Microbial Growth Factors

Microbial Growth Requirements

• Physical factors influencing microbial growth: temperature, pH, osmotic pressure.
• Most microorganisms thrive at human-favorable temperatures, categorized into psychrophiles, mesophiles, and thermophiles.

Temperature Classes

• Psychrophiles: Cold-loving microbes, optimal growth at -10 to 20°C.
• Mesophiles: Moderate-temperature loving microbes, optimal growth at 10 to 50°C.
• Thermophiles: Heat-loving microbes, optimal growth at 40 to 73°C.
• Hyperthermophiles: Optimal growth temperature of 80°C or higher, typically found in hot springs.

Temperature Ranges

• Maximum: Highest temperature for microbial growth.
• Optimum: Temperature at which growth is maximized.
• Minimum: Lowest temperature for growth; typically, max and min temperatures are about 30°C apart.

pH Levels

• Most bacteria prefer neutral pH (6.5 to 7.5).
• Acidophiles thrive in acidic environments.

Nutrient Acquisition

• Microorganisms require water to obtain nutrients dissolved in their surrounding environment.
• High osmotic pressure can result in plasmolysis, where cells lose water and shrink, affecting growth.

Salt Concentration Adaptations

• Extreme halophiles need high salt concentrations for growth.
• Obligate halophiles require salt, while facultative halophiles tolerate lower concentrations.

Culture Media Essentials

• Typical culture medium contains 1.5% agar and water.
• Nutrient media consist of carbon, nitrogen, sulfur, phosphorus, trace elements, and oxygen.

Sustainability of Microbial Life

• Carbon is a structural backbone of living matter, essential for all organic compounds.
• Nitrogen fixation allows the conversion of atmospheric nitrogen for incorporation into biological molecules.

Oxygen's Role and Effects

• Oxygen is necessary for aerobic respiration but toxic to obligate anaerobes.
• Singlet oxygen and superoxide radicals are reactive and require neutralization by enzymes like superoxide dismutase.

Growth Phases and Measurement

• Bacterial growth occurs in four phases: lag, log, stationary, and death.
• Direct measurement methods include plate counts, serial dilutions, and direct microscopic counts.
• Plate counts are a common method for viable cell quantification but labor-intensive.

Preservation Techniques

• Short-term preservation via refrigeration.
• Long-term techniques include deep-freezing and lyophilization (freeze-drying), allowing cultures to be stored for years.

Growth Media Types

• Selective media suppress unwanted bacteria, while differential media differentiate between species.
• Enrichment cultures favor growth conditions for specific microbes, useful in low-abundance scenarios.

Biofilms and Their Importance

• Microbes often exist in biofilms, complex communities providing protection and enabling cooperative growth.
• Biofilms are heavily involved in human infections, primarily in devices like catheters and heart valves.

Methods for Culturing Bacteria

• Inoculation introduces microbes to culture medium.
• Streak plate method aids in obtaining pure cultures from mixed samples.

Specialized Growth Conditions

• Anaerobic chambers and reducing media deplete oxygen for anaerobic organisms.
• Carbon dioxide incubators maintain specific CO2 levels for capnophiles, which prefer higher concentrations.

Applications for Growth Media

• Complex media provide varied nutrients; chemically defined media offer known compositions.
• Assays help determine specific nutritional requirements through controlled growth experiments.### Bacteria in Milk
• Viewing bacteria under an oil immersion lens allows for detailed observation of microbial structures.
• Bacteria can be counted in multiple fields of view, providing an average number of cells per viewing field.
• The Petroff-Hausser cell counter is a specific device used for counting bacterial cells.
• Live bacteria cannot be counted; dead cells are often counted, requiring a concentration of approximately 10 million cells per milliliter for accuracy.
• Advantage of this method: it does not require incubation time.

Estimating Bacterial Numbers Using Indirect Methods

• Common indirect estimation methods include turbidity, metabolic activity, and dry weight.

Turbidity

• Bacterial growth causes the medium to become cloudy, indicating an increase in cell numbers.
• As the bacterial population increases, light transmission decreases, leading to higher absorbance.
• A spectrophotometer is used to measure turbidity; a million cells per milliliter need to be present for initial turbidity detection.
• This method is ineffective for counting small bacterial populations.

Metabolic Activity

• This approach assumes a direct correlation between the amount of metabolic byproducts (e.g., acids, CO2) and the bacterial population present.

Dry Weight

• For filamentous fungi and molds, traditional measuring methods may be inadequate.
• Samples are removed from growth media, filtered to eliminate excess material, and dried before being weighed.
• A desiccator is typically used for the drying process.