A C. elegans plate is a specialized petri dish environment that serves as the standard housing for the nematode Caenorhabditis elegans. This tiny, transparent roundworm is valued in biological research for its simple body plan, rapid life cycle, and well-understood genetics. The plate itself is a carefully constructed ecosystem designed to support all stages of the worm’s life, from egg to adult. This controlled habitat allows scientists to conduct a vast array of experiments in a consistent and reproducible manner.
Components of a Standard Plate
The foundation of a C. elegans plate is a gel-like substance called Nematode Growth Medium (NGM). This medium is primarily composed of agar, a seaweed extract that solidifies the mixture, creating a firm surface for the worms to live on. Peptone is included to provide a rich source of amino acids and nitrogen for the bacteria they consume.
A component added to the NGM is cholesterol, a necessary nutrient for C. elegans growth, development, and reproduction, as they cannot synthesize it themselves. A mixture of salts is also incorporated to maintain the correct osmotic pressure of the medium, ensuring the worms’ cells are not damaged. These salts include:
- Sodium chloride
- Potassium phosphate
- Magnesium sulfate
- Calcium chloride
The final element is a lawn of bacteria, which serves as the primary food source for the worms. A specific strain of Escherichia coli known as OP50 is used. This strain is a uracil auxotroph, meaning its growth is limited on the NGM, preventing the bacterial lawn from becoming too thick, which would obscure the worms from view.
Preparing the Plate for Use
The preparation of a C. elegans plate is a precise process that begins with mixing the NGM ingredients with deionized water in a flask. This mixture is then sterilized in an autoclave, a machine that uses high-pressure steam to kill contaminating microbes. This step ensures that the only bacteria growing on the plate will be the specific food source intentionally added later.
After sterilization, the molten NGM is cooled in a water bath to about 55-60°C. At this stage, the heat-sensitive components, such as cholesterol and the salt solutions, are added. The still-liquid medium is then poured into sterile petri dishes, filling them about two-thirds full, and left at room temperature to solidify. This process is done a couple of days before use to allow excess moisture to evaporate and to confirm no contamination has occurred.
The final step before the plate is ready for worms is called “seeding.” A small volume of liquid culture containing E. coli OP50 is pipetted onto the center of the solidified agar. Care is taken to ensure the bacterial solution does not touch the edges of the plate. The plates are then incubated, allowing the bacteria to grow and spread across the center of the plate, forming a visible “lawn.”
Introducing and Observing Worms
Once the plates are prepared, worms can be introduced for study. The most common method for transferring individual nematodes is a technique called “worm picking.” This is performed using a simple tool, often handmade in the lab, consisting of a short piece of platinum wire melted into the end of a glass pipette, which is flattened at the tip to create a small, spatula-like surface.
To move a worm, a scientist looks through a dissecting stereomicroscope, which provides the necessary magnification to see the millimeter-long, transparent animals. The worm pick is used to gently lift a single worm off the surface of an existing plate. The worm adheres to the pick, allowing it to be carefully transferred and placed onto the fresh bacterial lawn of a new plate.
This process allows for the precise selection and transfer of worms of a specific age or genetic makeup for setting up controlled experiments. The microscope is not only used for picking but is the primary tool for all observations. Researchers can monitor the worms’ movement, development, egg-laying, and any physical or behavioral changes directly on the plate’s surface.
Experimental Applications
Lifespan assays are a common application where researchers track a population of worms from birth until death. By placing a synchronized population of worms on a plate, scientists can count the number of living individuals daily. This determines the average lifespan and how it might be affected by specific genes or drug treatments.
Stress resistance tests are another frequent use for these plates. Worms on a plate can be subjected to various environmental stressors, such as high temperatures or exposure to toxic chemicals mixed into the agar. Researchers can then observe how long the worms survive or monitor for specific physiological responses, providing insight into the biological pathways that protect against cellular damage.
Genetic screens are also widely performed on NGM plates. After exposing worms to mutagens or using techniques like RNA interference (RNAi) to silence specific genes, scientists can screen large numbers of their progeny for visible changes, or phenotypes. The solid surface of the plate allows for the examination of individual worms for alterations in size, shape, movement, or development, helping to connect genes to their functions.