3/20/2022
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MIC2011 Lecture
Microbial Physiology
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Do not remove this notice.
Chris Greening
chris.greening@monash.edu
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Learning outcomes
An important tip before we start…
At the end of this session, you should be able to:
1.
Outline the nutritional requirements for microorganisms
2.
Summarise how cells gain energy from sunlight, organic and inorganic compounds
3.
Differentiate aerobic respiration, anaerobic respiration, and fermentation
4.
Calculate parameters describing growth of microbial populations
5.
Describe how nutritional, physical, and chemical factors influence growth
6.
Categorise microorganisms based on physiological traits
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Many find microbial physiology challenging. One reason for that is that they
overlook the core concepts and instead get bogged down in the details.
For example, being able to remember the complete glycolysis pathway
shows you have a good memory. However, we’d be much more impressed if
you understand what this pathway does and why it is important.
So please focus on understanding concepts, not memorising details. We’ll be
assessing you on how you understand concepts, terminology, and basic
calculations. Not your ability to recall random facts!
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Lecture outline
A few reasons to study microbial physiology
My laboratory studies how microorganisms grow and persist in different environments. We’re using this to
tackle key questions related to planetary health, i.e. bridging environmental and medical microbiology.
•
Infectious disease: How can we reduce the burden of respiratory
and gastrointestinal pathogens in the developing world?
•
Climate change: How do microorganisms control the composition
of the atmosphere, for example by cycling methane gas?
•
Global biodiversity: How is life sustained in different
ecosystems, including extreme environments such as Antarctica?
1. Nutritional requirements
2. Aerobic and anaerobic growth
3. Population dynamics
4. Physical and chemical factors
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Nutritional requirements of microbial cells
Cells acquire nutrients through different strategies
Microorganisms grow by acquiring compounds from the environment and converting them into macromolecules
(e.g. proteins, lipids, carbohydrates, nucleic acids). Together with water, these macromolecules make up most
of the cell’s weight. They are critical for catalysis, transport, signalling, information storage, and cell structure.
Elemental composition of E. coli cell by dry weight
Carbon acquisition
Macromolecular composition of E. coli cell
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Nitrogen acquisition
Other nutrients
Macromolecules are formed from
the condensation of organic carbon
monomers. Autotrophs (primary
producers) fix carbon dioxide into
organic carbon. Heterotrophs
(consumers) recycle organic carbon
released by other organisms.
Nitrogen is a major component of
proteins and nucleic acids.
Diazotrophic bacteria and archaea
fix atmospheric N2 into aqueous
ammonium. Other microbes acquire
nitrogen from other sources (e.g.
ammonium, nitrate, dead biomass).
Microorganisms also have a range
of strategies to acquire other
important building elements,
including phosphorus (nucleic acids,
phospholipids), sulfur (amino acids,
vitamins), and metals (including Fe,
Mg, Na, K, Ca, Mn, Cu, Zn, Ni).
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Cells use ATP to convert nutrients into macromolecules
ATP
HYDROLYSIS
RELEASES FREE
ENERGY
ATP
Phosphoester bond
ADP
(biosynthesis,
motility, active
transport, etc.)
Most microbes make ATP via chemiosmosis
ATP (adenosine triphosphate) serves as the main
store of chemical energy in bacteria, archaea, and
microbial eukaryotes. Its phosphoester bond releases
a large amount of free energy when hydrolysed.
ATP synthase produces most ATP in most microorganisms. This
membrane-bound rotor motor uses the energy released from the
downhill flow of protons to convert ADP into ATP.
There is a steep proton gradient across the cell membrane of
bacteria and archaea (low in cytosol, high in periplasm). The
resultant concentration and charge gradient powers ATP synthesis.
In anabolism, cells use energy released by ATP
hydrolysis to synthesise macromolecules. ATP also
used for other work, e.g. motility and active transport.
