
• During cell division, each daughter cell receives a
chromosome and sufficient copies of all other cell
constituents to exist as an independent cell
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Figure 5.1
One generation
Cell
elongation
Septum
Septum
formation
Completion
of septum;
formation of
walls; cell
separation
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5.2 Fts Proteins and Cell Division
• Fts (filamentous temperature-sensitive) Proteins
(Figure 5.2)
– Essential for cell division in all prokaryotes
– Interact to form the divisome (cell division
apparatus)
• FtsZ: forms ring around center of cell; related to
tubulin
• ZipA: anchor that connects FtsZ ring to
cytoplasmic membrane
• FtsA: helps connect FtsZ ring to membrane and
also recruits other divisome proteins
– Related to actin
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5.2 Fts Proteins and Cell Division
• DNA replicates before the FtsZ ring forms
(Figure 5.3)
• Location of FtsZ ring is facilitated by Min proteins
– MinC, MinD, MinE
• FtsK protein mediates separation of
chromosomes to daughter cells
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Figure 5.2
Outer membrane
Peptidoglycan
Cytoplasmic
membrane
Divisome
complex
Cytoplasmic
membrane
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FtsZ ring
5.3 MreB and Determinants of Cell
Morphology
• Prokaryotes contain a cell cytoskeleton that is
dynamic and multifaceted
• MreB: major shape-determining factor in
prokaryotes
– Forms simple cytoskeleton in Bacteria and probably
Archaea
– Forms spiral-shaped bands around the inside of
the cell, underneath the cytoplasmic membrane
(Figure 5.4a and b)
– Not found in coccus-shaped bacteria
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5.3 MreB and Determinants of Cell
Morphology
• MreB (cont’d)
– Localizes synthesis of new peptidoglycan and
other cell wall components to specific
locations along the cylinder of a rod-shaped
cell during growth
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Figure 5.4a
FtsZ
Cell wall
Cytoplasmic
membrane
MreB
Sites of cell
wall synthesis
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Figure 5.4b
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5.3 MreB and Determinants of Cell
Morphology
• Most archaeal genomes contain FtsZ and
MreB-like proteins, thus cell morphology is
similar to that seen in Bacteria
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5.4 Peptidoglycan Synthesis and Cell
Division
• Production of new cell wall material is a major
feature of cell division
– In cocci, cell walls grow in opposite directions
outward from the FtsZ ring
– In rod-shaped cells, growth occurs at several
points along length of the cell
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5.4 Peptidoglycan Synthesis and Cell
Division
• Preexisting peptidoglycan needs to be
severed to allow newly synthesized
peptidoglycan to form
– Beginning at the FtsZ ring, small openings in
the wall are created by autolysins
– New cell wall material is added across the
openings
– Wall band: junction between new and old
peptidoglycan
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Figure 5.5
FtsZ ring
Wall bands
Septum
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Growth zone
Figure 5.4a
FtsZ
Cell wall
Cytoplasmic
membrane
MreB
Sites of cell
wall synthesis
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5.4 Peptidoglycan Synthesis and Cell
Division
• Bactoprenol: carrier molecule that plays
major role in insertion of peptidoglycan
precursors
– C55 alcohol (Figure 5.6)
– Bonds to N-acetylglucosamine/
N-acetylmuramic acid/pentapeptide
peptidoglycan precursor
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5.4 Peptidoglycan Synthesis and Cell
Division
• Glycolases: enzymes that interact with
bactoprenol (Figure 5.7a)
– Insert cell wall precursors into growing points
of cell wall
– Catalyze glycosidic bond formation
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Figure 5.7a
Peptidoglycan
Transglycosylase
activity
Cytoplasmic
membrane
Out
Growing point
of cell wall
Autolysin
activity
In
Pentapeptide
Bactoprenol
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5.4 Peptidoglycan Synthesis and Cell
Division
• Transpeptidation: final step in cell wall
synthesis (Figure 5.7b)
– Forms the peptide cross-links between
muramic acid residues in adjacent glycan
chains
– Inhibited by the antibiotic penicillin
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Figure 5.7b
Transpeptidation
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II. Population Growth
• 5.5 The Concept of Exponential Growth
• 5.6 The Mathematics of Exponential Growth
• 5.7 The Microbial Growth Cycle
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5.5 The Concept of Exponential Growth
• Most bacteria have shorter generation times
than eukaryotic microbes
• Generation time is dependent on growth
medium and incubation conditions
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5.5 The Concept of Exponential Growth
• Exponential growth: growth of a microbial
population in which cell numbers double
within a specific time interval
• During exponential growth, the increase in
cell number is initially slow but increases at a
faster rate (Figure 5.8)
