Probiotics are live, active bacteria such as bifidobacteria and lactobacilli that enter the gut by ingesting foods and supplements. As they are live bacteria, they can be affected by heat and acidity and can be destroyed on their way to the large intestine. Probiotic bacteria are live strains of bacteria which are found in fermented foods. Probiotic bacteria must be kept alive and must also reach the gut in sufficient numbers. Those reaching the lower gut must compete with over 1,000 bacterial species.