Lactobacillus casei bacteria. Scanning electron micrograph (SEM) of the bacteria Lactobacillus casei. L. casei is a bacterium that is found in the human intestines and mouth. It's most common application is in the production of dairy products, specifically yoghurts. Some of the L. casei bacteria are stated, by their producers, to be probiotic and beneficial to the intestinal microbial balance of the host but this has yet to be proven conclusively. Magnification x50000 (x12500 at 10cm wide).