LECTURE PRESENTATIONS
For BROCK BIOLOGY OF MICROORGANISMS, THIRTEENTH EDITION
Michael T. Madigan, John M. Martinko, David A. Stahl, David P. Clark
Chapter 5
Microbial Growth
Lectures by
John Zamora
Middle Tennessee State University
© 2012 Pearson Education, Inc.
I. Bacterial Cell Division
•
•
•
•
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
© 2012 Pearson Education, Inc.
5.1 Cell Growth and Binary Fission
• Growth: increase in the number of cells
• 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
Animation: Overview of Bacterial Growth
Animation: Binary Fission
© 2012 Pearson Education, Inc.
Figure 5.1
One generation
Cell
elongation
Septum
Septum
formation
Completion
of septum;
formation of
walls; cell
separation
© 2012 Pearson Education, Inc.
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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Figure 5.2
Outer membrane
Peptidoglycan
Cytoplasmic
membrane
Divisome
complex
Cytoplasmic
membrane
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FtsZ ring
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
© 2012 Pearson Education, Inc.
Figure 5.3
Min CD
Minutes
0
Cell wall
Cytoplasmic
membrane
Nucleoid
20
MinE
40
Divisome
complex
60
FtsZ ring
Septum
Nucleoid
80
MinE
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
5.3 MreB and Determinants of Cell
Morphology
• Crescentin: shape-determining protein
produced by vibrio-shaped cells of
Caulobacter crescentus
– Crescentin protein organizes into filaments
~10 nm wide that localize on the concave face
of the curved cells (Figure 5.4c)
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Figure 5.4c
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
Growth zone
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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Figure 5.6
Hydrophobic portion
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
Figure 5.7a
Peptidoglycan
Transglycosylase
activity
Cytoplasmic
membrane
Out
Growing point
of cell wall
Autolysin
activity
In
Pentapeptide
Bactoprenol
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
Figure 5.7b
Transpeptidation
© 2012 Pearson Education, Inc.
II. Population Growth
•
•
•
•
5.5
5.6
5.7
5.8
The Concept of Exponential Growth
The Mathematics of Exponential Growth
The Microbial Growth Cycle
Continuous Culture: The Chemostat
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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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1000
103
Logarithmic
plot
102
Arithmetic
500 plot
10
100
0
© 2012 Pearson Education, Inc.
1
4
3
2
Time (h)
5
1
Number of cells
(logarithmic scale)
Number of cells
(arithmetic scale)
Figure 5.8
5.6 The Mathematics of Exponential
Growth
• A relationship exists between the initial number of
cells present in a culture and the number present
after a period of exponential growth:
N = N0 2 n
N is the final cell number
N0 is the initial cell number
n is the number of generations during
the period of exponential growth
© 2012 Pearson Education, Inc.
5.6 The Mathematics of Exponential
Growth
• Generation time (g) of the exponentially
growing population is
g = t/n
t is the duration of exponential growth
n is the number of generations during
the period of exponential growth
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5.6 The Mathematics of Exponential
Growth
• Specific growth rate (k) is calculated as
k = 0.301/g
• Division rate (v) is calculated as
v = 1/g
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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):
–
–
–
–
Lag phase
Exponential phase
Stationary phase
Death phase
Animation: Bacterial Growth Curve
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
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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5.8 Continuous Culture: The Chemostat
• Continuous culture: an open-system microbial
culture of fixed volume
• Chemostat: most common type of continuous
culture device (Figure 5.11)
– Both growth rate and population density of culture
can be controlled independently and
simultaneously
• Dilution rate: rate at which fresh medium is pumped
in and spent medium is pumped out
• Concentration of a limiting nutrient
© 2012 Pearson Education, Inc.
Figure 5.11
Fresh medium
from reservoir
Flow-rate
regulator
Sterile air or
other gas
Gaseous
headspace
Culture
vessel
Culture
Overflow
Effluent containing
microbial cells
© 2012 Pearson Education, Inc.
5.8 Continuous Culture: The Chemostat
• In a chemostat
– The growth rate is controlled by dilution rate
– The growth yield (cell number/ml) is controlled
by the concentration of the limiting nutrient
• In a batch culture, growth conditions are
constantly changing; it is impossible to
independently control both growth
parameters (Figure 5.12)
© 2012 Pearson Education, Inc.
