The rumen bacteria

Abstract

This chapter will deal mainly with the characteristics of bacteria from the rumen that have been successfully cultivated in the laboratory. For some ecosystems, particularly those dominated by slow-growing or specialized microorganisms, it has become clear that only a very small fraction (often <1%) of the total microbial diversity has been recovered by cultural methods (Amann et al., 1995) and that descriptions of the ecosystem based on the available isolated strains can be highly misleading. These discrepancies are apparent both from comparison of direct microscopic and culturable counts, and from direct analyses of ribosomal RNA sequence diversity. In the rumen, organisms surviving in significant numbers must have growth rates sufficient to counteract the constant dilution due to turnover of rumen contents, and there are indications that the discrepancies may be less extreme. Leedle et al (1982) found that the culturable count fluctuated with time after feeding between 14% and 74% of the direct microscopic count in the rumens of animals fed on two different diets. Since the viability of several rumen species is known to change upon starvation, the lower figure could partly reflect changes in the viability of known, culturable species. Thus it is to be hoped that the major obstacles to cultivation of the most numerous rumen bacteria have been overcome by the development of sufficiently rigorous anaerobic methods and of suitable isolation media. It remains likely, however, that some functionally important groups (e.g. obligate syntrophs) may not have been recovered; Mclnerney et al (1981) used co-culture with Desulfovibrio in the presence of sulphate to isolate a fatty acid-oxidizing bacterium similar to Syntrophomonas wolfei from bovine rumen contents.