Developmental microbial ecology of the neonatal gastrointestinal tract

Ruminant animals have two metabolic systems that differ in their nutrient requirements: microbial metabolism in the rumen and mammalian metabolism in the tissues. Maximizing or optimizing ruminant productivity involves meeting requirements in proper amounts and balance for both metabolic systems. Ruminant nutritionists formulate diets with an intention of providing the animal with optimal levels of nutrients to achieve the optimal or maximal level of performance. Ideally, most of the dietary constituents should be digested, absorbed and utilized by the tissues as completely as possible. However, in reality, feed components are digested incompletely, more so with feedstuffs fed to ruminants than to non-ruminants. In ruminants, nutrient inputs are subjected first to fermentative digestion by microorganisms and then to glandular digestion by the host. Fermentative digestion is advantageous for substrates that cannot be digested by the host enzymes but is inefficient for digesting proteins, amino acids and sugars, because of losses in energy and nitrogen. Therefore, a proper balance between fermentative and glandular digestion needs to be achieved for optimal productivity.

Manipulation of ruminal fermentation

Effects of two microbial feed supplements on microbial activities in rumen-stimulating cultures and the rumens of steers fed a fescue hay-based roughage diet were evaluated. The yeast culture supplement contained Saccharomyces cerevisiae (1.4 to 4.2 x 10(9) colony-forming units [cfu]/g), whereas the mixed microbial supplement contained yeast, lactobacilli and enterococci (1.4 to 2.7 x 10(9) cfu/g, 1.2 to 2.3 x 10(9) cfu/g, and 1.5 to 2.6 x 10(10) cfu/g, respectively). Concentrations of viable yeast cells were increased consistently in continuous cultures and rumens of steers receiving either supplement (1 g/kg of feed). However, neither supplement consistently altered the relative concentrations of volatile fatty acids or ammonia in continuous cultures and rumens of steers. The pH tended to be greater (P = .13) in continuous cultures receiving yeast culture supplement than in cultures receiving the unsupplemented diet (6.50 vs 6.36), but pH in the rumens of steers was not affected by the supplements. Concentrations of cellulolytic microorganisms in cultures and the rumens of steers receiving supplements containing only yeast were from 5 to 40 times greater than those observed in cultures or steers receiving the unsupplemented diet. Supplements that had been treated with heat (121 degrees C for 15 min) to inactive yeast cells did not alter the concentrations of cellulolytic bacteria in rumen-stimulating cultures. These results suggest that live yeast culture supplements stimulate growth of cellulolytic microorganisms in the rumen.

Effects of microbial supplements containing yeast and lactobacilli on roughage-fed ruminal microbial activities

Yeasts and aerobic fungi have long been known to be normal inhabitants of the rumen (Clarke and DiMenna, 1961; Sivers, 1962a, b; Lund, 1974) but most species isolated are considered to be transient and non-functional, entering the rumen with the feed. Some aerobic fungi are capable of growth under anaerobic conditions, and Brewer et al. (1972) concluded that Aspergillus fumigatus, Mucor rouxii and Sporormia minima, which are implicated as causative agents in ovine ill-thrift, could survive in the rumen. Two other groups of fungi are now known to occur, one group parasitic upon ciliate protozoa, the other saprophytic on plant tissues.

The rumen anaerobic fungi

Sheep fed the forage Digitaria pentzii fertilized with sulfur were compared with those fed unfertilized forage for the rumen microbial population involved with fiber degradation. No differences were detected in the bacterial population as determined by anaerobic cultures on a habitat-simulating medium, xylan, or pectin, by 35S labeling techniques for microbial protein, or by transmission electron microscopic studies of bacterium-fiber interactions. Rumen volume and water flow from the rumen were not different for sheep fed each of the forages. Rumen fungi were prevalent in sheep fed sulfur-fertilized D. pentzii as shown by sporangia adhering to forage fiber and by colonies developing from zoospores in roll tubes with cellobiose plus streptomycin and penicillin. Fungi were absent or in extremely small numbers in sheep fed unfertilized forage. Nylon bag digestibility studies showed that the fungi preferentially colonized the lignified cells of blade sclerenchyma by 6 h and caused extensive degradation by 24 h. In the absence of bacteria in in vitro studies, extensive hyphal development occurred; other lignified tissues in blades (i.e., mestome sheath and xylem) were attacked, resulting in a residue with partially degraded and weakened cell walls. Nonlignified tissues were also degraded. Breaking force tests of leaf blades incubated in vitro with penicillin and streptomycin and rumen fluid from sheep fed sulfur-fertilized forage or within nylon bags in such sheep showed a residue at least twice as fragile as that from sheep fed unfertilized forage. In vitro tests for dry matter loss showed that rumen fungi, in the absence of actively growing bacteria, could remove about 62% of the forage material. The response of rumen fungi in sheep fed sulfur-fertilized D. pentzii afforded a useful in vivo test to study the role of these microbes in fiber degradation. Our data establish that rumen fungi can be significant degraders of fiber and further establish a unique role for them in attacking and weakening lignocellulosic tissues. The more fragile residues resulting from attack by fungi could explain the greater intake consistently observed by sheep eating sulfur-fertilized compared with unfertilized D. pentzii forage.

Rumen bacterial and fungal degradation of Digitaria pentzii grown with or without sulfur.

Summary: Only small numbers of fungi were found in the rumen fluid of cattle cultured on agar plates at 39°C, the counts being up to 3500 yeast colonies/ml, with a similar number of mould colonies. However, considerably larger numbers of yeast colonies appeared on plates incubated at 25°C. Nine species of yeasts were isolated belonging to Candida (including corresponding species of Pichia), Trichosporon, Torulopsis, Kluyveromyces, Saccharomycopsis, and Hansenula. The predominating species were Candida krusei, Trichosporon cutaneum and Trichosporon capitatum. The most common moulds were members of the Mucoraceae, of which Absidia corymbifera, A. ramosa, and Mucor pusillus were identified. Aspergillus fumigatus was isolated frequently. The fungal content of rumen fluid seemed to be dependent on the diet of the animal, and no particular fungal flora could be associated with the rumen per se. The predominating Candida krusei, and also the rarely-isolated species Torulopsis pintolopesii and Kluyveromyces bulgaricus, could reproduce under anaerobic conditions in vitro, but another commonly occurring yeast, Trichosporon capitatum, was unable to grow under the same conditions. The majority of yeast cells were obviously destroyed during passage through the alimentary tract, whereas large quantities of moulds could be excreted in a viable state.

A. Lund

Papers

Yeasts and Moulds in the Bovine Rumen