Chapter 6
Microbial Growth and Metabolism
Microbial Nutrition
Microbial Growth
Metabolism
6 – 1,Microbial Nutrition
Nutrient requirements
Nutritional types of microorganisms
Uptake of Nutrients by the Cell
Culture Medium
Isolation of Pure Cultures
Outline:
Microorganisms require about ten elements in large
quantities,because they are used to construct
carbohydrates,lipids,proteins,and nucleic acids,
Several other elements are needed in very small
amounts and are parts of enzymes and cofactors.
Concepts:
Nutrient
requirements
Macronutrients
95% or more of cell dry weight is made up of a
few major elements,carbon,oxygen,hydrogen,
nitrogen,sulfur,phosphorus,potassium,
calcium,magnesium and iron.
The first six ( C,H,O,N,P and S) are
components of carbonhadrates,lipids,proteins
and nucleic acids
Trace Elements
Microbes require very small amounts of other mineral
elements,such as iron,copper,molybdenum,and zinc;
these are referred to as trace elements,Most are
essential for activity of certain enzymes,usually as
cofactors,
Growth Factors
Amino acids are needed for protein synthesis,
purines and pyrimidines for nucleic acid synthesis,
Vitamins are small organic molecules that usually
make up all or part enzyme cofactors,and only
very small amounts are required for growth,
(1)amino acids,(2) purines and pyrimidines,(3) vitamins
Major nutritional
type
Sources of energy,
hydrogen/electrons,
and carbon
Representative
microorganisms
Photoautotroph
(Photolithotroph)
Light energy,inorganic
hydrogen/electron(H/e-)
donor,CO2 carbon source
Algae,Purple and green
bacteria,Cyanobacteria
Photoheterotroph
(Photoorganotroph)
Light energy,inorganic
H/e- donor,
Organic carbon source
Purple nonsulfur bacteria,
Green sulfur bacteria
Chemoautotroph
(Chemolithotroph)
Chemical energy source
(inorganic),Inorganic H/e-
donor,CO2 carbon source
Sulfur-oxdizing bacteria,
Hydrogen bacteria,
Nitrifying bacteria
Chemoheterotroph
(Chenoorganotroph)
Chemical energy source
(organic),Organic H/e-
donor,Organic carbon
source
Most bacteria,fungi,
protozoa
Nutritional types of microorganisms
Algae,Cyanobacteria
CO2 + H2O Light + Chlorophyll （ CH2O） +O2
Purple and green bacteria
CO2 + 2H2S Light + bacteriochlorophyll（ CH2O） +
H2O + 2S
Purple nonsulfur bacteria (Rhodospirillum)
CO2 + 2CH3CHOHCH3 Light + bacteriochlorophyll （ CH2O）
+ H2O + 2CH3COCH3
Photoautotroph:
Photoheterotroph:
Property cyanobacteria Green and
purple bacteria
Purple nonsulfur
bacteria
Photo - pigment Chlorophyll Bcteriochlorophyll Bcteriochlorophyll
O2 production Yes No No
Electron donors H2O H2,H2S,S H2,H2S,S
Carbon source CO2 CO2 Organic / CO2
Primary products
of energy
conversion
ATP + NADPH ATP ATP
Properties of microbial photosynthetic systems
Chemoautotroph:
Nitrifying bacteria
2 NH4+ + 3 O2 2 NO2- + 2 H2O + 4 H+ + 132 Kcal
Bacteria Electron
donor
Electron
acceptor
Products
Alcaligens and
Pseudomonas sp,H2 O2 H2O
Nitrobacter NO2- O2 NO3-,H2O
Nitrosomonas NH4+ O2 NO2-,H2O
Desulfovibrio H2 SO4 2- H2O,H2S
Thiobacillus denitrificans S0,H2S NO3- SO4 2-,N2
Thiobacillus ferrooxidans Fe2+ O2 Fe3+,H2O
are needed to grow microorganisms in the
laboratory and to carry out specialized
procedures like microbial identification,
water and food analysis,and the isolation
of particular microorganisms,A wide
variety of media is available for these and
other purposes.
Culture media
can be obtained through the use of
spread plates,streak plates,or pour
plates and are required for the careful
study of an individual microbial species.
Pure cultures
Nutrient molecules frequently cannot cross selectively
permeable plasma membranes through passive
diffusion and must be transported by one of three
major mechanisms involving the use of membrane
carrier proteins.
