Sauerkraut bacteria. Coloured scanning electron micrograph (SEM) showing the naturally occurring yeast and bacteria on a sample of sauerkraut. Sauerkraut is lightly pickled shredded cabbage. After being barrelled the naturally occurring yeast and bacteria present on the cabbage begin to ferment using the plant's sugars. The barrel must be sealed without any oxygen to reduce the risk of spoiling through contamination with other bacteria and moulds. Magnification: x7000, when printed 15cm wide.