The Boring Truth About Growing Bacteria for School Projects
I've run way too many bacterial culture experiments for science fairs to care about the hype. The process itself is straightforward, but most students ruin their results before they even finish day two because they don't understand what's actually happening on that agar plate. Let me walk you through what works and what doesn't. At its core, this project involves transferring microorganisms from a surface onto a solid growth medium, then watching what reproduces over 24 to 72 hours. The medium is usually agar powder mixed with nutrients like tryptone and sodium chloride, poured into petri dishes and allowed to solidify. You then swab whatever surface you want to test—doorknobs, phone screens, wallet straps—and streak it across the agar in a controlled pattern. The first thing most people get wrong is the timing. You don't need a fancy incubator. A warm closet or the top of a refrigerator that runs warm works fine for environmental samples. I keep mine at about 30 degrees Celsius, which is warm enough for mesophilic bacteria to grow but not so hot that you're selecting only the fastest growers. If you set it to 37 degrees, which some students do thinking "more heat means more growth," you'll mostly get Staphylococcus and other opportunistic skin flora dominating everything. That's not a bad result, but it's a less interesting one for a fair project.
I once had a student who prepared six plates, swabbed them, and sealed everything tightly with parafilm. Within 18 hours, two of those plates were completely overgrown with fuzzy green mold. The problem wasn't the swabbing technique or the agar quality. It was that he taped the petri dishes shut with regular plastic tape instead of using parafilm, and spores from the classroom air found their way in through the edges. Parafilm stretches and conforms to the dish rim. Regular tape doesn't. Switching to parafilm eliminated the mold entirely on subsequent attempts. That's a detail nobody mentions in the instructions. Here's another thing that catches people off guard: the agar needs to dry before you inoculate it. Freshly poured plates look pristine, but they have a thin layer of condensation on the surface. If you swab immediately, the bacteria spread into a continuous film instead of forming distinct colonies. You want isolated colonies because that's how you can tell different species apart. Let the plates sit open-lidded on the bench for about 30 minutes before you touch them. It sounds like wasted time, but it separates a readable plate from a useless one. For the actual growing process, I use nutrient agar or LB agar. You can buy pre-poured plates from several biological supply companies. If you're making your own, dissolve one packet of LB agar powder in 250 milliliters of distilled water, autoclave or boil for one minute, pour into petri dishes, and let them cool undisturbed for about 45 minutes. The whole process takes roughly 20 minutes of active work plus cooling time. Pre-poured plates from a supplier save you that time entirely and come sterile, which removes one variable from your experiment.
One counter-intuitive point that most beginners miss: slower growth can be more informative. If you incubate at room temperature—around 22 to 25 degrees Celsius—your culture will take longer to show results, maybe three or four days instead of one. But you'll see a much broader variety of colony morphologies. Fast incubation selects for aggressive growers that outcompete everything else. Slower incubation lets the slower species establish themselves. For a science fair project where variety and observation matter more than speed, this is worth considering. There are also downsides to this approach that deserve honesty. Growing bacteria at home or in a classroom means you're culturing unknown environmental organisms. Some of them may be pathogens, even if the risk is low. You should never open an incubated plate after growth appears. Seal everything in a plastic bag before disposal. Autoclaving isn't available to most students, so burying sealed plates in a garden or trash area is the standard workaround. Schools with biosafety level one labs can handle this more rigorously, but most middle and high school projects operate outside that infrastructure. Another limitation: agar plates dry out over time. If your project runs longer than five days, the edges of the plates will crack and the medium will shrink away from the plastic. This doesn't kill your bacteria, but it makes counting colonies and measuring growth zones harder. Work within a three to four day window if you can. If you need longer observation, you can spray a very light mist of sterile distilled water inside the lid area of the dish, but only do this once and don't let condensation form on the agar surface itself.
Get the Full Details
For a competition-ready project, the variable you're testing needs to be clear and measurable. Common approaches include comparing bacterial load on different surfaces, testing the effectiveness of handwashing techniques, or examining how different storage conditions affect food contamination. Whatever you choose, include a control plate that you swab with sterile water instead of a test sample. This proves that any growth you see is coming from your variable and not from contamination in the agar or the environment. If you want pre-made materials, several educational suppliers sell science fair bacterial culture kits. They typically include petri dishes, agar packets, sterile swabs, and labeling markers. A reasonably priced option runs about $30 to $50 for a class set. Individual components are cheaper if you buy them separately, but the convenience of a coordinated kit saves time on sourcing and reduces the chance of missing a supply mid-project. The hardest part of this project isn't the science. It's the patience and the discipline to follow sterile technique every single step. A rushed student produces a plate full of contamination. A careful student produces something that actually answers a question. The difference between those two outcomes is usually whether they wiped the outside of the petri dish with alcohol before opening it, whether theyFlame-sterilized the loop between streaks, and whether they incubated the plates upside down so condensation doesn't drip onto the agar surface.