Phosphoester bond
ATP
SYNTHESIS
USES FREE
ENERGY
(chemosynthesis,
photosynthesis)
ADP
ADP
Primary pumps generate this proton gradient. They use energy
derived from sunlight (photosynthesis) or organic/inorganic
chemicals (chemosynthesis) to pump protons to the periplasm.
In catabolism, cells use energy from the environment
to make ATP. Two main energy sources: light
(photosynthesis) and chemicals (chemosynthesis).
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Chemiosmosis occurs across three types of membranes
Cell membrane (bacteria and
archaea)
Mitochondrial inner
membrane (eukaryotes)
Thylakoid membrane
(cyanobacteria and algae)
Rhodopsins are a basic example of a proton pump
Photosynthetic and chemosynthetic microorganisms use numerous strategies to generate proton gradients. A
simple example are bacteriorhodopsins of archaea. These 250-residue proteins absorb energy from sunlight
(570 nm) and use the energy to pump protons out of the cell. Mediated by isomerisation of a retinal cofactor.
Note that these membranes are evolutionarily related through
endosymbiosis: mitochondria derived from heterotrophic
Alphaproteobacteria, chloroplasts from Cyanobacteria.
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Protons can be pumped through aerobic respiration
Aerobic bacteria and their mitochondrial descendants instead use the energy released by electron transfer to
pump protons. Electron transfer from reduced compounds (e.g.. NADH and FADH2) to oxidised compounds
(i.e. oxygen) releases much free energy. Through respiration, this energy is harnessed to pump protons.
Periplasm
Cytoplasmic
membrane
Electron transport chain
Cytoplasm
Organic and inorganic donors support aerobic respiration
Bacteria and archaea are much more metabolically flexible than eukaryotes. They can use a wide variety of
organic and inorganic energy sources for aerobic respiration. The same principle applies: electron transfer from
a reduced donor to oxidised acceptor yields energy that is used to pump protons. The enzymes involved differ.
Organic electron donors (organotrophy)
Inorganic electron donors (lithotrophy)
Carbohydrates (e.g. glucose)
Lipids (e.g. fatty acids)
Proteins
Lignin
Methane (CH4)
Hydrocarbons (e.g. benzene)
Xenobiotics (e.g. pesticides)
…and many more
Hydrogen gas (H2)
Carbon monoxide (CO)
Formate (HCO3-)
Ammonia (NH3)
Nitrite (NO2-)
Sulfide (S2-)
Iron (Fe2+)
…and many more
ATP synthase
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Microorganisms are categorised by nutritional preference
Classification 1 = Energy source
Phototroph = sunlight
Chemotroph = chemicals
Nostoc cyanobacteria:
Escherichia coli:
A photolithoautotroph adapted
for life in soil and freshwaters
A chemoorganoheterotroph
adapted for life in the human gut
Classification 1 = Energy source
Phototroph = sunlight
Chemotroph = chemicals
Classification 2 = Electron donor
Classification 2 = Electron donor
Lithotroph = inorganic compounds
Organotroph = organic compounds
Lithotroph = inorganic compounds
Organotroph = organic compounds
Classification 3 = Carbon source
Classification 3 = Carbon source
Autotroph = carbon dioxide
Heterotroph = organic carbon
In “Activity 1: Classify weird and wonderful
microorganisms”, you will classify bacteria, archaea,
and microbial eukaryotes based on their physiological
traits and ecological strategies
Autotroph = carbon dioxide
Heterotroph = organic carbon
Note that many microorganisms, for example purple sulfur bacteria (e.g.
Chromatium), can combine multiple strategies (mixotrophs).
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Nutritional requirements: summary
•
All microorganisms require energy, carbon, nitrogen, sulfur, phosphorus, various
metals, and water to grow. They acquire these elements in different ways.
•
Microorganisms are metabolically diverse. They can use light (photosynthesis)
and diverse organic/inorganic compounds (chemosynthesis) as energy sources.
•
Most microorganisms convert light and chemical energy into a proton gradient
across membranes. This gradient powers ATP production by ATP synthase.
•
Microorganisms can be categorised based on nutritional strategies, namely
energy source, electron donor, and carbon source.