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5.7 The Microbial Growth Cycle
• Batch culture: a closed-system microbial culture
of fixed volume
• Typical growth curve for population of cells grown
in a closed system is characterized by four
phases (Figure 5.10):
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–
–
–
Lag phase
Exponential phase
Stationary phase
Death phase
Animation: Bacterial Growth Curve
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Figure 5.10
Growth phases
Exponential
Stationary
Death
1.0
10
Log10 viable
organisms/ml
0.75
9
8
Turbidity
(optical density)
0.50
Viable count
0.25
7
6
0.1
Time
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Optical density (OD)
Lag
5.7 The Microbial Growth Cycle
• Lag phase
– Interval between when a culture is inoculated
and when growth begins
• Exponential phase
– Cells in this phase are typically in the
healthiest state
• Stationary phase
– Growth rate of population is zero
– Either an essential nutrient is used up or
waste product of the organism accumulates
in the medium
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5.7 The Microbial Growth Cycle
• Death Phase
– If incubation continues after cells reach
stationary phase, the cells will eventually die
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IV. Temperature and Microbial
Growth
• 5.12 Effect of Temperature on Growth
• 5.13 Microbial Life in the Cold
• 5.14 Microbial Life at High Temperatures
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Figure 5.18
Enzymatic reactions occurring
at maximal possible rate
Growth rate
Optimum
Enzymatic reactions occurring
at increasingly rapid rates
Minimum
Maximum
Temperature
Membrane gelling; transport
processes so slow that growth
cannot occur
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Protein denaturation; collapse
of the cytoplasmic membrane;
thermal lysis
5.12 Effect of Temperature on Growth
• Microorganisms can be classified into groups by
their growth temperature optima (Figure 5.19)
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–
–
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Psychrophile: low temperature
Mesophile: midrange temperature
Thermophile: high temperature
Hyperthermophile: very high temperature
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Figure 5.19
Thermophile
Example:
Geobacillus
stearothermophilus
Growth rate
Mesophile
Example:
Escherichia coli
Hyperthermophile Hyperthermophile
Example:
Pyrolobus fumarii
Example:
Thermococcus celer
60°
88°
106°
39°
Psychrophile
Example:
Polaromonas vacuolata
4°
0
10
20
30
40
50
60
70
Temperature (°C)
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80
90
100
110
120
5.12 Effect of Temperature on Growth
• Mesophiles: organisms that have midrange
temperature optima; found in
– Warm-blooded animals
– Terrestrial and aquatic environments
– Temperate and tropical latitudes
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5.13 Microbial Life in the Cold
• Extremophiles
– Organisms that grow under very hot or very cold
conditions
• Psychrophiles
– Organisms with cold temperature optima
– Inhabit permanently cold environments
(Figure 5.20)
• Psychrotolerant
– Organisms that can grow at 0ºC but have optima of
20ºC to 40ºC
– More widely distributed in nature than
psychrophiles
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5.13 Microbial Life in the Cold
• Molecular Adaptations to Psychrophily
– Production of enzymes that function optimally
in the cold; features that may provide more
flexibility
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More -helices than -sheets
More polar and less hydrophobic amino acids
Fewer weak bonds
Decreased interactions between protein
domains
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5.13 Microbial Life in the Cold
• Molecular Adaptations to Psychrophily (cont’d)
– Transport processes function optimally at low
temperatures
• Modified cytoplasmic membranes
– High unsaturated fatty acid content
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Figure 5.22
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Figure 5.23
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5.14 Microbial Life at High Temperatures
• Studies of thermal habitats have revealed
– Prokaryotes are able to grow at higher
temperatures than eukaryotes
– Organisms with the highest temperature
optima are Archaea
– Nonphototrophic organisms can grow at
higher temperatures than phototrophic
organisms
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5.14 Microbial Life at High Temperatures
• Molecular Adaptations to Thermophily
– Enzyme and proteins function optimally at high
temperatures; features that provide thermal
stability
• Critical amino acid substitutions in a few locations
provide more heat-tolerant folds
• An increased number of ionic bonds between basic
and acidic amino acids resist unfolding in the
aqueous cytoplasm
• Production of solutes (e.g., di-inositol phophate,
diglycerol phosphate) help stabilize proteins
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5.14 Microbial Life at High Temperatures
• Molecular Adaptations to Thermophily (cont’d)
– Modifications in cytoplasmic membranes to
ensure heat stability
• Bacteria have lipids rich in saturated fatty acids
• Archaea have lipid monolayer rather than bilayer
