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Microbial Growth
For microorganisms, growth is measured by
increase in cell number, due to their limited
increase in cell size.
Bacteria are classified by:
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Temperature Range
pH Range
Osmotic Pressure
Need for Oxygen
Other Classifications
• Facultative: Microorganisms that can grow in
more than one environment
• Obligate: Microorganisms that have specific
environments strictly needed for growth
Temperature Range
• Psychrophile: 0-30 degrees C range, 15 C
optimum growth temperature
• Mesophile: 25-40 degrees C range, 37 C
optimum growth temperature. These have
most pathogens, optimized for body
temperature.
• Thermophile: 40-110 degrees C range, 55 C
optimum growth temperature.
pH Range
• Acidophiles: Grow in acidic environment, pH
less than 6.5. Linked to stomach ulcers.
• Neutrophiles: Grow in neutral environment,
pH 6.5-7.5. Most pathogens are neutrophiles,
as your body is mostly within this pH range.
• Alkaliphiles: Grow in basic environments, pH
greater than 7.5.
pH
• PH a factor in food spoilage: Canned food high
in botulism
• Acid rain effect the environment
• Bacteria often produce acids that eventually
interfere w/ their own growth.
Osmotic Pressure Range
• Low osmotic pressure kills many prokaryotes.
• Halophiles: microbes that can survive in salty
environments.
• Obligate Halophiles: microbes that need salty
environments to survive.
Nutritional Requirements for Growth
• Carbon: All living things need a carbon source
• Nitrogen: used in protein and nucleic acid
synthesis
• Sulfur: used in protein synthesis
• Phosphorus: Used in ATP and nucleic acids
• Trace Elements: (Fe, Zn, Cu, etc.) Co-factors
and co-enzymes to activate enzymes.
Nutritional Requirements Cont.
• Trace Elements: (Fe, Zn, Cu, etc.) Co-factors
and co-enzymes to activate enzymes. Only in
small amounts.
• Organic Growth Factors are organic
compounds that are essential to the organism
but the organism is unable to synthesize it
itself (ex. Vitamins, amino acids, nucleotides).
Classifications for Oxygen Need
• Obligate Aerobes: Must have free oxygen for
aerobic respiraiton.
• Obligate Anaerobes: Killed by free oxygen.
Growth only occurs where there is no oxygen.
Oxygen Needs Cont.
• Facultative Anaerobes: Have both aerobic and
anaerobic growth. Growth is greater in the
presence of oxygen because full respiration
occurs.
• Aerotolerant anaerobes: Only anaerobic
growth, but can tolerate the presence of
oxygen. Growth occurs evenly throughout
media as oxygen has no effect.
Oxygen Needs Cont.
• Microaerophiles: Only aerobic growth. Oxygen
is required in low concentrations. Growth
occurs only where a low concentration of
oxygen has diffused into the medium.
Bacteria classified according to their Oxygen
needs
• Obligate aerobes
– Must have free oxygen for
aerobic respiration
– Grow only occurs on top of
media where there is high
concentration of oxygen
Bacteria classified according to their
Oxygen needs
• Falcutative Anaerobes
– Both aerobic and anaerobic
growth
– Growth is greater in the
presence of oxygen .
– Growth occurs throughout
media.
Bacteria classified according to their
Oxygen needs
• Obligate anaerobes
– killed by free oxygen
– Growth occurs only
where there is no oxygen
Bacteria classified according to their
Oxygen needs
• Aerotolerant Anaerobes
– Only anaerobic growth; but
can tolerate the presence of
oxygen
– Growth occurs evenly
throughout media as oxygen
has no effect
Bacteria classified according to their
Oxygen needs
• Microaerophiles
– Only aerobic growth
– oxygen is required in low
concentrations.
– Growth occurs only
where a low
concentration of oxygen
has diffused into
medium
Culture, Colony and Culture Media
• Culture: The actual bacteria being grown.
• Colony: A group of the same bacteria growing
together.
• Culture Media: What the bacteria are growing
on or in.
Culture Media
Two types of culture media
• Broth: A liquid nutrient media without agar
• Agar: a common solidifying agent for a culture
medium.
Agar
• It is added to nutrient medium to solidify it in
order to bacterial grow colonies.
• It is made form seaweed. It has long been
used in various foods from jellies, soups, and
ice cream.
• Few microbes can break down agar.
• Different freezing/melting points. Solid to
liquid at 94 C, liquid to solid at 42 C. Labs keep
it liquid at 50 C
Culture Media Cont.
• Chemically Defined: Medium in which the exact
chemical composition is known. Often used to grow
fastidious organisms. Expensive to buy.
• Complex: Used to grow most of the common
bacteria and fungi that would be used in an
introductory lab. Made of nutrients such as extracts
from yeast, meat and plants.
• Anaerobic Growth Media/Reducing Media: No free
oxygen and used to grown obligate anaerobes.
Culture Media Cont.
• Selective: Media that is designed to suppress
the growth of unwanted bacteria and
encourage the growth of wanted bacteria.
• Differential: Makes it easier to distinguish
colonies of the desired organism from other
colonies growing on the same plate.
• Enrichment: Used to encourage the growth of
a particular organism in a mixed culture.
Example soil, fecal samples etc...

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