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Chapter 6: Microbial Growth. 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

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microbial growth
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
physical requirements for growth temperature
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)

Physical Requirements for Growth: Temperature


Cause majority of food spoilage

Figure 6.1

hansen s disease leprosy
Hansen’s Disease(Leprosy)
  • Mycobacterium leprae
  • Optimal growth temperature: 30°C
    • Grows in peripheral nerves, nasal mucosa and skin cells

Figure 22.8

physical requirements for growth ph
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 (pH<5.5)
    • Alkaliphiles grow in basic environments (pH>8.5)
  • Acidic foods (pickles, sauerkraut) preserved by acids from bacterial fermentation
  • Growth media used in the laboratory contain buffers
physical requirements for growth osmotic pressure
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(>2% 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

physical requirements for growth osmotic pressure1
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

chemical requirements for growth
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-)
chemical requirements for growth1
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

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


chemical requirements for growth oxygen1
Chemical Requirements for Growth: Oxygen
  • Oxygen (O2)

Express SOD and catalase

Require oxygen, but at lower levels than in the air

Tolerate oxygen (express SOD/catalase) but incapable of using it for growth

Don’t express SOD/catalase

culture media
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
culture media agar
Culture Media:Agar
  • 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 100°C
    • Can incubate at a wide range of temperatures
anaerobic culture media broth cultures
Anaerobic Culture Media:Broth cultures
  • Reducing broth media
    • Contain chemicals (thioglycollate) that combine with dissolved O2 to deplete it from the media
anaerobic culture methods agar cultures
Anaerobic Culture Methods:Agar Cultures
  • Anaerobic jar
    • Oxygen and H2 combine to form water

Figure 6.5

culture media selective and differential media
Culture Media:Selective 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

obtaining pure cultures
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
streak plate method for isolation of a pure species
Streak Plate Method for Isolation of a Pure Species
  • Use loop to pick colony
  • Inoculate broth
  • Pure culture

Figure 6.10a, b

microbial growth in hosts biofilms
Microbial Growth in Hosts:Biofilms

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

microbial growth in hosts biofilms quorum sensing
Microbial Growth in Hosts:Biofilms & 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

reproduction in prokaryotes generation time
Reproduction in Prokaryotes:Generation Time

Generation time: the time required for one population doubling

  • Varies with species and environmental conditions
reproduction in prokaryotes generation number
Reproduction in Prokaryotes:Generation Number
  • Generation number: the number of times a cell population has doubled in a given time under given conditions

Figure 6.12b

reproduction in prokaryotes growth plot
Reproduction in Prokaryotes:Growth Plot



Figure 6.13

bacterial growth curve
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

measuring microbial growth
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
direct measurements of microbial growth viable cell count
Direct Measurements of Microbial Growth:Viable Cell Count
  • Plate Counts: Perform serial dilutions of a sample
  • How many cells are in 1 mL of original culture?








Figure 6.15, top portion

direct measurements of microbial growth viable count
Direct Measurements of Microbial Growth:Viable Count
  • Inoculate one agar plate with each serial dilution

Figure 6.16

direct measurements of microbial growth viable count1
Direct Measurements of Microbial Growth:Viable Count
  • After incubation, count colonies on plates that have 30-300 colonies (CFUs)

Figure 6.15

direct measurements of microbial growth
Direct Measurements of Microbial Growth
  • Filtration
    • Ideal when microbial density is low in a sample

Figure 6.17a, b

direct measurements of microbial growth1
Direct Measurements of Microbial Growth


-Likely to count dead cells

-Motile cells can be difficult to count

Figure 6.19