Pasteur demonstrated that fermentation is caused by the growth of micro-organisms, and the emergent growth of bacteria in nutrient broths is due to biogenesis rather than spontaneous generation. He exposed boiled broths to air in vessels that contained a filter to prevent all particles from passing through to the growth medium. Nothing grew in the broths unless the flasks were broken open, showing that the living organisms that grew in broths came from outside rather than spontaneously generated within the broth . This was one of the last and most prominent experiments disproving the theory of spontaneous generation. It also supported germ theory. Although he was not the first to propose germ theory, he developed it and conducted experiments that clearly showed its correctness and managed to convince most of Europe that it was true. Today, he is often called the father of germ theory.