A seventh-grade student has turned ordinary eggshells into crystals that slowed the growth of bacteria in her science project.Shrikari Punjala extracted calcium carbonate from eggshells using acetic acid. She then tested the crystals on four bacterial species: Bacillus subtilis, E. coli, Aeromonas sobria and Sporosarcina ureae.The bacteria were grown on agar plates and kept at 30 degrees Celsius for three days. After the calcium carbonate crystals were added, the bacterial coverage fell in all four species, according to the Society for Science.Coverage stood at 55 per cent for A. sobria, 65 per cent for S. ureae, 66.25 per cent for E. coli and 71.25 per cent for B. subtilis. The society concluded the calcium carbonate inhibited bacterial growth.The profile does not give starting coverage or plate counts, and the findings do not show how the crystals would work on other bacteria or under different conditions.Eggshells are mostly made of calcium carbonate. The project shows how food waste could be studied for useful properties.