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We’ll explore physiological classification in the workshop
Lecture outline
1. Nutritional requirements
2. Aerobic and anaerobic growth
3. Population dynamics
4. Physical and chemical factors
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Microorganisms vary in need for, and tolerance of, oxygen
Microorganisms vary in need for, and tolerance of, oxygen
Oxygen is essential for the life of humans and other vertebrates.
However, many microorganisms don’t require oxygen and some
are poisoned by it.
Let’s not forget microorganisms first evolved in anoxic
environments and then cyanobacteria oxygenated our atmosphere!
This is beautifully shown by Winogradsky columns. Easy to make
at home using water, mud, and a container (e.g. coke bottle).
Detailed explanation: https://media.hhmi.org/biointeractive/click/winogradsky/
How to guide: https://www.arcgis.com/apps/MapJournal/index.html?appid=e9856f7794734b7689ae5c619ac35366
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Microbes can be categorised by oxygen requirements
Obligate aerobes, for example many heterotrophic and ironoxidising bacteria, require oxygen to grow. Like ourselves, they use
aerobic respiration to grow. They thrive in environments such as
soils, freshwater, and marine waters.
How do aerobic and anaerobic respiration differ?
In anaerobic respiration, electrons from organic or inorganic compounds are transferred to electron acceptors
other than oxygen. Again, the same principle applies: electron transfer from a reduced donor to oxidised
acceptor yields energy that is used to pump protons. The resultant proton gradient powers ATP synthase.
Obligate anaerobes, for example sulfate-reducing bacteria,
methane-producing archaea, and green sulfur bacteria, grow only
in the absence of oxygen. They use anaerobic respiration and/or
fermentation to grow. They thrive in environments such as
gastrointestinal tracts, aquatic sediments, and the subsurface.
Aerobic respiration
Anaerobic respiration
Downhill electron flow to the electron acceptor
oxygen leads to proton pumping
Downhill electron flow to an anaerobic electron
acceptor (e.g. nitrate) leads to proton pumping
Facultative anaerobes, including E. coli and many pathogens, are
flexible organisms that grow by aerobic and anaerobic respiration.
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Some major electron acceptors of microorganisms
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Acceptor
Endproduct
Organism
Environmental example
Oxygen (O2)
Water (H2O)
Aerobes
Rhizobium
Iron (Fe3+)
Iron (Fe2+)
Iron reducers
Geobacter
Nitrate (NO3-)
Nitrite (NO2-)
Denitrifiers
Paracoccus
Sulfate (SO42-)
Sulfide (S2-)
Sulfate reducers
Desulfovibrio
Carbon dioxide (CO2)
Methane (CH4)
Methanogens
Methanobacterium (archaea)
Carbon dioxide (CO2)
Acetate (CH3COO-)
Acetogens
Acetobacterium
A golden rule in microbial physiology
If a reaction is thermodynamically
possible, there’ll be a
microorganism that does it
Microorganisms have evolved to use respiratory electron donors and electron acceptors in essentially all
possible combinations. Electron transfer from donor to acceptor must yield energy to pump protons with.
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Redox potentials determine which reactions are possible
Half-reaction
2
H+
Redox potential (Eo′)
/ H2
Strongest
e- donors
-420 mV
We’ll explore redox potentials a little more in the workshop
Electron transfer is thermodynamically favourable if an
electron donor with a lower redox potential (e.g. H2 or
NADH) reacts with an electron acceptor with a higher
redox potential (e.g. NO3- or O2). Example:
Half-reaction
2
H+
/ H2
In “Activity 2: What is and isn’t thermodynamically
possible?”, you will use this table of redox potentials to
determine which respiratory reactions are possible.
