This book represents a practical encyclopedia giving information on the unique characteristics and applications of one of the most useful groups of bacteria. This edition differs from those published previously by covering new data obtained by researchers during the last 25 years. Topics covered include: New phylogenetic taxonomy elaborated by modern methods of molecular biology. The peculiarities of the genetic, antimutagenic and reactive properties of propionibacteria in organisms inactivated by different stress factors. The unusual fermentation mechanism, linked with a short chain of electron transport, which generates more energy than the normal modes of fermentation. New functions of vitamin B12 in constructive metabolism, and the ability of B12-deficient cells to switch from an anaerobic to an aerobic style of existence, including the molecular regulation of this metamorphosis. Production of new bacteriocins and new means of propionic acid production, revealing strains producing large quantities of porphyrins, NO and superoxide dismutase. New bacterial combinations for starters in cheese-making and ensilage, increasing the regions of their practical applications. Immunomodulating properties of propionibacteria, and the probiotic characteristics that make them an object of medical and veterinary interest. The book is addressed not only to microbiologists, but also to general biologists, teachers and manufacturers who wish to learn more about the functions of propionic acid bacteria.