Microbial Growth

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Chapter 6

• Microbial Growth

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Microbial Growth

• Growth- refers to the increase in number of cells
• Unicellular organisms must stay small
• Cell size increases until S.A/Volume becomes
  unfavorable, then divide by binary fission

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Requirements for Growth

• Physical Requirements
• Temperature
• pH
• Osmotic pressure

• Chemical Requirements
• Carbon source
• Nitrogen source
• Oxygen
• Phosphorous
• Sulfur
• Trace elements
• Organic growth factors

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Physical Requirements

• Temperature
• Minimum growth temperature-
• Optimum growth temperature-
• Maximum growth temperature-

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Microbes are classified into groups based on
their preferred temperature range

• Psychrophiles
• Psychrotrophs
• Mesophiles
• Thermophiles
• Extreme thermophiles

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Food Spoilage Temperatures
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Food Spoilage- Continued
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Physical Requirements

• pH-
• Most bacteria grow best in a near-neutral pH
  range between 6.5-7.5
• Most bacteria inhibited at a pH below
• Optimum pH range of most molds and yeasts pH 5-6
• Fermentation as a food preservative
• Acidophiles

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Physical Requirements

• Osmotic pressure-
• Microorganisms are 80-90 water
• High osmotic pressures remove water from the
  cell, causing plasmolysis
• Food preservatives- salt, sugar
• Extreme halophiles-
• Obligate halophiles-
• Facultative halophiles-

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Chemical Requirements

• Carbon
• one of the most important requirements for
  microbial growth
• Approximately half the dry weight of a typical
  bacterial cell is carbon
• Chemoheterotrophs- get carbon from organic energy
  source (food)
• Chemoautotrophs, photoautotrophs- get carbon from
  carbon dioxide (inorganic)

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Chemical Requirements

• Nitrogen-
• 14 of dry weight
• Necessary for protein synthesis, DNA, RNA
• Nitrogen sources organic N (proteins, amino
  acids), ammonium, nitrates, atmospheric nitrogen
  N2 used by nitrogen fixing bacteria

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Chemical Requirements

• Sulfur-
• Needed to synthesize sulfur containing amino
  acids, thiamine, and biotin
• Hydrothermal vent bacteria-

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Chemical Requirements

• Phosphorous-
• Essential for phospholipid membrane
• ATP
• DNA
• RNA

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Chemical Requirements

• Trace elements-
• Nutrients essential for life but needed in very
  small amounts
• Iron, copper, zinc, molybdenum
• Most are enzyme cofactors-

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Chemical Requirements

• Oxygen-needed by some microbes some forms of
  oxygen are toxic to others (see table 6.1)
• Obligate aerobes-
• Facultative anaerobes-
• Obligate anaerobes-
• Aerotolerant anaerobes-
• Microaerophiles-

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Culture Media

• Nutrient media prepared for growing
  microorganisms in a laboratory
• Inoculum-
• Culture-
• Agar-

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Types of Culture Media

• Chemically defined media-
• Complex media-
• Examples

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Anaerobic Growth Media/Methods

• Reducing media-
• Anaerobic jars-
• Capnophiles-

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Selective Media

• Selects for the growth of certain microbes by
  inhibiting the growth of others
• Contains a substance that prevents growth of
  unwanted microbes- desired microbe is not
  affected by substance
• Examples

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Differential Media

• Allows the observer to differentiate between
  types of microorganisms based on the appearance
  of the growth on media
• Usually a test substance that cant be used by
  all bacteria
• A positive result will typically show a color
  change
• Examples

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Bacterial Colonies on Differential Media
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Enrichment Culture

• Used to promote the growth of bacteria that are
  present in small numbers in a sample
• Usually liquid, provides nutrients to increase
  growth of the rare microbes
• Fecal samples, soil samples

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Growth of Bacterial Cultures

• Growth refers to increase in number of cells
• Binary fission-
• Generation time-
• Exponential growth-

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Binary Fission
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Cell Division
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Phases of Microbial Growth

• Bacterial growth curve- shows the growth of cells
  over time
• Lag phase-
• Log phase-
• Stationary phase-
• Death phase-

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Bacterial Growth Curve
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Direct Measurement of Microbial Growth

• Plate count-
• Advantage- measures number of viable cells
• Disadvantage-
• Assumption- each bacterium divides and grows to
  form a colony
• CFU-

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Direct Measurement of Microbial Growth

• Serial Dilutions-
• Used for samples that would be too numerous to
  count on a plate
• Usually a 110 dilution is taken
• Diluted sample is cultured on a plate
• Successive dilutions are made and cultured until
  a plate has countable CFUs
• Original sample population is estimated by of
  colonies x reciprocal dilution of sample number
  of bacteria/ml

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Serial Dilutions
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Indirect Methods of Estimating Bacterial Numbers

• Turbidity-
• Metabolic activity-
• Dry weight-