Microbiology: A Systems
Approach, 2nd ed.
Chapter 7: Microbial Nutrition,
Ecology, and Growth
7.1 Microbial Nutrition
• Nutrition: a process by which chemical substances (nutrients) are
acquired from the environment and used in cellular activities
• All living things require a source of elements such as C, H, O, P, K, N,
S, Ca, Fe, Na, Cl, Mg- but the relative amounts vary depending on
the microbe
• Essential Nutrient: any substances that must be provided to an
organism
– Macronutrients: Required in relatively large quantities, play principal
roles in cell structure and metabolism (ex. C, H, O)
– Micronutrients: aka trace elements, present in smaller amounts and
involved in enzyme function and maintenance of protein structure (ex.
Mn, Zn, Ni)
• Nutrients are processed and transformed into the chemicals of the
cell after absorption
• Can also categorize nutrients according to C content
– Inorganic nutrients: A combination of atoms other than C and H
– Organic nutrients: Contain C and H, usually the products of living
things
Chemical Analysis of Microbial
Cytoplasm
Sources of Essential Nutrients
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Carbon sources
Nitrogen sources
Oxygen sources
Hydrogen sources
Phosphorus sources
Sulfur sources
Others
Carbon Sources
• The majority of C compounds involved in
normal structure and metabolism of all cells
are organic
• Heterotroph: Must obtain C in organic form
(nutritionally dependent on other living
things)
• Autotroph: Uses inorganic CO2 as its carbon
source (not nutritionally dependent on other
living things)
Nitrogen Sources
• Main reservoir- N2
• Primary nitrogen source for heterotrophsproteins, DNA, RNA
• Some bacteria and algae utilize inorganic
nitrogenous nutrients
• Small number can transform N2 into usable
compounds through nitrogen fixation
• Regardless of the initial form, must be converted
to NH3 (the only form that can be directly
combined with C to synthesize amino acids and
other compounds)
Oxygen Sources
• O is a major component of organic
compounds
• Also a common component of inorganic salts
• O2 makes up 20% of the atmosphere
Hydrogen Sources
• H is a major element in all organic and several
inorganic compounds
• Performs overlapping roles in the
biochemistry of cells:
– Maintaining pH
– Forming hydrogen bonds between molecules
– Serving as the source of free energy in oxidationreduction reactions of respiration
Phosphorus (Phosphate) Sources
• Main inorganic source of phosphorus is
phosphate (PO43-)
– Derived from phosphoric acid
– Found in rocks and oceanic mineral deposits
• Key component of all nucleotides
• Phospholipids in cell membranes
• Coenzymes
Sulfur Sources
• Widely distributed throughout the
environment in mineral form
• Essential component of some vitamins
• Amino acids- methionine and cysteine
Other Nutrients Important in microbial
Metabolism
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Potassium- protein synthesis and membrane function
Sodium- certain types of cell transport
Calcium- stabilizer of cell walls and endospores
Magnesium- component of chlorophyll and stabilizer of
membranes and ribosomes
• Iron- important component of cytochrome proteins
• Zinc- essential regulatory element for eukaryotic
genetics, and binding factors for enzymes
• Copper, cobalt, nickel, molybdenum, manganese,
silicon, iodine, and boron- needed in small amounts by
some microbes but not others
Growth Factors: Essential Organic
Nutrients
• Growth factor: An organic compound such as
an amino acid, nitrogenous base, or vitamin
that cannot be synthesized by an organism
and must be provided as a nutrient
• For example, many cells cannot synthesize all
20 amino acids so they must obtain them from
food (essential amino acids)
How Microbes Feed: Nutritional Types
Main Determinants of Nutritional Type
• Sources of carbon and energy
• Phototrophs- Microbes that photosynthesize
• Chemotrophs- Microbes that gain energy from
chemical compounds
Autotrophs and Their Energy Sources
• Photoautotrophs
– Photosynthetic
– Form the basis for most food webs
• Chemoautotrophs
– Chemoorganic autotrophs- use organic compounds for
energy and inorganic compounds as a carbon source
– Lithoautotrophs- rely totally on inorganic minerals
– Methanogens- produce methane from hydrogen gas
and carbon dioxide
• Archaea
• Some live in extreme habitats
Figure 7.1
Heterotrophs and Their Energy
Sources
– Majority are chemoheterotrophs that derive
both carbon and energy from organic compounds
• Saprobes
– Free-living microorganisms
– Feed primarily on organic detritus from dead organisms
– Decomposers of plant litter, animal matter, and dead
microbes
– Most have rigid cell wall, so they release enzymes to the
extracellular environment and digest food particles into
smaller molecules
» Obligate saprobes- exist strictly on dead organic matter in soil and
water
» Facultative parasite- when a saprobe infects a host, usually when
the host is compromised (opportunistic pathogen)
Figure 7.2
Other Chemoheterotrophs
• Parasites
– Derive nutrients from the cells or tissues of a host
– Also called pathogens because they cause damage
to tissues or even death