Uptake of nutrients
1,Phagocytosis – Protozoa
2,Permeability absorption – Most microorganisms
passive transport,simple diffusion
facilitated diffusion
active transport
group translocation
A few substances,such as glycerol,can cross the plasma
membrane by passive diffusion,Passive diffusion is the
process in which molecules move from a region of higher
concentration to one of lower concentration as a result of
random thermal agitation,
passive diffusion
The rate of diffusion across selectively permeable membranes
is greatly increased by the use of carrier proteins,sometimes
called permeases,which are embedded in the plasina
membrane,Since the diffusion process is aided by a carrier,it
is called facilitated diffusion,The rate of facilitated diffusion
increases with the concentratioti gradient much more rapidly
and at lower concentrations of the diffusing molecule than that
of passive diffusion
Facilitated diffusion
Active transport is the
transport of solute
molecules to higher
concentrations,or against a
concentration gradient,
with the use of metabolic
energy input.
Active transport
Group translocation
1,Phagocytosis – Protozoa
2,Permeability absorption – Most microorganisms
passive transport,simple diffusion
facilitated diffusion
active transport
group translocation
A few substances,such as glycerol,can cross the
plasma membrane by passive diffusion,Passive
diffusion is the process in which molecules move
from a region of higher concentration to one of
lower concentration as a result of random
thermal agitation,
passive diffusion
The rate of diffusion across selectively permeable membranes
is greatly increased by the use of carrier proteins,sometimes
called permeases,which are embedded in the plasina
membrane,Since the diffusion process is aided by a carrier,it
is called facilitated diffusion,The rate of facilitated diffusion
increases with the concentration gradient much more rapidly
and at lower concentrations of the diffusing molecule than that
of passive diffusion
Facilitated diffusion
The membrane carrier can change
conformation after binding an
external molecule and
subsequently release the molecule
on the cell interior,It then returns
to the outward oriented position
and is ready to bind another solute
molecule.
A model of facilitated diffusion
Because there is no energy input,molecules
will continue to enter only as long as their
concentration is greater on the outside.
Active transport is the
transport of solute
molecules to higher
concentrations,or against a
concentration gradient,
with the use of metabolic
energy input.
Active transport
The best-known group translocation system is the
phosphoenolpyruvate,sugar phosphotransferase system
(PTS),which transports a variety of sugars into procaryotic
cells while Simultaneously phosphorylating them using
phosphoenolpyruvate (PEP) as the phosphate donor.
Group translocation
PEP + sugar (outside) pyruvate + sugar-P (inside)
The phosphoenolpyruvate,sugar phosphotransferase system of E,
coli,The following components are involved in the system,
phosphoenolpyruvate,PEP; enzyme 1,E I; the low molecular
weight heat-stable protein,HPr; enzyme 11,E II,- and enzyme III,
E III.
Items Passive
diffusion
Facilitated
diffusion
Active
transport
Group
translocation
carrier proteins Non Yes Yes Yes
transport speed Slow Rapid Rapid Rapid
against gradient Non Non Yes Yes
transport
molecules
No specificity Specificity Specificity Specificity
metabolic
energy
No need Need Need Need
Solutes
molecules
Not changed Changed Changed Changed
Simple comparison of transport systems
Mode of action of antibacterial antibiotics
Symbiosis of peptidoglycan
6 – 2,Microbial growth
The Growth Curve
Cell life cycle
Measurement of Microbial Growth
Measurement of Cell Mass
Growth Yields and the Effects of a Limiting Nutrient
The Continuous Culture of Microorganisms
The Chemostat
The Influence of Environmental Factors on Growth
Outline
1,Growth is defined as an increase in cellular constituents
and may result in an increase in an organism's size,
population number,or both.
2,When microorganisms are grown in a closed system,
population growth remains exponential for only a few
generations and then enters a stationary phase due to
factors like nutrient limitation and waste accumulation,
If a population is cultured in an open system with
continual nutrient addition and waste removal,the
exponential phase can be maintained for long periods,
Concepts
3,A wide variety of techniques can be used to study
microbial growth by following changes in the total cell
number,the population of viable microorganisms,or the
cell mass.
4,Water availability,pH,temperature,oxygen
concentration,pressure,radiation,and a number of
other environmental factors influence microbial growth,
Yet many microorganisms,and particularly bacteria,
have managed to adapt and flourish under
environmental extremes that would destroy most
organisms.
