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Factors Affecting Microbial Growth

Factors Affecting Microbial Growth 2

FATTOMFood

Acid

Temperature

Time

Oxygen

Moisture

Factors Affecting Microbial Growth 3

Food

Factors Affecting Microbial Growth 4

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.