-420 mV
NAD+ / NADH
-320 mV
NAD+ / NADH
-320 mV
CO2 / CH4
-240 mV
Aerobic hydrogen oxidation: H2 + O2 → H2O
CO2 / CH4
-240 mV
FAD / FADH2
-220 mV
Redox change (ΔEo′): +820 mV – -420 mV = +1140 mV
FAD / FADH2
-220 mV
SO32- / S2-
-170 mV
Free energy released (ΔGo′): -237 kJ mol-1
Fe3+ / Fe2+
+200 mV
NO3- / NO2-
+420 mV
½ O 2 / H 2O
+820 mV
Less ATP is produced by anaerobic respiration than
aerobic respiration given a lower amount of free energy is
released. This reflects that O2 has the highest redox
potential of respiratory electron acceptors.
Strongest
e- donors
Redox potential (Eo′)
SO32- / S2-
-170 mV
Fe3+ / Fe2+
+200 mV
NO3- / NO2-
+420 mV
½ O 2 / H 2O
+820 mV
Strongest eacceptors
Strongest eacceptors
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Fermentation results in incomplete organic carbon oxidation
Many organotrophs conserve energy through fermentation.
This results in incomplete organic carbon oxidation.
Endproducts are excreted, e.g. ethanol (e.g. Saccharomyces),
lactate (e.g. Lactobacillus), and mixed acids (e.g. E. coli).
In fermentation, all ATP is generated by substrate-level
phosphorylation. This is primarily through the glycolysis
pathway. In this process, enzymes directly transfer phosphoryl
groups from an activated substrate to ADP.
Phosphoenolpyruvate
Fermentation yields much less ATP than respiration
Glucose
Pyruvate
Glycolysis
investment
2 ATP
2 ADP
Fructose 1,6bisphosphate
Glycolysis
payoff
Pyruvate
kinase
2 Pyruvate
Substrate-level
phosphorylation
2 NADH
2 NADH
+ 6 O2
+ 30 ADP
2 NAD+
Some microorganisms are facultative fermenters that survive
by fermentation when respiratory electron acceptors are
unavailable (e.g. E. coli). Others are obligate fermenters that
grow by this strategy, for example many clostridia.
Lactate
fermentation
2 Lactate
(2 ATP)
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4 ADP + 2 NAD+
4 ATP + 2 NADH
Aerobic
respiration
2 NADH
2 NAD+
2 NAD+ +
+ 30 ATP
6 CO2 + 6 H2O
(32 ATP, lower for anaerobic respiration)
Ethanol
fermentation
2 Ethanol + 2 CO2
(2 ATP)
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Pathogens use various energy conservation strategies
Human pathogens are primarily chemoorganoheterotrophs, i.e. they use host-derived organic carbon as their
primary energy, electron, and carbon sources. However, they vary in their energy conservation strategies. Most
pathogens that colonise anoxic niches, such as the intestines, are facultative or obligate anaerobes.
Obligate aerobes:
Facultative anaerobes:
Obligate anaerobes:
Mycobacterium tuberculosis
Helicobacter pylori
Corynebacterium diphtheriae
Legionella pneumophila
Plasmodium falciparum (protist)
Escherichia coli
Salmonella enterica
Staphylococcus aureus
Streptococcus pneumoniae
Candida albicans (fungus)
Clostridiodes difficile
Clostridium perfringens
Fusobacterium nucleatum
Bacteroides fragilis
Trichomonas vaginalis (protist)
Metabolically flexible microorganisms optimise ATP yield
Metabolically flexible microorganisms, such as E. coli, can switch between several energy conservation
strategies. E. coli prioritises aerobic respiration if oxygen is available in the environment, then nitrate respiration
if nitrate is available. As a last resort, it persists by fermentation if no respiratory electron acceptors are available.
Eo’ of electron
acceptor (mV)
ΔG0’ (kJ mol per
mole glucose
Max ATP yield per
mol glucose
Aerobic respiration
+820
-2800
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Nitrate respiration
+430
-860
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Mixed-acid fermentation
-412
-218
3
No growth
Process
Doubling time
(minutes)
This allows the bacterium to achieve the highest ATP yield and fastest growth rate depending on environmental
conditions. The bacterium achieves hierarchical control by sensing electron acceptors in the environment (e.g.
oxygen via FNR) and activating/repressing appropriate enzymes involved in respiration/fermentation.