Growth may be generally defined as a steady
increase in all of the chemical components of an
organism,Growth usually results in an increase
in the size of a cell and frequently results in cell
division
Growth definition:
G1 Primary growth phase of the cell during which cell enlargement occurs,a
gap phase separating cell growth from replication of the genome
S phase in which replication of the genome occurs
G2 Phase in which the cell prepares for separation of the replicated genomes,
this phase includes synthesis of microtubules and condensation of DNA to
form coherent chromosomes,a gap phase separating chromosome replication
from miosis.
M phase called miosis during which the microtubular apparatus is associated
and subsequently used to pull apart the sister chromosomes.
Cell life cycle in Eukaryotic cells
G1 S G2 M
G1 R D
Eukaryotic cell:
Prokaryotic cell:
Most bacterial cells reproduce asexually by binary fision,a
process in which a cell divides to produce two nearly equal-
sized progeny cells,Binary fision involves three processes,
Increase in cell size (cell elongation),
DNA replication
Cell division
Binary fision
Growth curve of bacteria
1,Lag Phase
2,Exponential Phase
3,Stationary Phase
4,Death Phase
Growth curve of bacteria
1,Lag Phase
2,Exponential Phase
3,Stationary Phase
4,Death Phase
(a) Lag phase,cells begin to synthesize inducible enzymes and use stored food
reserves.
(b) Logarithmic growth phase,the rate of multiplication is constant.
(c) Stationary phase,death rate is equal to rate of increase.
(d) Death phase,cells begin to die at a more rapid rate than that of reproduction.
Lag Phase
Stationary Phase
Death Phase
Logarithmic
growth phase
The time required for a cell to divide (and its
population to double) is called the generation time.
generation time
Suppose that a bacterial population increases from103 cells
to 109 cells in 10 hours,Calculate the generation time.
Nu
mb
er
of
cel
ls
Time
Nt = No x 2n
No = number of bacteria at beginning of time interval.
Nt = number of bacteria at end of any interval of time (t).
G = generation time
T = time,usually expressed in minutes
n = number of generation
G = t log2 / log Nt – log No
Chemostat used for continuous cultures,Rate of growth can be
controlled either by controlling the rate at which new medium
enters the growth chamber or by limiting a required growth
factor in the medium,
Continuous culture
of microorganisms
Chemostat
Bacteria grow over a range of temperatures; they do not
reproduce below the minimum growth temperattire nor
above the maximum growth temperature,Within the
temperature growth range there is an optimum growth
temperature at which bacterial reproduction is fastest.
Effect of temperature on bacterial growth rate
Enzymes exhibit a Q10 so that within a suitable
temperature range the rate of enzyme activity doubles for
every 10' C rise in temperature.
Microorganisms are classified as psychrophiles,
mesophiles.thermophiles,and extremethemophiles
based on their optimal growth temperature.
Effect of oxygen concentration – reduction potential
Effect of oxygen concentration on the growth of various
bacteria in tubes of solid medium
(a) Obligate aerobes-growth occurs only in the short
distance to which the oxygen diffuses into the medium,
(b) Facultative anaerobes growth is best near the surface,
where oxygen is available,but occurs throughout the tube,
(c) Obligate anaerobes-oxygen is toxic,and there is no
growth near the surface,
(d) Aerotolerant anaerobes-growth occurs evenly
throughout the tube but is not better at the surface because
the organisms do not use oxygen,
(e) Microaerophiles,aerobic organisms that do not tolerate
atmospheric concentrations of oxygen-growth occurs only
in a narrow band of optimal oxygen concentration.
Effect of pH value on microbial growth
Bacteria,Neutral condition
Fungi,Acidic condition
Actinomycetes,Alkaline condition
Water activity
The water activity of a solution is 1/100 the relative
humidity of the solution (when expressed as a percent),or
it is equivalent to the ratio of the solution's vapor
pressure to that of pure water,
aw = P solution / P water
Approximate lower aw limits for microbial growth:
0.90 – 1.00 for most bacteria,most algae and some fungi as
Basidiomycetes,Mucor,Rhizopus.
0.75 for Halobacterium,Aspergillus…
0.60 for some saccharomyces species
If the concentration of solutes,such as sodium chloride,is
higher in the surrounding medium (hypertonic),then water
tends to leave the cell,The cell membrane shrinks away
from the cell wall (an action called plasmolysis),and cell
growth is inhibited.
Plasmolysis
Normal cell Plasmolyzed cell
Control of microbial growth
Definitions:
Sterilization – the process of destroying all forms of
microbial life on an object or in a material.
Disinfection – the process of destroying vegetative
pathogens but not necessary endospores.