– Ectoparasites- live on the body
– Endoparasites- live in organs and tissues
– Intracellular parasites- live within cells
– Obligate parasites- unable to grow outside of a
living host
Transport Mechanisms for Nutrient
Absorption
• Cells must take nutrients in and transport
waste out
• Transport occurs across the cell membrane,
even in organisms with cell walls
The Movement of Water: Osmosis
• Osmosis: Diffusion of water through a selectively
permeable membrane
• The membrane is selectively permeable- having
passageways that allow free diffusion of water
but can block certain other dissolved molecules
• When the membrane is between solutions of
differing concentrations and the solute is not
diffusible, water will diffuse at a fast rate from
the side that has more water to the side that has
less water
Figure 7.3
Osmotic Relationships
• The osmotic relationship between cells and their environment is
determined by the relative concentrations of the solutions on either side
of the cell membrane
• Isotonic: The environment is equal in solute concentration to the cell’s
internal environment
– No net change in cell volume
– Generally the most stable environment for cells
• Hypotonic: The solute concentration of the external environment is lower
than that of the cell’s internal environment
– Net direction of osmosis is from the hypotonic solution into the cell
– Cells without cell walls swell and can burst
• Hypertonic: The environment has a higher solute concentration than the
cytoplasm
– Will force water to diffuse out of a cell
– Said to have high osmotic pressure
Figure 7.4
Adaptations to Osmotic Variations in
the Environment
• Example: fresh pond water- hypotonic
conditions
– Bacteria- cell wall protects them from bursting
– Amoeba- a water (or contractile) vacuole that
moves excess water out of the cell
• Example: high-salt environment- hypertonic
conditions
– Halobacteria living in the Great Salt Lake- absorb
salt to make their cells isotonic with the
environment
The Movement of Molecules:
Diffusion and Transport
• Diffusion: When atoms or molecules move in a gradient from
an area of higher density or concentration to an area of lower
density or concentration
– Random thermal movement of molecules will eventually
distribute the molecules from an area of higher
concentration to an area of lower concentration
– Evenly distributes the molecules
– Diffusion of molecules across the cell membrane is largely
determined by the concentration gradient and
permeability of the substance
– Simple or passive diffusion is limited to small nonpolar
molecules or lipid soluble molecules
Figure 7.5
Facilitated Diffusion
• Utilizes a carrier protein that binds a specific
substance, changes the conformation of the carrier
protein, and the substance is moved across the
membrane
• Once the substance is transported, the carrier protein
resumes its original shape
• Carrier proteins exhibit specificity
• Saturation: The rate of facilitated diffusion of a
substance is limited by the number of binding sites on
the transport proteins
• Competition: When two molecules of similar shape
can bind to the same binding site on a carrier protein
Figure 7.6
Active Transport
• Nutrients are transported against the diffusion gradient or
in the same direction as the natural gradient but at a rate
faster than by diffusion alone
• Requires the presence of specific membrane proteins
(permeases and pumps)
• Requires the expenditure of energy
• Items that require active transport: monosaccharides,
amino acids, organic acids, phosphates, and metal ions
• Specialized pumps- an important type of active transport
• Group translocation: couples the transport of a nutrient
with its conversion to a substance that is immediately
useful inside the cell
Figure 7.7
Endocytosis: Eating and Drinking by
Cells
• A form of active transport
• Transporting large molecules, particles, lipids, or other
cells
• Occurs in some eukaryotic cells
• The cell encloses the substance in its cell membrane,
simultaneously forming a vacuole and engulfing it
• Phagocytosis- amoebas and certain white blood cells;
ingesting whole cells or large solid matter
• Pinocytosis- Transport of liquids such as oils or
molecules in solution
7.2 Environmental Factors that
Influence Microbes
• Temperature Adaptations
– Microbial cells cannot control their temperature, so they assume the
ambient temperature of their natural habitat
– The range of temperatures for the growth of a given microbial species
can be expressed as three cardinal temperatures:
• Minimum temperature: the lowest temperature that permits a microbe’s
continued growth and metabolism
• Maximum temperature: The highest temperature at which growth and
metabolism can proceed
• Optimum temperature: A small range, intermediate between the minimum
and maximum, which promotes the fast rate of growth and metabolism
– Some microbes have a narrow cardinal range while others have a
broad one
– Another way to express temperature adaptation- to describe whether
an organism grows optimally in a cold, moderate, or hot temperature
range
Psychrophile
• A microorganism that has an optimum
temperature below 15°C and is capable of
growth at 0°C.
