The populations of dominant species within the human colonic microbiota can potentially be modified by dietary intake with consequences for health. Here we examined the influence of precisely controlled diets in 14 overweight men. Volunteers were provided successively with a control diet, diets high in resistant starch (RS) or non-starch polysaccharides (NSPs) and a reduced carbohydrate weight loss (WL) diet, over 10 weeks. Analysis of 16S rRNA sequences in stool samples of six volunteers detected 320 phylotypes (defined at 498% identity) of which 26, including 19 cultured species, each accounted for 41% of sequences. Although samples clustered more strongly by individual than by diet, time courses obtained by targeted qPCR revealed that 'blooms' in specific bacterial groups occurred rapidly after a dietary change. These were rapidly reversed by the subsequent diet. Relatives of Ruminococcus bromii (R-ruminococci) increased in most volunteers on the RS diet, accounting for a mean of 17% of total bacteria compared with 3.8% on the NSP diet, whereas the uncultured Oscillibacter group increased on the RS and WL diets. Relatives of Eubacterium rectale increased on RS (to mean 10.1%) but decreased, along with Collinsella aerofaciens, on WL. Inter-individual variation was marked, however, with 460% of RS remaining unfermented in two volunteers on the RS diet, compared to o4% in the other 12 volunteers; these two individuals also showed low numbers of R-ruminococci (o1%). Dietary non-digestible carbohydrate can produce marked changes in the gut microbiota, but these depend on the initial composition of an individual's gut microbiota.

A commercial blend of essential oil (EO) compounds was added to a grass, maize silage, and concentrate diet fed to dairy cattle in order to determine their influence on protein metabolism by ruminal microorganisms. EO inhibited (P < 0.05) the rate of deamination of amino acids. Pure-culture studies indicated that the species most sensitive to EO were ammonia-hyperproducing bacteria and anaerobic fungi.The drive toward a decrease in the use of antibiotics in animal production has intensified the search for natural products that enhance production, in the case of ruminants, by modulating ruminal fermentation. Essential oils (EO) are natural products that give plants and spices their characteristic odor and color. Oh et al. (12) found that EO from the Douglas fir pine needle have an antibacterial effect in the rumens of sheep and deer. Fernandez et al. (6) used a commercial blend of EO compounds to manipulate rumen fermentation, inhibiting the breakdown of protein, thus potentially increasing the dietary protein available to the ruminant. The aim of the present study was to investigate the influence of EO on protein metabolism by mixed ruminal microorganisms and on different ruminal bacteria, protozoa, and fungi.In order to identify the mechanisms and microbial species affected by EO, the effects of EO were measured in ruminal microorganisms obtained from four Holstein-Friesian cows, each fitted with a ruminal cannula and fed ad libitum a diet consisting of grass, maize silage, and concentrate mix (330, 220, and 450 g kg Ϫ1 on a dry-matter basis). The concentrate contained soybean meal, rapeseed meal, sunflower meal, maize gluten feed, maize distillers grain, cottonseed meal, ground wheat, palm kernel meal, molassed sugar beet pulp, wheat feed, and minerals (106, 106,

Two suggested modes of action of yeast in stimulating rumen fermentation were investigated. The first, that yeast respiratory activity protects anaerobic rumen bacteria from damage by 0,, was tested using different strains of yeast that had previously been shown to have differing abilities to increase the viable count of rumen bacteria. Succhurumyces cerevisiue NCYC 240, NCYC 1026, and the commercial product Yea-Sac@, added to rumen fluid in vitro at 1-3 mg/ml, increased the rate of 0, disappearance by between 46 and 89 YO. The same three preparations also stimulated bacterial numbers in an in vitru fermenter (Rusitec). S. cerevisiae NCYC 694 and NCYC 1088, which had no influence on the viable count in Rusitec, also had no effect on 0, uptake. Respirationdeficient (RD) mutants of S. cerevisiue NCYC 240 and NCYC 1026 were enriched by repeated culturing in the presence of ethidium bromide. S. cerevisiue NCYC 240 and NCYC 1026 stimulated the total and cellulolytic bacterial populations in Rusitec, while the corresponding RD mutants did not. Rigorous precautions to exclude air from Rusitec resulted in S. cevevisiue NCYC 240 no longer stimulating total bacterial numbers, although it still increased numbers of cellulolytic bacteria. The second hypothesis, that yeast provides malic and other dicarboxylic acids which stimulate the growtb of some rumen bacteria, was examined by comparing the effects of yeast and malic acid on rumen fermentation in sheep. Three mature sheep were given 085 kg barley/d plus 0.55 kg chopped ryegrass hay/d either unsupplemented, or supplemented with 4 g S. cerevisiue NCYC 240/d or 100 mg L-malic acid/d either mixed with the diet or in aqueous solution infused continuously into the rumen. Yeast increased the total viable count of bacteria (P < 0.05) whereas malic acid did not, and no other effect of the treatments reached statistical significance. It was concluded, therefore, that the stimulation of rumen bacteria by S. cerevisiue is at least partly dependent on its respiratory activity, and is not mediated by malic acid.

There is increasing interest in exploiting natural products as feed additives to solve problems in animal nutrition and livestock production. Essential oils and saponins are two types of plant secondary compounds that hold promise as natural feed additives for ruminants. This paper describes recent advances in research into these additives. The research has generally concentrated on protein metabolism. Dietary essential oils caused rates of NH 3 production from amino acids in ruminal fluid taken from sheep and cattle receiving the oils to decrease, yet proteinase and peptidase activities were unchanged. Hyper-ammonia-producing (HAP) bacteria were the most sensitive of ruminal bacteria to essential oils in pure culture. Essential oils also slowed colonisation and digestion of some feedstuffs. Ruminobacter amylophilus may be a key organism in mediating these effects. Saponin-containing plants and their extracts appear to be useful as a means of suppressing the bacteriolytic activity of rumen ciliate protozoa and thereby enhancing total microbial protein flow from the rumen. The effects of some saponins seems to be transient, which may stem from the hydrolysis of saponins to their corresponding sapogenin aglycones, which are much less toxic to protozoa. Saponins also have selective antibacterial effects which may prove useful in, for example, controlling starch digestion. These studies illustrate that plant secondary compounds, of which essential oils and saponins comprise a small proportion, have great potential as 'natural' manipulators of rumen fermentation, to the potential benefit of the farmer and the environment.

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