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Lecture outline
Aerobic and anaerobic growth: summary
•
Microorganisms differ in their oxygen preferences. Whereas some grow by
aerobic respiration, others grow through anaerobic respiration and fermentation.
•
In respiration, energy from electron transfer (from a donor to an acceptor) is
used to pump protons across membranes. Much ATP is made by ATP synthase.
•
In fermentation, organic compounds are incompletely oxidised. This results in
production of a small amount of ATP by substrate-level phosphorylation.
•
The diverse metabolism of microorganisms has tremendous ecological,
environmental, and industrial significance. We will explore in Weeks 10 & 11.
1. Nutritional requirements
2. Aerobic and anaerobic growth
3. Population dynamics
4. Physical and chemical influences
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Two key proteins control E. coli cell division
Bacteria and archaea divide by binary fission
Rod-shaped bacteria and archaea divide by binary fission. In
this process, cells elongate to approximately twice their
normal length, concurrent with the replication of DNA and
synthesis of other macromolecules. The cells are then
partitioned into two equal daughter cells.
An actin-like protein promotes cell elongation
A tubulin-like protein promotes fission
MreB, a protein homologous to eukaryotic actin, forms a simple
cytoskeleton around E. coli cells. This maintains their size and
shape. This protein promotes cell wall synthesis and
chromosome separation during growth.
FtsZ, a protein homologous to eukaryotic tubulin, polymerises
into a ring around the centre of the cell following elongation. The
constriction of this ring through depolymerisation triggers
partitioning into two daughter cells (cytokinesis).
In microbial eukaryotes, cell division occurs through mitosis
(asexual) or meiosis (sexual). While mitosis shares some
common principles with binary fission, this process is more
complicated and involves spindle formation.
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Microbial populations grow exponentially
Four common methods to measure microbial growth
Microorganisms grow exponentially through binary fission (bacteria and archaea) and mitosis (eukaryotes).
Thus, the number of cells present in a population doubles with every generation when conditions are
favourable. Generation times vary depending on the microbial species and the environmental conditions.
Population growth for a bacterium with a
generation time of 30 minutes
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Optical density measurements
Plate colony counting
Spectrophotometers measure the amount of light that passes
through a solution. Microbial cells cause turbidity that prevents
passage of light. The more cells, the higher the OD.
Microorganisms can be serially diluted and spotted on to agar
plates. This measures number of viable cells. Spot plates and
pour plates can both be used.
Microorganisms greatly vary in
generation times
Microorganism
Generation time (h)
Escherichia coli
0.28
Streptococcus lactis
0.43
Staphylococcus aureus
0.5
Clostridium botulinum
0.58
Rhizobium japonicum
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Microscopy counting
PCR-based
Mycobacterium tuberculosis
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Treponema pallidum
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Methanobrevibacter curvatus
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The number of microorganisms on a microscope cover slip can
be directly counted. Commonly performed for microbial
eukaryotes, but unreliable for bacteria.
Quantitative PCR can measure the number of DNA copies of a
particular microorganism. This is particularly useful in mixed
cultures or environmental samples.
Saccharomyces cerevisiae
2
Giardia lambia
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Four major phases of microbial growth
Growth is generally best visualised on a logarithmic scale
Batch growth on linear scale
Lag phase:
Cells prepare for growth
(e.g. enzyme synthesis). No
duplication occurs despite
high nutrient supply.
Exponential phase:
Cells grow rapidly by
binary fission and consume
available nutrients. Most
cells are live.
Stationary phase:
Cell numbers peak as
nutrients become scarce.
Number of live cells equal
to number of dead cells.
Batch growth on logarithmic scale
Death phase:
Cell numbers decline and
more dead cells than live.
Nutrients exhausted, acids
and wastes increase.
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Calculation of generation time of microorganisms
We’ll explore growth parameters more in the workshop
The generation time of a microbial culture can be readily
determined based on a growth curve visualised on a
logarithmic scale. You simply measure the time taken for
the population to double during exponential phase.