• True psychrophiles are obligate with respect to
cold and cannot grow above 20°C.
• Psychrotrophs or facultative psychrophilesgrow slowly in cold but have an optimum
temperature above 20°C.
Figure 7.8
Figure 7.9
Mesophile
• An organism that grows at intermediate
temperatures
• Optimum growth temperature of most: 20°C
to 40°C
• Temperate, subtropical, and tropical regions
• Most human pathogens have optima between
30°C and 40°C
Thermophile
• A microbe that grows optimally at
temperatures greater than 45°C
• Vary in heat requirements
• General range of growth of 45°C to 80°C
• Hyperthermophiles- grow between 80°C and
120°C
Gas Requirements
• Atmospheric gases that most influence microbial
growth- O2 and CO2
• Oxygen gas has the greatest impact on microbial
growth
• As oxygen enters into cellular reactions, it is
transformed into several toxic products
– Most cells have developed enzymes that go about
scavenging and neutralizing these chemicals
• Superoxide dismutase
• Catalase
– Essential for aerobic organisms
Several General Categories of Oxygen
Requirements
• Aerobe: can use gaseous oxygen in its metabolism and possesses
the enzymes needed to process toxic oxygen products
• Obligate aerobe: cannot grow without oxygen
• Facultative anaerobe: an aerobe that does not require oxygen for
its metabolism and is capable of growth in the absence of it
• Microaerophile: does not grow at normal atmospheric
concentrations of oxygen but requires a small amount of it in
metabolism
• Anaerobe: lacks the metabolic enzyme systems for using oxygen in
respiration
• Strict, or obligate, anaerobes: also lack the enzymes for processing
toxic oxygen and cannot tolerate any free oxygen in the immediate
environment and will die if exposed to it.
• Aerotolerant anaerobes: do not utilize oxygen but can survive and
grow to a limited extent in its presence
Figure 7.10
Figure 7.11
Carbon Dioxide
• All microbes require some carbon dioxide in
their metabolism
• Capnophiles grow best at a higher CO2 tension
than is normally present in the atmosphere
Effects of pH
• Majority of organisms live or grow in habitats
between pH 6 and 8
• Obligate acidophiles
– Euglena mutabilis- alga that grows between 0 and
1.0 pH
– Thermoplasma- archaea that lives in hot coal piles
at a pH of 1 to 2, and would lyse if exposed to pH
7
Osmotic Pressure
• Most microbes live either under hypotonic or
isotonic conditions
• Osmophiles- live in habitats with a high solute
concentration
• Halophiles- prefer high concentrations of salt
• Obligate halophiles- grow optimally in
solutions of 25% NaCl but require at least 9%
NaCl for growth
Miscellaneous Environmental Factors
• Nonphotosynthetic microbes tend to be damaged by the
toxic oxygen products produced by contact with light
– Some produce yellow carotenoid pigments to protect against
the damaging effects of light by absorbing and dismantling toxic
oxygen
• Other types of radiation that can damage microbes are
ultraviolet and ionizing rays
• Barophiles: deep-sea microbes that exist under hydrostatic
pressures ranging from a few times to over 1,000 times the
pressure of the atmosphere
• All cells require water- only dormant, dehydrated cell
stages tolerate extreme drying
Ecological Associations Among
Microorganisms
• Most microbes live in shared habitats
• Interactions can have beneficial, harmful, or
no particular effects on the organisms
involved
• They can be obligatory or nonobligatory to the
members
• They often involve nutritional interactions
Symbiosis
• A general term used to denote a situation in which two
organisms live together in a close partnership
– Members are termed symbionts
– Three main types of symbionts
• Mutualism: when organisms live in an obligatory but mutually
beneficial relationship
• Commensalism: the member called the commensal receives
benefits, while its coinhabitant is neither harmed nor benefited
– Satellitism: when one member provides nutritional or protective factors
needed by the other
• Parasitism: a relationship in which the host organism provides the
parasitic microbe with nutrients and a habitat
Figure 7.12
Nonsymbiotic Relationship
• Organisms are free-living and relationships
are not required for survival
– Synergism: an interrelationship between two or
more free-living organisms that benefits them but
is not necessary for their survival
– Antagonism: an association between free-living
species that arises when members of a
