OptiBac Probiotics Bifidobacteria & Fibre - 30 Sachets OptiBac Probiotics Bifidobacteria & Fibre contains 5 billion live cultures as well as 4g of FOS fibres. Bifidobacterium lactis is a high-quality strain that has been scientifically proven to survive stomach acidity and biliary salts to reach the gut alive. FOS is a soluble fibre, which may act as a food source for the body's natural bacteria and as roughage in the bowels. This in turn may help relieve symptoms of constipation and IBS.