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Title: Chapter 6: Microbial Growth


1
Chapter 6Microbial Growth
2
Microbial Growth
  • Microbial growth growth in population
  • Increase in number of cells, not cell size
  • Two main categories of requirements for microbial
    growth
  • Physical requirements (environmental conditions)
  • Temperature, pH, osmotic pressure
  • Chemical requirements

3
Physical Requirements for Growth Temperature
  • Temperature
  • Minimum growth temperature
  • Optimum growth temperature
  • Maximum growth temperature
  • Three main classifications
  • Psychrophiles (optimum 120C)
  • Psychrotrophs (optimum 230C)
  • Mesophiles (optimum 370C)
  • Thermophiles (optimum above 500C)

4
Physical Requirements for Growth Temperature
Refrigeration
Cause majority of food spoilage
Figure 6.1
5
Hansens Disease(Leprosy)
  • Mycobacterium leprae
  • Optimal growth temperature 30C
  • Grows in peripheral nerves, nasal mucosa and skin
    cells

Figure 22.8
6
Physical Requirements for Growth pH
  • pH
  • Most bacteria grow between pH 6.5 and 7.5
  • Molds and yeasts grow optimally between pH 5 and
    6
  • Acidophiles grow in acidic environments (pHlt5.5)
  • Alkaliphiles grow in basic environments (pHgt8.5)
  • Acidic foods (pickles, sauerkraut) preserved by
    acids from bacterial fermentation
  • Growth media used in the laboratory contain
    buffers

7
Physical Requirements for Growth Osmotic Pressure
  • Osmotic Pressure
  • Hypertonic environments (high osmotic pressure),
    increased salt or sugar, cause plasmolysis
  • Obligate halophiles require high osmotic
    pressure
  • Facultative halophiles tolerate high osmotic
    pressure (gt2 salt)
  • Nutrient agar has a high percentage of water to
    maintain low osmotic pressure (bacterial cells
    are 80-90 water)

Low osmotic pressure
High osmotic pressure
Water flow
High solute concentration/ Low water concentration
Low solute concentration/ High water concentration
8
Physical Requirements for Growth Osmotic Pressure
  • Plasmolysis cell growth is inhibited when the
    plasma membrane pulls away from the cell wall
  • Added salt or sugar is another method of
    preserving food

Hypertonic solution (high osmotic pressure)
Isotonic solution
Figure 6.4
9
Chemical Requirements for Growth
  • Carbon
  • Structural organic molecules, energy source
  • Heterotrophs use organic carbon sources
  • Autotrophs use CO2
  • Nitrogen, Sulfur, Phosphorus
  • For synthesis of amino acids, nucleotides,
    vitamins, phospholipids
  • Most bacteria decompose proteins to obtain N
  • Inorganic ions are sources for these elements
    (NH4, NO3-, PO43-, SO42-)

10
Chemical Requirements for Growth
  • Trace Elements (Iron, Copper, Zinc)
  • Inorganic elements required in small amounts,
    usually as enzyme cofactors
  • Often present in tap water
  • Organic Growth Factors
  • Organic compounds obtained from the environment
    (i.e. the organism cannot synthesize them)
  • Vitamins, amino acids

11
Chemical Requirements for Growth Oxygen
  • Oxygen (O2)

Obligate aerobes Facultative anaerobes Obligate anaerobes Aerotolerant anaerobes Microaerophiles

12
Chemical Requirements for Growth Oxygen
  • Aerotolerance of individual organisms depends on
    their ability to handle oxygen toxicity
  • Oxygen radical species O2-, O22-, OH
  • Presence/lack of enzymes that neutralize toxic
    oxygen species
  • SOD (Superoxide dismutase)
  • Catalase/peroxidase

.
13
Chemical Requirements for Growth Oxygen
  • Oxygen (O2)

Obligate aerobes Facultative anaerobes Obligate anaerobes Aerotolerant anaerobes Microaerophiles

Require oxygen, but at lower levels than in the
air
Express SOD and catalase
Dont express SOD/catalase
Tolerate oxygen (express SOD/catalase) but
incapable of using it for growth
14
Culture Media
  • Culture Medium Nutrients prepared for microbial
    growth
  • Source of energy, carbon, nitrogen, sulfur,
    phosphorus, trace elements and organic growth
    factors
  • Sterile No living microbes
  • Inoculum Introduction of microbes into medium to
    initiate growth
  • Culture Microbes growing in/on culture medium

