The streak plate technique is a foundational method in microbiology, used to obtain a pure culture from a mixed population of microorganisms. This process involves physically diluting bacteria across a solid growth surface until individual cells are separated and grow into distinct colonies. The streak plate is a composite structure: a physical container (Petri dish) holding a specialized nutrient-rich gel called the culture medium. The combination of these components creates the sterile, stable environment necessary for successful microbial isolation and growth.
The Physical Container: Petri Dish Composition
The physical structure of the plate is a shallow, circular container consisting of a base and a slightly larger lid. Modern laboratories primarily use disposable plates made of clear, high-quality polystyrene plastic. These offer excellent transparency for observing growth and reduce the risk of cross-contamination inherent in cleaning and reusing labware.
Alternatively, some laboratories use reusable Petri dishes made from heat-resistant borosilicate glass. This material can withstand the high temperatures required for repeated sterilization in an autoclave. The lid protects the medium from airborne contaminants and minimizes the dehydration of the nutrient gel inside.
The Solidifying Component: Agar
The culture medium requires a solidifying agent to provide a stable, flat surface for microbial separation and growth. This component is agar, a complex polysaccharide extracted from red algae. Agar is nearly ideal because it is inert; almost no known microorganisms can metabolize it, ensuring the physical structure of the medium remains intact during incubation.
Agar also exhibits thermal hysteresis: it melts at a high temperature (85 to 95°C) but solidifies into a gel at a much lower temperature (32 to 42°C). This wide temperature difference allows the medium to be sterilized by heat and poured into plastic dishes without melting the container. For most general-purpose solid media, agar is incorporated at a concentration of approximately 1.5% to 2.0% by weight.
The Nutritional Components: Base Medium
The nutritional base medium is the water-soluble portion dissolved within the agar gel. This base medium must contain all the necessary chemical elements and compounds to support the growth of the target microorganisms. A general-purpose medium, such as Nutrient Agar or Tryptic Soy Agar, includes several different types of ingredients to meet diverse nutritional needs.
Organic Nutrients (Peptones and Extracts)
A primary source of carbon, nitrogen, and complex organic compounds is provided by protein hydrolysates, commonly referred to as peptones. These are derived from the enzymatic digestion of animal or plant proteins, yielding a mixture of amino acids and small peptides that are easily absorbed by bacteria. The medium also includes various extracts, often derived from yeast or beef, which supply B vitamins, trace elements, and cofactors required for metabolic processes.
Inorganic Salts and Solvent
Simple inorganic salts are also a necessary inclusion. Sodium chloride is frequently added to help maintain an osmotic balance, preventing water from moving too quickly into or out of the bacterial cells. Distilled water acts as the solvent, making up the bulk of the medium and allowing all the nutritional components to remain dissolved and accessible to the growing culture.