Bacteria are single-celled organisms, too small to be seen individually without a microscope, yet they are ubiquitous, found in nearly every environment on Earth, including within our own bodies. They reproduce rapidly, often forming visible colonies when provided with the right conditions. Cultivating bacteria at home for a science experiment allows for firsthand observation of these microscopic life forms, offering insights into microbial life and its presence in everyday surroundings.
Prioritizing Safety
Engaging in home-based bacteria cultivation requires careful attention to safety to prevent contamination or potential exposure to harmful microorganisms. Always treat all microorganisms as potentially pathogenic, even those collected from common household surfaces.
Thorough handwashing with soap and water for at least 20 seconds is necessary before and after handling any materials. Avoid eating or drinking near the experiment area, and do not store food where microorganisms are kept. Keep pets and young children away from the designated workspace and all experimental materials to prevent accidental contact or ingestion. Working in a clean, designated area with minimal airflow helps reduce airborne contaminants. Covering any cuts or open wounds on your hands before beginning, and wearing gloves, provides protection.
Gathering Your Supplies
Petri dishes, typically clear plastic or glass, serve as shallow, lidded containers for bacterial growth. Nutrient agar, a gelatinous substance derived from red algae, is the most common medium for supporting the growth of various bacteria and fungi.
Sterile cotton swabs are necessary for collecting samples from different surfaces without introducing contaminants. A sterile environment for preparing agar plates can be achieved by thoroughly cleaning your workspace with rubbing alcohol or a 10% bleach solution. For preparing the agar, a heat source like a microwave or stovetop is needed to dissolve the powder. These items can be sourced from online science supply stores or large general stores.
The Cultivation Process
Begin by preparing the nutrient agar medium, mixing agar powder with distilled water according to manufacturer’s instructions, often around 1.2 grams (½ teaspoon) of agar powder per 60 milliliters (0.25 cups) of hot water. Heat this mixture, stirring continuously, until the agar powder is completely dissolved and the liquid appears clear, usually by boiling for at least one minute. Boiling also helps sterilize it, eliminating most unwanted microorganisms.
Once dissolved, let the agar mixture cool slightly to around 50-55°C, until warm to the touch. Pour the warm liquid agar into sterile petri dishes, filling just enough to cover the bottom (about 10-13 mL for standard 90-100 mm dishes). Work quickly in a clean area to minimize airborne contaminants, lifting the petri dish lid only enough to pour. Immediately replace the lid and gently tilt the dish to ensure the agar evenly coats the entire bottom surface. Allow the agar to solidify at room temperature for at least an hour, or until firm.
With the agar plates prepared, collect samples using sterile cotton swabs. Choose various environmental surfaces like doorknobs, phone screens, or kitchen counters, avoiding harmful sources like raw meat. Gently rub a fresh sterile swab across the chosen surface to collect microorganisms. Use a separate swab for each sample to prevent cross-contamination.
To inoculate the petri dish, carefully lift the lid just enough to access the agar surface. Gently rub the collected sample onto the hardened agar in a zigzag pattern, ensuring the swab makes contact without breaking the gel. Avoid pressing too hard, as this can damage the agar. Immediately replace the lid on the petri dish to prevent further contamination.
After inoculation, seal the petri dishes with tape to secure the lid and prevent accidental opening, but do not make them airtight. Leaving a small gap allows for gas exchange, which supports bacterial growth and can prevent the growth of anaerobic bacteria. Place the sealed petri dishes upside down in a warm, dark place for incubation. An ideal temperature range for many common bacteria is between 20-37°C (70-98°F). Visible bacterial growth should appear within 2-7 days, depending on the sample source and incubation conditions.
Observing and Disposing
Once bacterial colonies have grown, observe your results without opening the petri dishes. Note the different shapes, colors, and sizes of the colonies that have formed on the agar. Different types of microorganisms produce colonies with distinct characteristics, such as circular, irregular, or filamentous shapes, and varying colors like white, yellow, or pink. Document your observations, perhaps by drawing or taking photographs of the sealed dishes.
For safe disposal, deactivate the grown cultures to prevent the release of microorganisms into your home environment. Do not open the petri dishes once growth is observed, as this can release spores or concentrated bacteria into the air. A common deactivation method involves adding a disinfectant, such as a 10% bleach solution, to the sealed dish. Carefully pour the bleach solution into the dish through a small opening or by briefly lifting the lid, ensuring it covers the entire agar surface. Allow the bleach to sit in the sealed dish for at least 20 minutes to thoroughly kill the bacteria. After decontamination, place the sealed and disinfected petri dish into a plastic bag, seal it, and dispose of it in your regular household trash. Always wash your hands thoroughly with soap and water after handling the sealed dishes, even after disinfection.