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5.14 Microbial Life at High Temperatures
• Hyperthermophiles produce enzymes widely
used in industrial microbiology
– Example: Taq polymerase, used to automate
the repetitive steps in the polymerase chain
reaction (PCR) technique
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V. Other Environmental Factors
Affecting Growth
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5.15 Acidity and Alkalinity
5.16 Osmotic Effects on Microbial Growth
5.17 Oxygen and Microorganisms
5.18 Toxic Forms of Oxygen
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5.15 Acidity and Alkalinity
• The pH of an environment greatly affects
microbial growth (Figure 5.24)
• Some organisms have evolved to grow best
at low or high pH, but most organisms grow
best between pH 6 and 8 (neutrophiles)
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Figure 5.24
Acidophiles
pH Example
Increasing
acidity
Alkaliphiles
Neutrality
Increasing
alkalinity
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Moles per liter of:
Volcanic soils, waters
Gastric fluids
Lemon juice
Acid mine drainage
Vinegar
Rhubarb
Peaches
Acid soil
Tomatoes
American cheese
Cabbage
Peas
Corn, salmon, shrimp
OH
H
1
1014
101
1013
102
1012
103
1011
104
1010
105
109
106
108
Pure water
107 107
Seawater
108
106
Very alkaline
natural soil
Alkaline lakes
Soap solutions
Household ammonia
Extremely alkaline
soda lakes
Lime (saturated solution)
109
105
1010
104
1011
103
1012
102
1013
101
1014
1
5.15 Acidity and Alkalinity
• Acidophiles: organisms that grow best at low
pH (<6)
– Some are obligate acidophiles; membranes
destroyed at neutral pH
– Stability of cytoplasmic membrane critical
• Alkaliphiles: organisms that grow best at high
pH (>9)
– Some have sodium motive force rather than
proton motive force
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5.15 Acidity and Alkalinity
• The internal pH of a cell must stay relatively
close to neutral even though the external pH
is highly acidic or basic
– Internal pH has been found to be as low as
4.6 and as high as 9.5 in extreme acido- and
alkaliphiles, respectively
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5.15 Acidity and Alkalinity
• Microbial culture media typically contain
buffers to maintain constant pH
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5.16 Osmotic Effects on Microbial Growth
• Typically, the cytoplasm has a higher solute
concentration than the surrounding
environment, thus the tendency is for water to
move into the cell (positive water balance)
• When a cell is in an environment with a higher
external solute concentration, water will flow
out unless the cell has a mechanism to
prevent this
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5.16 Osmotic Effects on Microbial Growth
• Halophiles: organisms that grow best at reduced
water potential; have a specific requirement for
NaCl (Figure 5.25)
• Extreme halophiles: organisms that require high
levels (15–30%) of NaCl for growth
• Halotolerant: organisms that can tolerate some
reduction in water activity of environment but
generally grow best in the absence of the added
solute
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Figure 5.25
Halophile
Example:
Staphylococcus
aureus
Example:
Aliivibrio fischeri
Extreme
halophile
Example:
Halobacterium
salinarum
Growth rate
Halotolerant
Nonhalophile
Example:
Escherichia coli
0
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5
10
NaCl (%)
15
20
5.16 Osmotic Effects on Microbial Growth
• Osmophiles: organisms that live in
environments high in sugar as solute
• Xerophiles: organisms able to grow in very
dry environments
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5.16 Osmotic Effects on Microbial Growth
• Mechanisms for combating low water activity in
surrounding environment involve increasing the
internal solute concentration by
– Pumping inorganic ions from environment
into cell
– Synthesis or concentration of organic solutes
• compatible solutes: compounds used by cell to
counteract low water activity in surrounding
environment
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5.17 Oxygen and Microorganisms
• Aerobes: require oxygen to live
• Anaerobes: do not require oxygen and may even
be killed by exposure
• Facultative organisms: can live with or without
oxygen
• Aerotolerant anaerobes: can tolerate oxygen and
grow in its presence even though they cannot
use it
• Microaerophiles: can use oxygen only when it is
present at levels reduced from that in air
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5.17 Oxygen and Microorganisms
• Thioglycolate broth (Figure 5.26)
– Complex medium that separates microbes
based on oxygen requirements
– Reacts with oxygen so oxygen can only
penetrate the top of the tube
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Figure 5.26
Oxic zone
Anoxic zone
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5.17 Oxygen and Microorganisms
• Special techniques are needed to grow
aerobic and anaerobic microorganisms
(Figure 5.27)
• Reducing agents: chemicals that may be
added to culture media to reduce oxygen
(e.g., thioglycolate)
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Figure 5.27
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