Chapter 6
Microbial Growth
BIO205
Ted Brewster
Requirements for Growth

Physical
Temperature
 pH
 Osmotic Pressure
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Chemical
Nutrients
 Organic and Inorganic

Temperature

Temperatures for growth and metabolism
 Minimum
 Lowest temp a species can grow
 Maximum
 Highest temp at which an organism will
grow
 Optimum
 Temp at which the species grows best

Most grow at temperatures we also favor.
Others can grow in extreme environments.

Temperature Classification

Organisms are classified by
preferred temperature range
for growth
 Psychrophiles
 Cold-loving
 Mesophiles
 Moderate-loving
 Thermophiles
 Heat-loving
Psychrophile
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0oC - 20oC
Optimum below 15oC
Snowfields, polar ice, deep ocean
Psychrotrophs or facultative psychrophiles

can grow above 20oC in refrigerator
Mesophile
•
•
•
•
•
10 – 50 oC
Optimum 25 – 40 oC (37oC)
Most common type of organism
Organisms that can live in animal bodies
Cause most food spoilage and disease
Thermophile

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Organisms that are capable of growth at high
temperatures
Optimum growth between 50 - 60 oC
Many cannot grow below 45 oC
Hyperthermophile
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Optimum growth temperature of 80 oC
Record temperature 110 oC
Extremophiles
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Can live in harsh
habitats
Usually can't live in
moderate ones
Acidophiles
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Remarkably tolerant of acid
Some can function and survive in an
environment with a pH of 1 (mining).
Many microbes will produce acid, thereby
limiting their own growth.
Osmotic Pressure


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Cells are 80-90% water
High osmotic pressure removes water from
the cell
Osmotic loss of water causes Plasmolysis
or shrinkage of the cell.
Extreme Halophiles
Require high salt concentrations for growth
 Obligate Halophiles
 Facultative Halophiles

Halophiles


Can tolerate, favor, or require high salt
concentrations for growth
Extreme Halophiles
Require high salt concentrations for growth
 Obligate Halophiles


Facultative Halophiles

Do not require high salt concentrations for
growth, but can tolerate high salt levels
Chemical Requirements

Essential nutrients needed to survive, grow
and reproduce
Carbon
 Nitrogen
 Sulfur
 Phosphorous
 Oxygen
 Hydrogen
 Trace Elements
 Organic Growth Factors

Carbon



Structural backbone of living matter
Needed for all organic compounds
½ the weight of a microbial cell is carbon
Nitrogen

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N2 gas from atmosphere
Converted from one form to another
Proteins, DNA, RNA, ATP
Sulfur
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Found in rocks and sediments
Some amino acids
Vitamins
Structural stability
Shape of proteins by disulfide bonds
Phosphorous

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Found in rocks
Part of DNA, RNA, ATP, and coenzymes
Oxygen


Atmosphere, Salts, water
Needed for metabolism in many organisms

Aerobes

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Anaerobes

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Require oxygen to live
Facultative Anaerobes

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Aerobic, but use air in a concentration that is lower than the air
Obligate Aerobes
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Unable to use oxygen for energy production, but can tolerate it
Microaerophiles
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Organisms that do not or cannot use oxygen
Aerotolerant Anaerobes
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Organisms that use molecular oxygen
Can use oxygen when it is present, but not required
Respiration and fermentation pathways
Obligate Anaerobes

Unable to use oxygen for energy production
Toxic Forms of Oxygen

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Singlet oxygen
Superoxide free radicals
Peroxide anion
Hydrogen peroxide and benzoyl peroxide
 Enzymes neutralize hydrogen peroxide

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Catalase – converts it to water and oxygen
Hydroxyl radical
Hydrogen
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Water, salts, gases
Maintains pH
Forms H bonds
Source of free energy in redox reactions of
respiration
Other Nutrients
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Ca
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Fe
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Part of cytochrome system in cell respiration
Na

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Stabilizer of cell wall and endospores
Important for some types of cell transport
Mg

Part of chlorophyll, a stabilizer of membranes
and ribosomes
Trace Elements

Zn, Mn, Ni, Cu, Co, Mo, Si, I, B
Organic Growth Factors

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Essential organic nutrients
Amino acids or vitamins that can't be
synthesized in the organism
Fastidious bacteria
 Organisms that need certain growth
factors
Culture Media

Culture medium

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Inoculum

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Microbes that are introduced into a medium
Culture

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Nutrient material prepared for the growth of
microbes in a laboratory
Microbes that grow and multiply on a medium
Sterile