Figure 5.12
Rate and
yield affected
Growth yield (
Growth rate (
)
)
Only yield affected
0
0.1
0.2
0.3
0.4
Nutrient concentration (mg/ml)
© 2012 Pearson Education, Inc.
0.5
5.8 Continuous Culture: The Chemostat
• Chemostat cultures are sensitive to the dilution
rate and limiting nutrient concentration
(Figure 5.13)
– At too high a dilution rate, the organism is
washed out
– At too low a dilution rate, the cells may die from
starvation
– Increasing concentration of a limiting nutrient
results in greater biomass but same growth
rate
© 2012 Pearson Education, Inc.
Steady state
5
Bacterial concentration
6
4
3
4
2
2
1
0
0
0
0.25
0.5
Dilution rate (h1)
© 2012 Pearson Education, Inc.
0.75
1.0
Washout
Doubling time (h)
Steady-state bacterial concentration (g/l)
Figure 5.13
III. Measuring Microbial Growth
• 5.9 Microscopic Counts
• 5.10 Viable Counts
• 5.11 Turbidimetric Methods
© 2012 Pearson Education, Inc.
5.9 Microscopic Counts
• Microbial cells are enumerated by microscopic
observations (Figure 5.14)
– Results can be unreliable
© 2012 Pearson Education, Inc.
Figure 5.14
To calculate number
per milliliter of sample:
12 cells  25 large squares  50  103
Ridges that support coverslip
Coverslip
Number/mm2 (3  102)
Sample added here. Care must be
taken not to allow overflow; space
between coverslip and slide is 0.02 mm
( 501 mm. Whole grid has 25 large
squares, a total area of 1 mm2 and
a total volume of 0.02 mm3.
© 2012 Pearson Education, Inc.
Microscopic observation; all
cells are counted in large
square (16 small squares):
12 cells. (In practice, several
large squares are counted and
the numbers averaged.)
Number/mm3 (1.5  104)
Number/cm3 (ml) (1.5  107)
5.9 Microscopic Counts
• Limitations of microscopic counts
– Cannot distinguish between live and dead cells
without special stains
– Small cells can be overlooked
– Precision is difficult to achieve
– Phase-contrast microscope required if a stain is
not used
– Cell suspensions of low density (<106 cells/ml)
hard to count
– Motile cells need to immobilized
– Debris in sample can be mistaken for cells
© 2012 Pearson Education, Inc.
5.9 Microscopic Counts
• A second method for enumerating cells in
liquid samples is with a flow cytometer
– Uses laser beams, fluorescent dyes, and
electronics
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5.10 Viable Counts
• Viable cell counts (plate counts):
measurement of living, reproducing
population
– Two main ways to perform plate counts:
• Spread-plate method (Figure 5.15)
• Pour-plate method
• To obtain the appropriate colony number, the
sample to be counted should always be
diluted (Figure 5.16)
© 2012 Pearson Education, Inc.
Figure 5.15
Spread-plate method
Surface
colonies
Incubation
Sample is pipetted onto
surface of agar plate
(0.1 ml or less)
Sample is spread evenly
over surface of agar
using sterile glass
spreader
Typical spread-plate
results
Pour-plate method
Surface
colonies
Solidification
and incubation
Sample is pipetted into
sterile plate
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Sterile medium is
added and mixed
well with inoculum
Subsurface
colonies
Typical pour-plate
results
Figure 5.16
Sample to
be counted
1 ml
1 ml
1 ml
1 ml
1 ml
1 ml
9-ml
broth
1/10
Total
dilution (101)
1/100
(102)
1/103
(103)
1/104
(104)
1/105
(105)
1/106
(106)
Plate 1-ml samples
Too many colonies
to count
159
17
2
0
colonies colonies colonies colonies
159  103
1.59  105

Plate Dilution
Cells (colony-forming units)
count factor
per milliliter of original sample
© 2012 Pearson Education, Inc.
5.10 Viable Counts
• Plate counts can be highly unreliable when
used to assess total cell numbers of natural
samples (e.g., soil and water)
– Selective culture media and growth conditions
target only particular species
© 2012 Pearson Education, Inc.
5.10 Viable Cell Counting
• The Great Plate Anomaly: direct microscopic
counts of natural samples reveal far more
organisms than those recoverable on plates
• Why is this?
– Microscopic methods count dead cells
whereas viable methods do not
– Different organisms may have vastly different
requirements for growth
© 2012 Pearson Education, Inc.