Antisepsis – chemical disinfection of skin,mucous
membranes or other living tissues
Microbial Growth and Metabolism
Microbial Nutrition
Microbial Growth
Metabolism
6 – 1,Microbial Nutrition
Nutrient requirements
Nutritional types of microorganisms
Uptake of Nutrients by the Cell
Culture Medium
Isolation of Pure Cultures
Outline:
Microorganisms require about ten elements in large
quantities,because they are used to construct
carbohydrates,lipids,proteins,and nucleic acids,
Several other elements are needed in very small
amounts and are parts of enzymes and cofactors.
Concepts:
Nutrient
requirements
Macronutrients
95% or more of cell dry weight is made up of a
few major elements,carbon,oxygen,hydrogen,
nitrogen,sulfur,phosphorus,potassium,
calcium,magnesium and iron.
The first six ( C,H,O,N,P and S) are
components of carbonhadrates,lipids,proteins
and nucleic acids
Trace Elements
Microbes require very small amounts of other mineral
elements,such as iron,copper,molybdenum,and zinc;
these are referred to as trace elements,Most are
essential for activity of certain enzymes,usually as
cofactors,
Growth Factors
Amino acids are needed for protein synthesis,
purines and pyrimidines for nucleic acid synthesis,
Vitamins are small organic molecules that usually
make up all or part enzyme cofactors,and only
very small amounts are required for growth,
(1)amino acids,(2) purines and pyrimidines,(3) vitamins
Major nutritional
type
Sources of energy,
hydrogen/electrons,
and carbon
Representative
microorganisms
Photoautotroph
(Photolithotroph)
Light energy,inorganic
hydrogen/electron(H/e-)
donor,CO2 carbon source
Algae,Purple and green
bacteria,Cyanobacteria
Photoheterotroph
(Photoorganotroph)
Light energy,inorganic
H/e- donor,
Organic carbon source
Purple nonsulfur bacteria,
Green sulfur bacteria
Chemoautotroph
(Chemolithotroph)
Chemical energy source
(inorganic),Inorganic H/e-
donor,CO2 carbon source
Sulfur-oxdizing bacteria,
Hydrogen bacteria,
Nitrifying bacteria
Chemoheterotroph
(Chenoorganotroph)
Chemical energy source
(organic),Organic H/e-
donor,Organic carbon
source
Most bacteria,fungi,
protozoa
Nutritional types of microorganisms
Algae,Cyanobacteria
CO2 + H2O Light + Chlorophyll （ CH2O） +O2
Purple and green bacteria
CO2 + 2H2S Light + bacteriochlorophyll（ CH2O） +
H2O + 2S
Purple nonsulfur bacteria (Rhodospirillum)
CO2 + 2CH3CHOHCH3 Light + bacteriochlorophyll （ CH2O）
+ H2O + 2CH3COCH3
Photoautotroph:
Photoheterotroph:
Property cyanobacteria Green and
purple bacteria
Purple nonsulfur
bacteria
Photo - pigment Chlorophyll Bcteriochlorophyll Bcteriochlorophyll
O2 production Yes No No
Electron donors H2O H2,H2S,S H2,H2S,S
Carbon source CO2 CO2 Organic / CO2
Primary products
of energy
conversion
ATP + NADPH ATP ATP
Properties of microbial photosynthetic systems
Chemoautotroph:
Nitrifying bacteria
2 NH4+ + 3 O2 2 NO2- + 2 H2O + 4 H+ + 132 Kcal
Bacteria Electron
donor
Electron
acceptor
Products
Alcaligens and
Pseudomonas sp,H2 O2 H2O
Nitrobacter NO2- O2 NO3-,H2O
Nitrosomonas NH4+ O2 NO2-,H2O
Desulfovibrio H2 SO4 2- H2O,H2S
Thiobacillus denitrificans S0,H2S NO3- SO4 2-,N2
Thiobacillus ferrooxidans Fe2+ O2 Fe3+,H2O
are needed to grow microorganisms in the
laboratory and to carry out specialized
procedures like microbial identification,
water and food analysis,and the isolation
of particular microorganisms,A wide
variety of media is available for these and
other purposes.
Culture media
can be obtained through the use of
spread plates,streak plates,or pour
plates and are required for the careful
study of an individual microbial species.
Pure cultures
Nutrient molecules frequently cannot cross selectively
permeable plasma membranes through passive
diffusion and must be transported by one of three
major mechanisms involving the use of membrane
carrier proteins.
