A stepwise adaptation was carried out on eight sheep through diets containing 10, 24, 44, 60 to a final diet containing 71 % maize meal and molasses. The numbers of protozoa in the rumen increased in proportion to amount of readily fermentable carbohydrate fed, up to and including the 60% grain and molasses diet, while the numbers of total culturable bacteria remained essentially constant. However, the proportions of amylolytic and lactate-utilizing bacteria increased, and there was an orderly shift from acid-sensitive to more acid-tolerant species, particularly amongst the lactate-utilizers in response to the gradual decrease in the ruminal pH. Up to this stage the protozoa probably controlled the rate of fermentation by engulfing starch grains and bacteria and were thus able to maintain the pH of the rumen above 5-5. Lactic acid appeared only transiently and the peak values tended to diminish as adptation progressed.The first day the final diet was fed the ruminal pH decreased to 5-4 or below for several hours. Within 7 days the number of protozoa had decreased by 50-80 % and the number of total culturable bacteria increased sharply. Conditions in the rumen became unstable: peak values of D-and L-laotic acid increased by ca. 0-5 HIM, the acetate/propionate ratio decreased to ca. 2 and peak glucose concentration increased to 3-2-9-5 mM. One animal refused all food for 1 day. Acid-tolerant species of lactateutilizing bacteria multiplied rapidly in response to the increased production of ruminal lactic acid and the ratio of amylolytics to lactate-utilizers decreased from a mean of 10-7 to 3-6. This controlled the increase in lactic acid and the decrease in ruminal pH, allowing the ciliate protozoa to proliferate and regain control of the fermentation.The types of oellulolytic bacteria did not change during the experiment. Despite their acid sensitivity, the number of cellulolytic bacteria per gram of ingesta was of the same order after 54 days on the 71 % grain and molasses diet (0-5-13

Changes in the numbers and types of lactate-producing and lactate-utilizing bacteria in the rumen of sheep were followed during stepwise adaptation from a low- to a high-concentrate diet. The mean numbers of bacteria increased after each change in diet when increasing amounts of maize grain were substituted for maize stover. A surge in number of amylolytic bacteria always preceded an increase in lactate-utilizing bacteria, and with the final diet containing 71% grain and molasses the two groups tended to balance each other, which resulted in low lactic acid accumulation. The lactate utilizers thus played a key role in controlling the fermentation. Orderly shifts occurred among the predominating amylolytic and lactate-utilizing bacteria in response to the gradual decrease in ruminal pH as the amount of maize meal in the diet increased. Among the lactate utilizers, the succession began with acid-sensitive Veillonella and Selenomonas , which were superseded by more acid-tolerant Anaerovibrio and Propionibacterium . Among the amylolytic bacteria, Bacteroides was superseded by more acid-tolerant Lactobacillus and Eubacterium . The ecological succession of predominating genera was shown to be correlated significantly with ruminal pH and, more specifically, with the length of time as well as the extent to which the pH remained below a certain critical undefined value in the rumen, arbitrarily set at pH 6.00.

Three organisms, Clostridium kluyveri (Barker and Taha, 1942), Rhodospirillum rubrm (Kohlmiller and Gest, 1951) and an organism isolated from silage by Rosenberger (1952) and identified as Clotridium scatologenes, have been shown to synthesize fatty acids higher than n-butyrate, as end products of fermentative metabolism. In this paper we give an account of the isolation and properties of an anaerobic, gram negative organism which ferments certain carbohydrates and DL-lactic acid with the formation of hydrogen, carbon dioxide, acetate, propionate, n-butyrate, n-valerate, and n-hexanoate. The organism was isolated from the rumen contents of the sheep. MATERIALS AND METHODS Media. Except where otherwise stated the basal medium contained 0.4 per cent yeast extract (Difco), 0.05 per cent K1H2P04, 0.05 per cent NH4Cl, 0.03 per cent MgCl2.6H20, 0.03 per cent thioglycolic acid in tapwater; the pH was adjusted to 7.4. Medium A consisted of the basal medium plus 2 per cent soluble starch. Medium B consisted of basal medium plus 2 vol per cent of 70 per cent DL-sodium lactate. Rumen contents. The samples of rumen contents used were obtained from Scotch Blackface sheep fitted with rumen cannulae, and fed meadow hay and concentrates. Cultural techniques. Liquid enrichment cultures were carried out in 60-ml glass-stoppered bottles completely filled with medium. Agar plates were incubated in desiccators filled with H25 per cent C02 gas mixture and containing a capsule of