In “Activity 3: Growth parameters in a changing
environment”, you will calculate microbial growth
parameters and relate findings to the energetics of
aerobic and anaerobic growth.
Time at OD600 0.25 = 23.3 h
Time at OD600 0.50 = 27.0 h
Generation time = 27.0 – 23.3 = 3.7 h
Specific growth rate = 1/3.7 = 0.27 h-1
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Growth ceases when cells become limited for a nutrient
Many bacteria persist in stress-resistant dormant states
Microorganisms stop growing if they are limited for a nutrient or other compound they require for growth (e.g.
carbon, nitrogen, oxygen, metals). For example, the obligate aerobe and chemoorganoheterotroph
Mycobacterium stops growing when limited for either organic carbon or oxygen.
In natural environments, few microorganisms are actively growing. Instead, most exist in some sort of
dormant states, where they expend energy exclusively for maintenance processes (e.g. repair) rather than
growth. Dormant cells and spores are extraordinarily stress-resistant and can often survive for years.
Growth yield = OD600 ~3.0
Aerated cultures stop growing because
cells run out of organic carbon
Growth yield = OD600 ~1.0
Sealed cultures stop growing because
cells run out of oxygen
Fungal
exospores
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Cyanobacterial
filaments
Mycobacterial
persister cells
Mixed bacterial
biofilm
Bacilli
endospores
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Population dynamics: summary
•
Bacterial and archaeal cells grow by binary fission. This depends on cytoskeletal
organisation by actin-like proteins and cytokinesis by tubulin-like proteins.
•
Microorganisms grow exponentially when optimal resources are available to
them. They will become stationary when these resources are exhausted.
•
There are two key microbial growth parameters: specific growth rate (inverse of
generation time) and maximum growth yield (total biomass produced).
•
Many microorganisms enter dormant states following resource limitation. Many
of these states, for example spores and biofilms, are highly stress-resistant.
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Lecture outline
1. Nutritional requirements
2. Aerobic and anaerobic growth
3. Population dynamics
4. Physical and chemical influences
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Physical and chemical factors also influence growth
Growth is a temperature-dependent performance trait
Microorganisms can be detected in nearly all environments on earth, from the upper atmosphere through to the
deep subsurface. In each of these environments, they’re specifically adapted to a broad range of physical,
chemical, and biological factors that influence growth and survival. These include:
Microbial growth varies in a temperature-dependent manner in common with most biological performance
traits. At temperatures below the optimum, growth is limited by rates of transport processes or enzyme activity.
At temperatures above the optimum, growth rapidly ceases due to protein denaturation and membrane lysis.
Nutrient availability
Oxygen concentration
Temperature
Moisture content
pH
Salinity
Pressure
Interspecies interactions
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Some microorganisms grow at extreme temperatures
Animals and plants only grow up to moderate temperatures, with none able to grow above 50°C. In contrast,
microorganisms vary in their temperature preferences. While many are also adapted to moderate temperatures
(mesophiles), others can grow at very cold (psychophiles) or very high (thermophile) temperatures. Bacteria
and archaea have been isolated that can grow up to 95°C and 122°C respectively!
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Extremophiles have distinct macromolecular characteristics
Optimal temperature
Protein content
Membrane content
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Adaptations of psychrophiles
Adaptations of thermophiles
<15°C (e.g. polar deserts, alpine, deep sea)
>40°C (e.g. hot deserts, hot springs, vents)
More fluid (more hydrophobic residues)
More rigid (more polar residues, disulfide bonds)
More fluid (short and unsaturated fatty acids)
More rigid (long and saturated fatty acids)
DNA content
More AT bases (two hydrogen bonds)
More GC bases (three hydrogen bonds)
Chaperones
Cryoprotectant solutes to reduce ice formation
Heat-shock proteins to reduce denaturation
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Amazing examples at each extreme of life
The hyperthermophilic methane-producing archaeon
Methanopyrus grows up to 122°C. It was isolated from deepsea hydrothermal vents (black smokers) like these.
