Rumen symbioses

Ruminants hold enormous significance for man as a source of milk and meat. Their remarkable ability to convert indigestible plant mass into these digestible food products is the outcome of a symbiosis that resides in the reticulorumen-an anaerobic double-chambered compartment in the ruminant digestive system. The reticulorumen houses a complex microbiota which is responsible for the degradation of plant material consisting mainly of indigestible sugar polymers such as cellulose and hemicelluloses, consequently enabling the conversion of plant fibers into chemical compounds that are absorbed and digested by the animal. This cooperative relationship between the ruminant and its resident ruminal microorganisms evolved over millions of years and has implications for our everyday lives with respect to food, environment, renewable energy and economics. Ruminants hold enormous significance for man as they can convert energy stored in plant mass, which is mainly indigestible for humans, to digestible food products. Because of this trait, ruminants have been extremely important in the evolution of human civilizations via their effects on the development of hunting and agricultural societies (White LA (2007) The evolution of culture: the development of civilization to the fall of Rome. Left Coast Press, Walnut Creek, CA). Today, a significant proportion of domesticated animal species worldwide-the source for most meat and dairy products-are ruminants. Hence, an understanding of this complex ecosystem is of major interest. This chapter discusses several aspects of this ecosystem: the physiology of the ruminant digestive system and its suitability for cooperative interaction with its resident microbiota, the composition of overall rumen metabolism and its role in ruminant well-being, the ruminal microbial populations, their interactions with each other, their importance and effects on the host, and their acquisition after birth.