community compete
• One microbe secretes chemical substances into the
surrounding environment that inhibit or destroy
another microbe in the same habitat
Interrelationships Between microbes
and Humans
• Normal microbiotia: microbes that normally
live on the skin, in the alimentary tract, and in
other sites in humans
• Can be commensal, parasitic, and synergistic
relationships
7.3 The Study of Microbial Growth
• Growth takes place on two levels
– Cell synthesizes new cell components and
increases in size
– The numer of cells in the population increases
• The Basis of Population Growth: Binary
Fission
Figure 7.13
Figure 7.14
The Rate of Population Growth
– Generation or doubling time: The time required for a
complete fission cycle
– Each new fission cycle or generation increases the
population by a factor of 2
– As long as the environment is favorable, the doubling
effect continues at a constant rate
– The length of the generation time- a measure of the
growth rate of an organism
• Average generation time- 30 to 60 minutes under optimum
conditions
• Can be as short as 10 to 12 minutes
– This growth pattern is termed exponential
Graphing Bacterial Growth
• The data from growing bacterial populations are
graphed by plotting the number of cells as a
function of time
– If plotted logarithmically- a straight line
– If plotted arithmetically- a constantly curved slope
• To calculate the size of a population over time:
Nf = (Ni)2n
– Nf is the total number of cells in the population at
some point in the growth phase
– Ni is the starting number
– N denotes the generation number
The Population Growth Curve
• A population of bacteria does not maintain its potential
growth rate and double endlessly
• A population displays a predictable pattern called a
growth curve
• The method to observe the population growth pattern:
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Place a tiny number of cells in a sterile liquid medium
Incubate this culture over a period of several hours
Sampling the growth at regular intervals during incubation
Plating each sample onto solid media
Counting the number of colonies present after incubation
Stages in the Normal Growth Curve
• Data from an entire growth period typically
produce a curve with a series of phases
• Lag Phase
• Exponential Growth Phase
• Stationary Growth Phase
• Death Phase
Lag Phase
• Relatively “flat” period
• Newly inoculated cells require a period of
adjustment, enlargement, and synthesis
• The cells are not yet multiplying at their
maximum rate
• The population of cells is so sparse that the
sampling misses them
• Length of lag period varies from one
population to another
Exponential Growth (Logarithmic or
log) Phase
• When the growth curve increases
geometrically
• Cells reach the maximum rate of cell division
• Will continue as long as cells have adequate
nutrients and the environment is favorable
Stationary Growth Phase
• The population enters a survival mode in
which cells stop growing or grow slowly
– The rate of cell inhibition or death balances out
the rate of multiplication
– Depleted nutrients and oxygen
– Excretion of organic acids and other biochemical
pollutants into the growth medium
Death Phase
• The curve dips downward
• Cells begin to die at an exponential rate
Figure 7.15
Potential Importance of the Growth
Curve
• Implications in microbial control, infection,
food microbiology, and culture technology
• Growth patterns in microorganisms can
account for the stages of infection
• Understanding the stages of cell growth is
crucial for working with cultures
• In some applications, closed batch culturing is
inefficient, and instead, must use a chemostat
or continuous culture system
Other Methods of Analyzing
Population Growth
– Turbidometry- a tube of clear nutrient solution
becomes turbid as microbes grow in it
Figure 7.16
Enumeration of Bacteria
• Direct or total cell count- counting the
number of cells in a sample microscopically
– Uses a special microscope slide (cytometer)
– Used to estimate the total number of cells in a
larger sample
Figure 7.17
Automated Counting
• Coulter counter- electronically scans a culture
as it passes through a tiny pipette
• Flow cytometer also measures cell size and
differentiates between live and dead cells
• Real-time PCR allows scientists to quantify
bacteria and other microorganisms that are
present in environmental or tissue samples
without isolating or culturing them
Figure 7.18