The lactobacilli produce themselves organic acids, hydrogen peroxide and antibacterial peptides (lactocidin, acidophilin, lactacin B, etc.). Most probiotics are producing lactic acid, which lowers the local pH and thus prevents the growth of sensitive bacteria in acid and renders permeable the outer membrane Gram-negative bacteria. Although many studies have been conducted on animals, the effects of lactic acid bacteria on human subjects remain controversial. However, there is evidence of their usefulness.