Something that does not contain any living
organisms
Agar

A complex polysaccharide agent derived
from marine algae.
Chemically Defined Media

Media whose exact chemical composition is
known
Complex Media

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What we use in an intro lab class
Made up of nutrients such as:
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Extracts of yeast
Meat
Plants
Digests of Proteins
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Composition is variable for each batch made.
Nutrient Broth
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Main source of C, P, N, S, Etc. for microbial growth
Liquid
Nutrient Agar
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Solid
Anaerobic Growth Media

Reducing media

Contain ingredients such as:
Sodium
 Thyoglycolate

Combines with dissolved oxygen in media
 Depletes oxygen from the media

Anaerobic Growth Tools

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Tight-capped test
tubes or flasks
Anaerobic Jars
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Utilize packets that
contain chemicals
which absorb oxygen in
the sealed jar
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Animals
Carbon dioxide
incubators
Candle Jars

Capnophiles

Microbes that grow best at
high CO2 levels
Selective and Differential Media

Selective Media
Suppress the growth of unwanted organisms
 Contain inhibiting agents
 pH, salts, etc.

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Differential Media
Used to distinguish colonies of desired
organisms from others on the same plate
 Pure cultures have identifiable reactions on
certain medias

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i.e., Blood Agar, Methylene blue, EMB
Enrichment Culture

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A media that favors the growth of only
certain microbes
Also a selective medium
Assists in growing smaller organisms that
are difficult to grow under normal
conditions
Obtaining Pure Cultures

Colony
Arises from a single microorganism
 Can have distinctive shapes and characteristics

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Streak Plate Method
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Used to separate mixed cultures of
microorganisms
Preservation

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Refrigeration
Deep-freezing
Lyophilization (freeze-drying)
Growth of Bacterial Cultures

Bacterial Division
Binary Fission
 Budding

Generation Time
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Time required for a
cell to divide
Dependent on many
factors
Chemical
 Physical
 Environmental
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Growth is
exponential
Many bacteria in a
short time
Phases of Growth
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Bacterial Growth Curve

Shows the growth of cells over time
Four Phases of Growth
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Lag
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Log
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Growth or logarithmic phase
Stationary
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Little or no cell division
Period of equilibrium (new cell growth = cell
death)
Death

Death exceeds new cell growth
Direct Measurement of Microbial
Growth

Several methods to count cell populations
Plate Counts
 Serial Dilutions
 Pour Plates
 Spread Plates
 Filtration
 Most probable Number Method
 Direct microscopic count
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Plate Counts
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Most frequently used method
Measures the number of viable cells
Takes time
Assumes that each living bacterium grows and
divides to produce a single colony
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Not always true – clumps or chains
Often reported as Colony-forming Units (CFU)
Too many colonies can limit our ability to count
the cells (25-250 or 30-300 is preferred).
Serial Dilutions

The dilution of bacterial sample in order to obtain
population counts
Pour Plates

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Bacteria cells are inoculated into a medium
while it is liquid, which then solidifies
Colonies grow within a nutrient medium
and on the surface
Some organisms can get damaged by the
heat of the medium as it is poured into the
plate
Spread Plates


An inoculum is
spread uniformly
over the surface of a
medium with a
specially shape,
sterilized glass rod
(hockey stick)
Positions all the
organisms on the
surface of the plate
Filtration
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Water testing uses filters to count low
numbers of bacteria in samples
Filtered bacteria are trapped in the filter
Filter is then transferred to a petri dish and
placed on a nutrient pad
Colonies arise on the filter
Coliform bacteria – indicates fecal matter
polution
Most Probable Number Method
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Statistical estimation
Good for identifying organisms that will not
grow on a solid medium
95% chance that the population falls within
a certain number range
Direct Microscopic Count
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Measures cells over a specified distance
using the microscope and a special slide
with a grid on it
Staining is involved
Uses an average of numbers in different
“quadrants” of the slide grid
Petroff-Hausser cell counter

Special slide that is used for counting cells
Estimating Bacterial Numbers by
Indirect Methods

Turbidity
Bacteria growth increases turbidity in a liquid
medium
 Using light, the absorbancy of the liquid can be
measured

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Metabolic Activity

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Measures CO2, acidity, etc. resulting from cell
growth
Dry Weight

Dried and then weighed (molds and fungi)
Exam Review
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Read Chapter 6
Complete the Chapter 6 Quiz
Study the Chapter 6 Exam Notes
Review the following YouTube video on
Hydrothermal Vents for possible essay
questions on the Exam.
Thermal vents