Microbial Metabolism: Nutritional Requirements and Laboratory Culture

Microbial Metabolism

Laboratory Culture of Microorganisms

Microorganisms require specific nutrients and conditions for growth, which are provided in laboratory culture media. Understanding the chemistry and nutrition of microbes is essential for cultivating and studying them.

• Nutrients: Substances that supply monomers or precursors required by cells for growth and metabolism.

• Macronutrients: Nutrients required in large amounts, essential for cellular structure and function.

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

Macronutrients

Macronutrients are vital elements needed by all cells, forming the major components of macromolecules.

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

• Nitrogen (N): About 13% of bacterial cell dry weight; found in proteins, nucleic acids, and other cell constituents.

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

• Sulfur (S): Present in sulfur-containing amino acids (cysteine, 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; required for many enzymes.

• Calcium (Ca): Helps stabilize cell walls in microbes; 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 essential for specific cellular functions.

• 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 and Periodic Table

The elemental composition of microbial cells reflects their nutritional requirements. The periodic table can be used to classify elements based on their essentiality for microorganisms.

Macromolecule	Percent of Dry Weight
Protein	55
Lipid	9.1
Polysaccharide	5.0
Lipopolysaccharide	3.4
DNA	3.1
RNA	20.5

Elemental composition of an E. coli cell (dry weight):

• 50% Carbon

• 20% Oxygen

• 14% Nitrogen

• 8% Hydrogen

• 3% Phosphorus

• 2% Sulfur, K, Mg, Ca, Cl

• 0.3% Other elements

Media and Laboratory Culture

Microbial growth in the laboratory requires nutrient solutions called culture media, which can be classified by their composition and application.

• Defined Media: Precise chemical composition is known.

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

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

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

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

Pure Culture Techniques

Obtaining pure cultures is essential for studying individual microbial species.

• Pure Culture: Culture containing only a single kind of microbe.

• Contaminants: 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 (colonies).