Recent Topics in Rumen Microbiology with Particular Reference to Animal Production in Japan

Abstract

Microbes with which ruminants have established symbiotic relationships include bacteria, protozoa and fungi. Microbiological studies in Japan have emphasized the ecology of the ruminal system including the physiological and biochemical interrelationships between individual microbes. The difference in the distribution of various types of microbes in the rumen was studied first and the importance of the attachment of microbes to solid feed substrates was clarified. The existence of substrate-binding factors in bacteria has also been demonstrated. Many genes encoding fibrolytic enzymes have been cloned and characterized from ruminal microbes to improve the digestion of fiber. Some highly fibrolytic bacteria were produced and the survival of recombinant bacteria in the rumen was studied. The manipulation of ruminal bacteria has not been successful and is still at a challenging stage. Both microbial genetics and ecological approaches are needed to improve ruminant productivity through microbial manipulation. As for the study of microbial protein and amino acid metabolism, a particular emphasis was given to the role of ciliate protozoa. The synthesis of lysine from diaminopimelate by protozoa was demonstrated. The metabolic pathways for many kinds of amino acids were well studied, and it was shown that protozoa actively decompose feed protein and excrete amino acids as metabolic products. Therefore, protozoa tended to reduce the microbial amino acid flow to the intestine. A specific role for pipecolic acid in the physiology and behavior of the host animal was postulated. Protozoal function is still an important problem and deserves further research. Concerning the metabolism of carbohydrate, the reduction of ruminal methane production has been studied because methane is viewed as a pollutant greenhouse gas. Some possible methods of reducing methane production with the least adverse effect have been developed. In this review, some recent topics in ruminal microbiology are described.