Bacteria and yeast from sauerkraut. Coloured scanning electron micrograph (SEM) of bacteria and yeast (yellow) cultured from sauerkraut. Fermentation starts when yeast and bacteria that are naturally present on the cabbage and your hands, as well as in the air, come into contact with the sugars in the cabbage. During the fermentation process, beneficial probiotics, or 'live bacteria', are produced, and these probiotics are what give sauerkraut most of its health benefits. Lactic acid bacteria present in sauerkraut are the main source of fermentation. Magnification: x4000 when printed at 10cm wide.