Green, low-cost, and reproducible Lactobacillus-mediated biosynthesis of metal and oxide nanoparticles are reported. Silver and titanium dioxide nanoparticles are synthesized using Lactobacillus sp. procured from yoghurt and probiotic tablets. The synthesis is performed akin to room temperature in the laboratory ambience. X-ray and transmission electron microscopy analyses are performed to ascertain the formation of metallic and oxide nanoparticles. Individual nanoparticles having the dimensions of 10-25 nm (n-Ag) and 10-70 nm (n-TiO(2)) are found. The mechanism involved for the synthesis of metallic and oxide nanoparticles has also been discussed.