Uptake of nutrients
1,Phagocytosis – Protozoa
2,Permeability absorption – Most microorganisms
passive transport,simple diffusion
facilitated diffusion
active transport
group translocation
A few substances,such as glycerol,can cross the plasma
membrane by passive diffusion,Passive diffusion is the
process in which molecules move from a region of higher
concentration to one of lower concentration as a result of
random thermal agitation,
passive diffusion
The rate of diffusion across selectively permeable membranes
is greatly increased by the use of carrier proteins,sometimes
called permeases,which are embedded in the plasina
membrane,Since the diffusion process is aided by a carrier,it
is called facilitated diffusion,The rate of facilitated diffusion
increases with the concentratioti gradient much more rapidly
and at lower concentrations of the diffusing molecule than that
of passive diffusion
Facilitated diffusion
Active transport is the
transport of solute
molecules to higher
concentrations,or against a
concentration gradient,
with the use of metabolic
energy input.
Active transport
Group translocation
1,Phagocytosis – Protozoa
2,Permeability absorption – Most microorganisms
passive transport,simple diffusion
facilitated diffusion
active transport
group translocation
A few substances,such as glycerol,can cross the
plasma membrane by passive diffusion,Passive
diffusion is the process in which molecules move
from a region of higher concentration to one of
lower concentration as a result of random
thermal agitation,
passive diffusion
The rate of diffusion across selectively permeable membranes
is greatly increased by the use of carrier proteins,sometimes
called permeases,which are embedded in the plasina
membrane,Since the diffusion process is aided by a carrier,it
is called facilitated diffusion,The rate of facilitated diffusion
increases with the concentration gradient much more rapidly
and at lower concentrations of the diffusing molecule than that
of passive diffusion
Facilitated diffusion
The membrane carrier can change
conformation after binding an
external molecule and
subsequently release the molecule
on the cell interior,It then returns
to the outward oriented position
and is ready to bind another solute
molecule.
A model of facilitated diffusion
Because there is no energy input,molecules
will continue to enter only as long as their
concentration is greater on the outside.
Active transport is the
transport of solute
molecules to higher
concentrations,or against a
concentration gradient,
with the use of metabolic
energy input.
Active transport
The best-known group translocation system is the
phosphoenolpyruvate,sugar phosphotransferase system
(PTS),which transports a variety of sugars into procaryotic
cells while Simultaneously phosphorylating them using
phosphoenolpyruvate (PEP) as the phosphate donor.
Group translocation
PEP + sugar (outside) pyruvate + sugar-P (inside)
The phosphoenolpyruvate,sugar phosphotransferase system of E,
coli,The following components are involved in the system,
phosphoenolpyruvate,PEP; enzyme 1,E I; the low molecular
weight heat-stable protein,HPr; enzyme 11,E II,- and enzyme III,
E III.
Items Passive
diffusion
Facilitated
diffusion
Active
transport
Group
translocation
carrier proteins Non Yes Yes Yes
transport speed Slow Rapid Rapid Rapid
against gradient Non Non Yes Yes
transport
molecules
No specificity Specificity Specificity Specificity
metabolic
energy
No need Need Need Need
Solutes
molecules
Not changed Changed Changed Changed
Simple comparison of transport systems
Mode of action of antibacterial antibiotics
Symbiosis of peptidoglycan
6 – 2,Microbial growth
The Growth Curve
Cell life cycle
Measurement of Microbial Growth
Measurement of Cell Mass
Growth Yields and the Effects of a Limiting Nutrient
The Continuous Culture of Microorganisms
The Chemostat
The Influence of Environmental Factors on Growth
Outline
1,Growth is defined as an increase in cellular constituents
and may result in an increase in an organism's size,
population number,or both.
2,When microorganisms are grown in a closed system,
population growth remains exponential for only a few
generations and then enters a stationary phase due to
factors like nutrient limitation and waste accumulation,
If a population is cultured in an open system with
continual nutrient addition and waste removal,the
exponential phase can be maintained for long periods,
Concepts
3,A wide variety of techniques can be used to study
microbial growth by following changes in the total cell
number,the population of viable microorganisms,or the
cell mass.
4,Water availability,pH,temperature,oxygen
concentration,pressure,radiation,and a number of
other environmental factors influence microbial growth,
Yet many microorganisms,and particularly bacteria,
have managed to adapt and flourish under
environmental extremes that would destroy most
organisms.