Microbial growth is sensitive to osmotic effects
Water is the solvent for microbial activity and the electron donor for
oxygenic photosynthesis. Water diffuses from regions of low to high
solute concentration (high to low water concentration) via osmosis.
Too much water causes swelling, too little causes desiccation.
Microorganisms regulate osmosis to maintain optimal water content.
For example, they can increase water uptake by synthesizing
internal solutes (e.g. sugars, glycerol).
Microorganisms rapidly lose water through osmosis when placed in
environments where water availability is very low or salt, sugar, or
other solutes are very high. For these reasons, drying, salting, and
sugaring are very effective the food preservation approaches.
The psychrophilic lichen Xanthorian is able to photosynthesise
at temperatures below -24°C. It can colonise rocks throughout
continental Antarctica.
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Osmotic adaptations of microorganisms
pH strongly affects microbial growth
All microorganisms have an optimal and often narrow range of external pH values that they can grow in. While
microorganisms grow at a range of external pHs, almost all of them maintain their cytoplasmic pH in a narrow
range (pH 7.3 to 7.8). This depends on mechanisms such as cytoplasmic buffering and proton active transport.
Many bacteria, archaea, and fungi (pictured) can survive in environments with low water (xerophiles) or high salt
(halophiles). These organisms all synthesise compatible solutes, such as trehalose, glycine betaine, and glycerol.
These serve two roles: (i) promoting osmosis into the cell and (ii) replacing water as a solvent for macromolecules.
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Time to marvel at some other extremophiles…
Acidophiles grow optimally at pH below 5.5
These include Acidithiobacillus (growth at pH 1) in
acid mine drainage (pictured: Rio Tinto River, Spain)
Cultivation differentiates microbes by physiological traits
Microorganisms can be cultivated on solid (agar) medium based on physiological characteristics such as
nutritional, oxygen, temperature, pH, and salt preferences. Selective media promotes growth of some
microorganisms over others. Differential media contains ingredients that allows potential identification.
Alkaliphiles grow optimally at pH above 8.5
These include Natronomonas archaea (growth at pH 11)
in soda lakes (pictured: Mono Lake, California)
Mannitol salt agar
Selective: High salt selects for the halotolerant genus
Staphylococcus and inhibits most other bacteria (e.g. E. coli)
Differential: Mannitol is metabolised by pathogen S. aureus
(yellow colonies), but not commensal S. epidermidis (red colonies)
S. aureus
S. epidermis
E. coli (no growth)
MacConkey agar
Other extremophiles can withstand high pressures, radiation, and even space vacuums. Polyextremophiles
tolerate multiple pressures at once, e.g. Antarctic bacteria must tolerate both cold and dry conditions.
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Selective: Contains bile acids that are toxic to Gram-positives
(e.g. S. aureus), but expelled by Gram-negatives (e.g. E. coli)
Differential: Contains lactose that is used by E. coli and others
(pink colonies), but not by S. enterica and others (yellow colonies)
E. coli
Salmonella
S. aureus (no growth)
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Physical and chemical factors: summary
Check your understanding
•
Microorganisms grow and survive by adapting to local environmental conditions,
including temperature, moisture, pH, salinity, and nutrient availability.
Check your understanding against the learning objectives:
•
Microorganisms use molecular homeostatic mechanisms to maintain cellular
viability in response to changes in temperature, water content, and pH.
1.
Outline the nutritional requirements for microorganisms
2.
Summarise how cells gain energy from sunlight, organic and inorganic compounds
Extremophiles grow in conditions incompatible with multicellular life, for example
high temperature or acidic pH. These include many bacteria, archaea, and fungi.
3.
Differentiate aerobic respiration, anaerobic respiration, and fermentation
4.
Calculate parameters describing growth of microbial populations
Microorganisms are selected in natural ecosystems and enriched in selective
media when physical and chemical conditions match their growth preferences.
5.
Describe how nutritional, physical, and chemical factors influence growth
6.
Categorise microorganisms based on physiological traits
•
•
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