Microorganisms are ubiquitous, but scientists cannot properly study them in their natural, mixed environments. To understand how bacteria function, researchers must isolate and grow them in a controlled setting, a process known as microbial culture. This requires providing the necessary nutritional components and a stable physical structure. A petri dish filled with a solid, nutrient-rich medium encourages bacterial multiplication outside its natural habitat. This solid medium transforms an invisible population of microbes into distinct, visible masses for analysis.
What Is Agar and Where Does It Originate?
Agar is a complex carbohydrate, or polysaccharide, that serves as the gelling agent in the culture medium. This long-chain molecule is derived naturally from the cell walls of certain species of red seaweed, belonging to the phylum Rhodophyta. The raw material is typically extracted from genera like Gelidium and Gracilaria, where it functions as a structural component for the algae.
Chemically, agar is primarily a mixture of two components: agarose and agaropectin. Agarose is the linear, neutral fraction of the polymer responsible for the gelling properties. Agaropectin is a more complex, charged molecule containing groups like sulfate, which vary depending on the red algae source.
The Role of Solidification
Agar became the standard for microbial growth media due to its superior physical characteristics compared to other gelling agents, such as gelatin. Agar exhibits thermal hysteresis, meaning there is a significant difference between the temperature at which it melts and the temperature at which it solidifies. It must be heated to approximately 85°C–90°C to dissolve, but it remains liquid until it cools down to about 35°C–40°C before forming a firm gel.
This wide temperature difference is significant because most bacteria are incubated at 37°C. If gelatin were used, the medium would melt at this temperature, but agar remains a stable solid. Furthermore, most bacteria cannot metabolize or break down the agar molecule itself. This inert nature allows the gel to maintain its firm structure and provide a stable surface for bacterial growth throughout the experiment.
The Essential Nutrients in the Medium
Bacteria proliferate within the aqueous, nutrient-rich solution, or broth, held in place by the solidifying agent. The agar functions like a scaffold, turning the liquid broth into a semi-solid surface. This prevents the microbes from washing away or mixing together, allowing for localized growth.
This nutrient broth is a complex mixture designed to meet the diverse metabolic needs of the bacteria. The medium typically contains peptones, which are partially digested proteins providing organic nitrogen, amino acids, and polypeptides. Beef or yeast extract is included to supply water-soluble vitamins, carbohydrates, and necessary growth factors. Salts like sodium chloride are also added to maintain the correct osmotic balance.
Why Isolation Matters
The solid surface created by the agar is necessary for achieving a pure culture, a goal of most microbial studies. When a mixed sample is spread across the plate, the medium physically separates individual bacterial cells. As each isolated cell multiplies, it remains fixed in that location, forming a distinct, visible mass called a colony.
Since a colony is the clone of a single bacterium, scientists can select one and transfer it to a new plate. This technique, often called streak plating, results in a pure culture containing only one species of microorganism. Isolating these pure cultures is necessary for accurately identifying a species, studying its characteristics, and performing reliable tests like determining antibiotic sensitivity.