Factors Affecting Microbial Growth
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Transcript of Factors Affecting Microbial Growth
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Factors Affecting Microbial Growth
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FATTOMFood
Acid
Temperature
Time
Oxygen
Moisture
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Food
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Food Nutrients needed by microorganisms include:
Carbon – carbon containing compounds are needed as
an energy source (ex. glucose) for building blocks.
Nitrogen needed for amino acids and nucleotides; some can synthesize all 20 amino acids; others have to have some provided in their
medium.
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Food Sulfur – needed for amino acids, coenzymes, Phosphorus – needed for ATP, phospholipids, and nu
cleotides Vitamins
a vitamin is an organic substance that an organism requires in small amounts and that is typically used as a coenzyme;
many bacteria make their own, but some are required in the medium;
microbes living in the human intestine manufacture vitamin K, needed for blood clotting, and some of the B vitamins, thus benefiting their host.
Certain trace elements – ex. copper, iron, zinc, sodium, chloride, potassium, calcium, etc.;
often serve as cofactors in enzymatic reactions.
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Acid (pH)
Measures alkalinity or acidity Most spoilage bacteria grow best near neutral
pH Most bacteria grow between pH 6.5-7.5 Molds and yeasts grow between pH 5 and 6 Most bacteria produce organic acids as they
grow and metabolize Buffers
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Effects of acids on organisms
enzyme activity affected proteins, DNA, other molecules denatured longer lag, less rapid growth
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Classificationof microbesaccordingto toleranceof pH extremes
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Typical growth rates of different types of microorganisms as affected by pH
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Limits of pH for growth of pathogens
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TTemperatureemperature
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TTemperatureemperature
Type of microorganism
Growth Range Optimum
Psychrophiles -10 - 20°C 10 - 15°C
Psychrotrophs 0 - 30°C 20 - 25°C
Mesophiles 10 - 50°C 36 - 38°C
Thermophiles 40 - 72°C 55 - 65°C
Thermodurics 35 - 80°C 45 - 55°C
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Temperatureeffectson growth rate.
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Typical growth rates of different types of microorganisms in response to temperature
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Food spoilage temperature
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Time
Not more than 2-4 hours at temperature danger (growth) zone
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Oxygen
Poisonous at high concentrations Often combined with hydrogen to form
water Microorganisms have variation in need to
metabolize oxygen
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Oxygen
Type of Organism Description Oxygen Level Example
Obligate aerobe With O2 20% B.subtilis
Obligate anaerobe Without O2 0% C. botulinum
Facultative anaerobe With/without O2 0-20% E.coli
Microaerophiles With O2 2-10% Campylobacter
Aerotolerant With/without O2 0-20%
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Oxygen and GrowthObligate aerobic
anaerobes
facultativeaerobes
Microaerophiles
Aerotolerant anaerobes
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Oxygen Requirements: The Shake Tube
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Moisture
Types of waterBound water – not available for
microorganismsFree water – available for microorganisms
Water activityAmount of free waterPotentially hazardous – 0.97-0.99Lowest Aw to prevent bacterial growth: 0.85
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Effects of low water levels
longer lag, slower growth impaired transport loss of membrane fluidity
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Approximate Aw values of selected food categories
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Approximate Aw values of selected food categories
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Addendum: Osmotic Pressure
High osmotic pressures remove necessary water from the cell
Plasmolysis – shrinkage of the cell’s plasma membrane
Growth of cell is inhibited as the plasma membrane pulls away from the cell wall
Served as the basis of food preservation methods using salt and sugar
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Plasmolysis
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Osmotic Pressure
HalophilesCan withstand high salt concentrationObligate – require high salt concentrations
(30%)Facultative – do not require high salt
concentration, but can survive high salt concentrations (2-15%)
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Classificationof microbesaccordingto salinity tolerances.