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Many of the colors you see on the moldy bread are due to the spores the fungi have produced. Molds reproduce by making spores at the end of stalks that rises above the surface of the bread, giving molds a fuzzy appearance. Spores are like seeds-they spread molds to new places so that they can continue to grow. Spores are usually colorful. Some fungi, such as Rhizopus nigricans (rye-zoh-puss neye-grih-cans) and Aspergillus niger (As-per-jill-us neye-jer), make black spores. Neurospora crassa (new-rah-spore-ah crah-sah) produces spores that appear pink. And the Penicillium (pen-ih-sill-ee-um) molds, the molds that make penicillin, are blue-green.
Some of the colors on your bread may be the result of growing colonies of bacteria, which also sometimes grow on old food. For example, a bacterium called Serratia marcescens (ser-ay-shuh mar-seh-sens) forms reddish colonies. You can tell bacteria colonies apart from molds because bacteria colonies appear smooth while molds look fuzzy.