Lactobacillus casei for healing those gastrointestinal complaints. Lactobacillus casei is a lactic acid bacteria with a number of strains having been proven to be probiotic. It is found naturally in both the human intestine and the mouth. L casei is resistant to both gastric acid and bile and is able to survive the journey through the gastrointestinal tract.1 Like most probiotics it does not take up permanent residence in the intestines but can be found for around 10 days after being taken.