Physiological characteristics of rumen microbes and relation to diet and fermentation patterns.

Control of the rumen fermentation needs a basic knowledge of the rumen microbes. This short paper provides a few examples of work on the types and metabolic characteristics of the organisms and the influence of external conditions on their growth and metabolism. The term ‘fermentation patterns’ in the rumen is generally understood to denote principally the proportions of three volatile fatty acids (VFA) ; acetic, propionic and butyric. The principal source of these fatty acids is carbohydrate fermentation. Lactate and succinate are also fermented, but the organisms responsible for fermentation of the latter are unknown. Deamination of amino acids usually appears to accompany carbohydrate fermentation, and Aciduminococcus fementans, recently isolated by Elias (1971), is the first bacterium deriving energy from amino acid fermentations and unable to ferment carbohydrate known to occur as an appreciable proportion of a rumen flora. Control of carbohydrate fermentation will thus control the rumen fermentation pattern. The growth of bacteria in media with excess of all growth factors except carbohydrate or nitrogen will be considered and it will be assumed that conditions are the same in the rumen but, for instance, inadequate minerals in the food could alter growth of the micro-organisms. Carbohydrate fermentation is a conversion of sugars to VFA, lactic and succinic acids, ethanol, carbon dioxide and hydrogen. Some of these products may be further utilized by other bacteria with formation of VFA. The mixture of organisms producing these fermentation products consists of many types and one might ask whether it is possible to say that certain organisms are responsible for particular fermentation products? Identification of isolated bacteria with bacteria present in the rumen is often difficult because of pleomorphism. One can never be sure that all the main types have been isolated. Because of practical difficulties usually only a few strains of bacteria, grouped on the basis of a few tests, are selected for detailed study and determination of fermentation products. Fermentation products are usually complex and proportions of the different fermentation products often apparently vary widely between strains of a species, and which strain is predominant in the rumen may be difficult to tell. Different products may be formed by the same bacterium in fermenting different substrates, such as lactate or glucose, or even a polysaccharide or the monosaccharide derived from it.