Growth may be generally defined as a steady
increase in all of the chemical components of an
organism,Growth usually results in an increase
in the size of a cell and frequently results in cell
division
Growth definition:
G1 Primary growth phase of the cell during which cell enlargement occurs,a
gap phase separating cell growth from replication of the genome
S phase in which replication of the genome occurs
G2 Phase in which the cell prepares for separation of the replicated genomes,
this phase includes synthesis of microtubules and condensation of DNA to
form coherent chromosomes,a gap phase separating chromosome replication
from miosis.
M phase called miosis during which the microtubular apparatus is associated
and subsequently used to pull apart the sister chromosomes.
Cell life cycle in Eukaryotic cells
G1 S G2 M
G1 R D
Eukaryotic cell:
Prokaryotic cell:
Most bacterial cells reproduce asexually by binary fision,a
process in which a cell divides to produce two nearly equal-
sized progeny cells,Binary fision involves three processes,
Increase in cell size (cell elongation),
DNA replication
Cell division
Binary fision
Growth curve of bacteria
1,Lag Phase
2,Exponential Phase
3,Stationary Phase
4,Death Phase
Growth curve of bacteria
1,Lag Phase
2,Exponential Phase
3,Stationary Phase
4,Death Phase
(a) Lag phase,cells begin to synthesize inducible enzymes and use stored food
reserves.
(b) Logarithmic growth phase,the rate of multiplication is constant.
(c) Stationary phase,death rate is equal to rate of increase.
(d) Death phase,cells begin to die at a more rapid rate than that of reproduction.
Lag Phase
Stationary Phase
Death Phase
Logarithmic
growth phase
The time required for a cell to divide (and its
population to double) is called the generation time.
generation time
Suppose that a bacterial population increases from103 cells
to 109 cells in 10 hours,Calculate the generation time.
Nu
mb
er
of
cel
ls
Time
Nt = No x 2n
No = number of bacteria at beginning of time interval.
Nt = number of bacteria at end of any interval of time (t).
G = generation time
T = time,usually expressed in minutes
n = number of generation
G = t log2 / log Nt – log No
Chemostat used for continuous cultures,Rate of growth can be
controlled either by controlling the rate at which new medium
enters the growth chamber or by limiting a required growth
factor in the medium,
Continuous culture
of microorganisms
Chemostat
Bacteria grow over a range of temperatures; they do not
reproduce below the minimum growth temperattire nor
above the maximum growth temperature,Within the
temperature growth range there is an optimum growth
temperature at which bacterial reproduction is fastest.
Effect of temperature on bacterial growth rate
Enzymes exhibit a Q10 so that within a suitable
temperature range the rate of enzyme activity doubles for
every 10' C rise in temperature.
Microorganisms are classified as psychrophiles,
mesophiles.thermophiles,and extremethemophiles
based on their optimal growth temperature.
Effect of oxygen concentration – reduction potential
Effect of oxygen concentration on the growth of various
bacteria in tubes of solid medium
(a) Obligate aerobes-growth occurs only in the short
distance to which the oxygen diffuses into the medium,
(b) Facultative anaerobes growth is best near the surface,
where oxygen is available,but occurs throughout the tube,
(c) Obligate anaerobes-oxygen is toxic,and there is no
growth near the surface,
(d) Aerotolerant anaerobes-growth occurs evenly
throughout the tube but is not better at the surface because
the organisms do not use oxygen,
(e) Microaerophiles,aerobic organisms that do not tolerate
atmospheric concentrations of oxygen-growth occurs only
in a narrow band of optimal oxygen concentration.
Effect of pH value on microbial growth
Bacteria,Neutral condition
Fungi,Acidic condition
Actinomycetes,Alkaline condition
Water activity
The water activity of a solution is 1/100 the relative
humidity of the solution (when expressed as a percent),or
it is equivalent to the ratio of the solution's vapor
pressure to that of pure water,
aw = P solution / P water
Approximate lower aw limits for microbial growth:
0.90 – 1.00 for most bacteria,most algae and some fungi as
Basidiomycetes,Mucor,Rhizopus.
0.75 for Halobacterium,Aspergillus…
0.60 for some saccharomyces species
If the concentration of solutes,such as sodium chloride,is
higher in the surrounding medium (hypertonic),then water
tends to leave the cell,The cell membrane shrinks away
from the cell wall (an action called plasmolysis),and cell
growth is inhibited.
Plasmolysis
Normal cell Plasmolyzed cell
Control of microbial growth
Definitions:
Sterilization – the process of destroying all forms of
microbial life on an object or in a material.
Disinfection – the process of destroying vegetative
pathogens but not necessary endospores.
Antisepsis – chemical disinfection of skin,mucous
membranes or other living tissues
