Laboratory Culture of Microorganisms

Chemistry and Nutrition

Microorganisms require a variety of chemical nutrients for growth and metabolism. These nutrients are classified based on the quantity required and their function in cellular processes.

• Nutrients: Supply of monomers (or precursors of monomers) required by cells for growth.

• Macronutrients: Nutrients required in large amounts, essential for the synthesis of cellular components.

• Micronutrients: Nutrients required in trace amounts, often serving as cofactors for enzymatic reactions.

Macronutrients

Macronutrients are the primary elements and compounds needed for microbial growth and are integral to the structure and function of cells.

• Carbon (C): Required by all cells; major element in all classes of macromolecules. A typical bacterial cell is approximately 50% carbon by dry weight. Heterotrophs use organic carbon, while autotrophs use carbon dioxide (CO2).

• Nitrogen (N): Constitutes about 13% of a bacterial cell's dry weight. It is a key element in proteins, nucleic acids, and other cell constituents.

• Phosphorus (P): Essential for the synthesis of nucleic acids and phospholipids.

• Sulfur (S): Found in sulfur-containing amino acids (e.g., cysteine and methionine) and vitamins (e.g., thiamine, biotin, lipoic acid) and coenzyme A.

• Potassium (K): Required by enzymes for activity.

• Magnesium (Mg): Stabilizes ribosomes, membranes, and nucleic acids; also required for many enzymes.

• Calcium (Ca): Helps stabilize cell walls in microbes and plays a key role in heat stability of endospores.

• Sodium (Na): Required by some microbes, such as marine microorganisms.

Micronutrients

Micronutrients are required in much smaller quantities but are vital for specific cellular functions, particularly in enzymatic activity and electron transport.

• Iron (Fe): Key component of cytochromes and FeS proteins involved in electron transport.

• Growth Factors: Organic compounds required in small amounts by certain organisms. Examples include vitamins, amino acids, purines, and pyrimidines.

• Vitamins: Most commonly required growth factors; most function as coenzymes.

Elemental Composition of Microbial Cells

The elemental composition of a typical Escherichia coli cell (by dry weight) is as follows:

• Protein: 55%

• Lipid: 9.1%

• Polysaccharide: 5%

• Lipopolysaccharide: 3.4%

• DNA: 3.1%

• RNA: 20.5%

Major elements include carbon (50%), oxygen (20%), nitrogen (14%), hydrogen (8%), phosphorus (3%), sulfur (1%), and others (4%).

Media and Laboratory Culture

Culture Media

Culture media are nutrient solutions used to grow microbes in the laboratory. They are classified by their chemical composition and intended application.

• Defined Media: Precise chemical composition is known.

• Complex Media: Composed of digests of chemically undefined substances (e.g., yeast and meat extracts).

Media can also be classified by application:

• Enriched Media: Contain complex media plus additional nutrients for nutritionally demanding organisms.

• Selective Media: Contain compounds that selectively inhibit growth of some microbes but not others.

• Differential Media: Contain an indicator, usually a dye, that detects particular chemical reactions occurring during growth.

Pure Culture

A pure culture contains only a single kind of microbe, which is essential for studying the properties of individual species. Contaminants are unwanted organisms in a culture.

• Cells can be grown in liquid or solid culture media.

• Solid media are prepared by addition of a gelling agent (agar or gelatin).

• When grown on solid media, cells form isolated masses called colonies.

Periodic Table and Microbial Nutrition

The periodic table can be used to categorize elements based on their essentiality for microorganisms:

• Essential for all microorganisms

• Essential cations/anions for most microorganisms

• Trace metals, some essential

• Used for special functions

• Unessential, but metabolized

• Unessential, not metabolized

Refer to the table above for specific examples and their roles in microbial metabolism.

Example: Escherichia coli Cell Composition

The elemental composition of an E. coli cell illustrates the relative abundance of major elements required for microbial growth and metabolism.

• Carbon: 50%

• Oxygen: 20%

• Nitrogen: 14%

• Hydrogen: 8%

• Phosphorus: 3%

• Sulfur: 1%

• Other elements: 4%

This distribution reflects the biochemical needs of the cell for macromolecule synthesis and metabolic activity.

Additional info: These notes provide foundational knowledge for understanding microbial metabolism, laboratory culture techniques, and the nutritional requirements of microorganisms, which are essential topics in college-level Microbiology.