Microbial Nutrition, Growth, and Culturing: Study Notes

Microbial Nutrition and Growth

Growth Requirements

Microbial growth refers to the increase in the population of microbes, typically resulting in the formation of discrete colonies. Growth is dependent on reproduction and the availability of nutrients, which are used for energy and to build cellular structures.

• Nutrients: Essential elements include carbon, oxygen, nitrogen, and hydrogen.

• Sources: Microbes obtain nutrients from a variety of sources, which can be classified based on their carbon and energy requirements.

Classification by Carbon and Energy Source

Microorganisms are classified based on their sources of carbon and energy:

• Autotrophs: Use carbon dioxide as a carbon source.

• Heterotrophs: Use organic compounds as a carbon source.

• Phototrophs: Use light as an energy source.

• Chemotrophs: Use chemical compounds as an energy source.

Oxygen Requirements

Oxygen is essential for some microbes (obligate aerobes) but toxic for others (obligate anaerobes). Toxic forms of oxygen include singlet oxygen, superoxide radicals, peroxide anion, and hydroxyl radical, which can cause irreparable cellular damage.

• Aerobes: Require oxygen.

• Anaerobes: Cannot tolerate oxygen.

• Facultative anaerobes: Can grow with or without oxygen.

• Aerotolerant anaerobes: Tolerate oxygen but do not use it.

• Microaerophiles: Require low levels of oxygen.

Nitrogen, Phosphorus, Sulfur, and Trace Elements

Nitrogen is vital for anabolism and is acquired from organic and inorganic sources. Nitrogen fixation by certain bacteria is essential for life. Phosphorus, sulfur, and trace elements are required in small amounts. Growth factors are organic chemicals that some organisms cannot synthesize and must obtain from the environment.

Physical Requirements for Growth

Temperature

Temperature affects protein function and membrane fluidity. Microbes are classified based on their optimal growth temperatures:

• Psychrophiles: Grow best at low temperatures.

• Mesophiles: Grow best at moderate temperatures.

• Thermophiles: Grow best at high temperatures.

• Hyperthermophiles: Grow best at extremely high temperatures.

pH

Microbes are sensitive to changes in acidity, which affects hydrogen bonding. Neutrophiles grow best at neutral pH, acidophiles in acidic environments, and alkalinophiles in alkaline conditions.

Water, Osmotic Pressure, and Hydrostatic Pressure

Water is essential for dissolving nutrients and enzymes. Osmotic pressure restricts organisms to certain environments:

• Obligate halophiles: Require high salt concentrations.

• Facultative halophiles: Can tolerate high salt but do not require it.

• Barophiles: Live under extreme pressure; their membranes and enzymes depend on pressure for proper function.

Associations and Biofilms

Microbial Relationships

Microbes often live in association with other species, forming antagonistic, synergistic, or symbiotic relationships. Biofilms are complex communities of microorganisms that develop an extracellular matrix, adhere to surfaces, sequester nutrients, and protect individuals within the biofilm.

Culturing Microorganisms

Obtaining Pure Cultures

Pure cultures are composed of cells arising from a single progenitor, called a colony-forming unit (CFU). Aseptic technique is used to prevent contamination. Common isolation techniques include streak plates and pour plates.

Culture Media

Culture media are used to grow microorganisms in the laboratory. Types include:

• Defined media: Exact chemical composition is known.

• Complex media: Contains nutrients from partial digestion of yeast, beef, soy, or proteins.

• Selective media: Favors growth of particular microbes.

• Differential media: Distinguishes between different types of microbes.

• Anaerobic media: Supports growth of anaerobes.

• Transport media: Used to move specimens without contamination.

Special Culture Techniques

Special techniques include animal and cell culture, low-oxygen culture, and enrichment culture. Candle jars are used to create low-oxygen environments for culturing certain microbes.

Preserving Cultures

• Refrigeration: Short-term storage.

• Deep-freezing: Long-term storage (years).

• Lyophilization: Very long-term storage (decades).

Growth of Microbial Populations

Binary Fission

Most bacteria reproduce by binary fission, a process in which a cell divides into two genetically identical daughter cells.

Generation Time

Generation time is the period required for a bacterial cell to grow and divide. It depends on chemical and physical conditions.

Microbial Growth Curve

A typical microbial growth curve consists of four phases:

• Lag phase: Cells adjust to environment.

• Log (exponential) phase: Rapid cell division.

• Stationary phase: Growth rate slows as nutrients deplete.

• Death (decline) phase: Cells die due to lack of nutrients and accumulation of waste.

Measuring Microbial Reproduction

Direct Methods

• Serial dilution and viable plate counts: Estimate cell numbers by counting colonies after dilution.

• Membrane filtration: Concentrates cells from large volumes for counting.

• Most probable number (MPN): Statistical estimation based on dilution and growth in tubes.

• Microscopic counts: Direct counting under a microscope.

• Electronic counters: Automated cell counting.

Indirect Methods

• Metabolic activity: Measures products of metabolism.

• Dry weight: Measures mass of cells.

• Turbidity: Measures cloudiness of culture.

Genetic Methods

Genetic methods involve isolating DNA sequences of unculturable prokaryotes to estimate their numbers in a sample.

Summary Table: Microbial Nutritional Types

Carbon Source	Energy Source	Type	Examples
CO2 (auto-)	Light (photo-)	Photoautotrophs	Plants, algae, cyanobacteria
CO2 (auto-)	Chemical compounds (chemo-)	Chemoautotrophs	Hydrogen, sulfur, nitrifying bacteria
Organic compounds (hetero-)	Light (photo-)	Photoheterotrophs	Green and purple nonsulfur bacteria
Organic compounds (hetero-)	Chemical compounds (chemo-)	Chemoheterotrophs	Animals, fungi, protozoa, many bacteria

Additional info: Table recreated from image_1 and expanded for clarity.