Laboratory Culture of Microorganisms

Chemistry and Nutrition

Microorganisms require a variety of chemical elements and compounds for growth and metabolism. These nutrients are classified based on the amounts required and their biological roles.

• Nutrients: Substances that supply monomers (or their precursors) required by cells for growth and maintenance.

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

• Micronutrients: Nutrients required in trace amounts, often serving as cofactors for enzymes or in specialized cellular functions.

Macronutrients

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

• Nitrogen (N): Makes up ~13% of a typical bacterial cell by dry weight. It is a key element in proteins, nucleic acids, and many other cell constituents.

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

• Sulfur (S): Found in sulfur-containing amino acids (cysteine and methionine), 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 the heat stability of endospores.

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

Micronutrients

• Iron (Fe): A 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: The most commonly required growth factors, most of which function as coenzymes.

Elemental Composition of Microbial Cells

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

Macromolecular composition of a cell:

Additional info: The periodic table can be used to identify which elements are essential, beneficial, or non-essential for microbial growth. Essential elements are required by all microorganisms, while others may be required only by specific groups.

Media and Laboratory Culture

Culture Media

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

• Defined media: The precise chemical composition is known.

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

By application:

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

• Selective media: Contain compounds that selectively inhibit the 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

• Pure culture: A 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 (such as agar or gelatin).

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

Example: Escherichia coli can be grown on both liquid and solid media. On solid agar plates, individual cells multiply to form visible colonies, which can be used to isolate pure cultures.