In a sense, propionic acid bacteria are domesticated bacteria. They might have been used for cheese making as early as 9000 years Be. In the last 40 years their pmctical uses have expanded to include vitamin B12 and propionic acid production, bread baking, starters for ensilage and some pharmaceutical prepamtions. New prospects for their future uses are also emerging, based on the useful properties recently discovered. This monograph is the result of many years of investigating propioni­ bacteria by the faculty, staff and postgraduate students in the Department of Microbiology at the Moscow State University, as well as a number of scientists in other countries. The encouragement and various contributions of my colleagues has made this book possible, which might as well be entitled "My life with propionic acid bacteria", since these bacteria were the subject of our investigations for more than 40 years. I hope that this book will be of interest not only to scientists of biological specialties, but also to those associated with industrial firms and medical institutions. L.l. Vorobjeva IX Acknowledgements Writing a monograph is impossible without the cooperation of many people I am very grateful to all my postgraduate students - thirty of them - who work at present not only in Russia, but also in various other countries, including Canada, Cuba, Egypt, India, Iran and Vietnam. Many thanks are due to my colleagues who shared my scientific interests and enthusiasm in experimental research. Special thanks are to N. Baranova, E. lordan, N.