5.11 Turbidimetric Methods
• Turbidity measurements are an indirect, rapid,
and useful method of measuring microbial
growth (Figure 5.17a)
– Most often measured with a spectrophotometer
and measurement referred to as optical density
(O.D.)
© 2012 Pearson Education, Inc.
Figure 5.17a
Light
Prism
Incident light, I0
Filter
Sample containing
cells ( )
Unscattered light, I
Photocell (measures
unscattered light, I )
Spectrophotometer
Optical density (OD)
 Log
© 2012 Pearson Education, Inc.
I0
I
5.11 Turbidimetric Methods
• To relate a direct cell count to a turbidity
value, a standard curve must first be
established (Figure 5.17c)
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Figure 5.17c
Optical density
0.8
0.7
Theoretical
Actual
0.6
0.5
0.4
0.3
0.2
0.1
Cell numbers or mass
(dry weight)
© 2012 Pearson Education, Inc.
5.11 Turbidimetric Methods
• Turbidity measurements
– Quick and easy to perform
– Typically do not require destruction or
significant disturbance of sample
– Sometimes problematic (e.g., microbes that
form clumps or biofilms in liquid medium)
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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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5.12 Effect of Temperature on Growth
• Temperature is a major environmental factor
controlling microbial growth
• Cardinal temperatures: the minimum,
optimum, and maximum temperatures at
which an organism grows (Figure 5.18)
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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
© 2012 Pearson Education, Inc.
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)
–
–
–
–
Psychrophile: low temperature
Mesophile: midrange temperature
Thermophile: high temperature
Hyperthermophile: very high temperature
© 2012 Pearson Education, Inc.
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)
© 2012 Pearson Education, Inc.
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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Figure 5.20
© 2012 Pearson Education, Inc.
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
•
•
•
•
More -helices than -sheets
More polar and less hydrophobic amino acids
Fewer weak bonds
Decreased interactions between protein
domains
© 2012 Pearson Education, Inc.
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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5.14 Microbial Life at High Temperatures
• Above ~65°C, only prokaryotic life forms exist
• Thermophiles: organisms with growth temperature
optima between 45°C and 80°C
• Hyperthermophiles: organisms with optima greater
than 80°C
– Inhabit hot environments including boiling hot
springs and seafloor hydrothermal vents that can
have temperatures in excess of 100°C
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5.14 Microbial Life at High Temperatures
• Hyperthermophiles in Hot Springs
– Chemoorganotrophic and chemolithotrophic
species present (Figure 5.22)
– High prokaryotic diversity (both Archaea and
Bacteria represented)
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Figure 5.22
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5.14 Microbial Life at High Temperatures
• Thermal gradients form along edges of hot
environments
• Distribution of microbial species along the
gradient is dictated by organism’s biology
(Figure 5.23)
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Figure 5.23
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
V. Other Environmental Factors
Affecting Growth
•
•
•
•
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)
© 2012 Pearson Education, Inc.
Figure 5.24
Acidophiles
pH Example
Increasing
acidity
Alkaliphiles
Neutrality
Increasing
alkalinity
© 2012 Pearson Education, Inc.
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
• Water activity (aw): water availability;
expressed in physical terms
– Defined as ratio of vapor pressure of air in
equilibrium with a substance or solution to the
vapor pressure of pure water
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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
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
Figure 5.25
Halophile
Example:
Staphylococcus
aureus
Example:
Aliivibrio fischeri
Extreme
halophile
Example:
Halobacterium
salinarum
Growth rate
Halotolerant
Nonhalophile
Example:
Escherichia coli
0
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
5.18 Toxic Forms of Oxygen
• Several toxic forms of oxygen can be formed
in the cell (Figure 5.28):
–
–
–
–
Single oxygen
Superoxide anion
Hydrogen peroxide
Hydroxyl radical
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Figure 5.28
Reactants
Products
(superoxide)
(hydrogen peroxide)
(hydroxyl radical)
(water)
Outcome:
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5.18 Toxic Forms of Oxygen
• Enzymes are present to neutralize most of
these toxic oxygen species (Figure 5.29)
–
–
–
–
Catalase (Figure 5.30)
Peroxidase
Superoxide dismutase
Superoxide reductase
© 2012 Pearson Education, Inc.
Figure 5.29
Catalase
Peroxidase
Superoxide dismutase
Superoxide dismutase/catalase in combination
Superoxide reductase
© 2012 Pearson Education, Inc.
Figure 5.30
© 2012 Pearson Education, Inc.