15
Culture MediaAgar
  • Complex polysaccharide
  • Used as solidifying agent for culture media in
    Petri plates, slants, and deeps
  • Generally not metabolized by microbes
  • Agar is not a nutrient
  • Liquefies above 100C
  • Can incubate at a wide range of temperatures

16
Culture Media
17
Anaerobic Culture MediaBroth cultures
  • Reducing broth media
  • Contain chemicals (thioglycollate) that combine
    with dissolved O2 to deplete it from the media

18
Anaerobic Culture MethodsAgar Cultures
  • Anaerobic jar
  • Oxygen and H2 combine to form water

Figure 6.5
19
Culture MediaSelective and Differential Media
  • Selective media suppress growth of unwanted
    microbes and encourage growth of desired microbes
  • Differential media make it easy to distinguish
    colonies of different microbes

Enterobacter aerogenes on EMB
E. coli on EMB
Figure 6.9b, c
20
Obtaining Pure Cultures
  • A pure culture contains only one species or
    strain
  • A colony is a population of cells arising from a
    single cell or spore or from a group of attached
    (identical) cells
  • One colony arises from one colony-forming unit
    (CFU)
  • Specimens (pus, sputum, food) typically contain
    many different microorganisms
  • Common way to isolate a single species from a
    mixture of microorganisms Streak plate method

21
Streak Plate Method for Isolation of a Pure
Species
  • Use loop to pick colony
  • Inoculate broth
  • Pure culture

Figure 6.10a, b
22
Microbial Growth in HostsBiofilms
  • Microbial communities
  • 3-dimensional slime
  • i.e. dental plaque, soap scum
  • Share nutrients
  • Sheltered from harmful factors
  • Cell-to-cell communication quorum sensing

Figure 6.5
Bacterial biofilm growing on a micro-fibrous
material
23
Microbial Growth in HostsBiofilms Quorum
Sensing
  • Quorum sensing allows a form of bacterial
    communication
  • Individual cells can sense the accumulation of
    signaling molecules (autoinducers)
  • Informs individual cells about surrounding cell
    density
  • May change the behavior (gene expression) of
    individual cells
  • Results in a coordinated response by the whole
    population

http//biofilmbook.hypertextbookshop.com/public_ve
rsion/
24
Prokaryotic ReproductionBinary Fission
Figure 6.11
25
Reproduction in ProkaryotesGeneration Time
  • Generation time the time required for one
    population doubling
  • Varies with species and environmental conditions

26
Reproduction in ProkaryotesGeneration Number
  • Generation number the number of times a cell
    population has doubled in a given time under
    given conditions

Figure 6.12b
27
Reproduction in ProkaryotesGrowth Plot
Logarithmic
Arithmetic
Figure 6.13
28
Bacterial Growth Curve
  • Lag little/no cell division
  • Adapting to new medium
  • Metabolically active
  • Log exponential growth
  • Most metabolically active
  • Gen. time at constant minimum
  • Stationary equilibrium phase
  • Growth rate death rate
  • Nutrients exhausted, waste accumulation, pH
    changes
  • Death logarithmic decline

Figure 6.14
29
Measuring Microbial Growth
  • To determine the size of a bacterial population
    in a specimen, cell counting techniques are used
  • Often there are too many cells per ml or gram of
    specimen
  • A small proportion of the specimen (a dilution)
    is counted
  • The number of cells in the original specimen can
    be calculated based on the count in the small
    dilution

30
Direct Measurements of Microbial GrowthViable
Cell Count
  • Plate Counts Perform serial dilutions of a
    sample
  • How many cells are in 1 mL of original culture?

DF1
DF
10-3
10-5
10-2
10-4
10-1
Figure 6.15, top portion
31
Direct Measurements of Microbial GrowthViable
Count
  • Inoculate one agar plate with each serial dilution

Figure 6.16
32
Direct Measurements of Microbial GrowthViable
Count
  • After incubation, count colonies on plates that
    have 30-300 colonies (CFUs)

Figure 6.15
33
Direct Measurements of Microbial Growth
  • Filtration
  • Ideal when microbial density is low in a sample

Figure 6.17a, b
34
Direct Measurements of Microbial Growth
Disadvantages -Likely to count dead
cells -Motile cells can be difficult to count
Figure 6.19
35
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