Sk Asraf Hossain & Nilufar Haque
Dairy Cattle Nutrition Division ;Dairy Cattle Physiology Division; National Dairy Research Institute, Karnal;
Email of corresponding author: email@example.com
The work on feeding of yeast as probiotic was initiated as early as 1950's, although yeast biomass as a byproduct of alcohol industry was earlier used as a protein source (Eckles and Williams, 1925). Yeast was used only at a lower level as a feed supplement, which was later named as probiotic. It was presumed that the yeast supplemented in diet, as a probiotic multiplied in the gastrointestinal tract and released useful products, which may affect nutrient utilization from the feed. Alternately if the yeast culture was not able to multiply in the prevailing conditions of the intestine, at least the components of yeast cells may affect the fermentation process favorably.
Properties of microorganisms for use as feed additive:
The microbial culture which is to be used as a potential probiotic should have the following characteristics (Kamra and Pathak, 2005):
Appropriate selection of microbes to be used as probiotics is a very essential step in the process of development of a probiotic product. The efficiency of a microbial feed additive can be tested in ruminants by conducting production trials. The yeast cells do not survive in the rumen in both in vitro and in vivo experiments however, they remain metabolically active for some time (El Hassan et al., 1993). Newbold et al. (1990) reported that yeast cells can remain metabolically active up to 6 hr in the rumen. These studies indicate that the additive is effective only till it is fed. Therefore a constant feeding of yeast cells to the ruminants appears to be essential to achieve any beneficial effect on the productivity of the animals.
Fungal DFM for ruminants
Fungal DFM have been popular additions to ruminant diets for many years. In general, three types of fungal additives are available. First, some products contain live yeast cultures (LYC) viz: Saccharomyces cerevisiae (Denev, 1996). Second, other additives contain S. cerevisiae and culture extracts, but make no guarantee for live organisms. Third, there are fungal additives based on Aspergillus niger and Aspergillus oryzae fermentation end products that also make no claim for supplying live microbes.
Saccharomyces cerevisiae, the yeast cultures, is considered as a promising probiotics for efficient nutrient utilization in ruminants leading to improvement in animal productivity in terms of growth. A yeast culture (YC) is a yeast-fermented feed additive that contains both live and dead yeast cells (Linn and Raeth-Knight, 2006). In the dairy industry, these products were first used in cow rations to increase dry matter intake during the transition period or periods of stress (Garrett, 2000). Yeast does not grow in rumen fluid but retains metabolic activity and viability (Newbold et al., 1996). The most common YC used in ruminant diets is Saccharomyces cerevisiae.
Dawson (1990) studied the mode of action of live yeast in ruminants. The overall mode of action of live yeast is illustrated briefly in Figure 1.
Fig. 1 Mode of action of live yeast in ruminants
Altered microbial protein synthesis
Altered amino acid profile
of duodenal digesta
Increase feed intake
Stimulation of rumen microbes
Altered VFA production
Mode of action of yeast in the rumen:
Yeast cells are unable to develop in a durable manner in the ruminal milieu, so that they have to be fed every day. However, they can stay alive in the rumen upto about 30 hours. Several modes of action have been demonstrated. Moreover, yeast action can utilize part of free sugar in the rumen and limit a fermentation shift due to rapid degradation of these compounds.
Only live yeast can use ruminal sugars and oxygen. Saccharomyces cerevisiae can carry some metabolites that are useful for ruminal microorganisms. Yeast cultures contain B vitamins, amino acids and organic acids, particularly malate, which stimulates growth of ruminal bacteria that digest cellulose (Callaway and Martin, 1997). Malate has been shown to be a potent growth promotor for lactate-fermenting bacteria in-vitro (Nisbet and Martin, 1991), but is not sufficient to increase the number of ruminal bacteria in-vivo. It is suggested that addition of Yea-sacc to the rumen increases the number of cellulolytic bacteria possibly by assisting hydrogen transfer (Williams and Lyons, 1988). Further, yeast culture provides soluble growth factors that stimulate growth of ruminal bacteria that utilize lactate and digest cellulose.
(1) Oxygen scavenger:
Yeast respiratory activity lowers the redox potential (Dawson et al., 1990). Live yeast use oxygen to metabolize sugars and small oligosaccharides solubilized from feed particles and produce peptides and amino acids as end products which are used by the bacteria in the vicinity of yeast. Because main ruminal microorganisms are strictly anaerobic, removal of oxygen improves anaerobiosis in the microenvironment of solid feed particals, protects anaerobic rumen bacteria from damage by O2, increases the number of cellulolytic bacteria, provide better conditions for the growth of cellulolytic bacteria and improves ruminal digestion (Jouany, 2001).
(2) Supply of growth factors: (Girard, 1996)
Metabolically active yeast cells
Short chain peptides
Stationary phase of bacteria
Protein synthesis in bacteria
Transitition to exponential growth
Increases bacterial growth and activity
(3) pH stabilization:
Yeast increase lactate utilization. Yeast is a source of dicarboxylic acid and malic acid which is utilized by S. ruminantium to produce propionate from lactate. S. Ruminantium HD4 showed 3.8 fold more uptake of lactic acid in presence of yeast (Nisbet and Martin, 1991).
(4) Stimulation of rumen microbes:
Oxygen uptake, supply of growth factors and pH stabilization synergistically act for stimulation of rumen microbes. YC also directly stimulates rumen fungi, which may improve fiber digestion (Chaucheryas et al., 1995). They increase the number of rumen protozoa and neutral detergent fiber digestion (Plata et al., 1994). Addition of yeast culture increases concentration of cellulolytic and total anaerobic bacteria by creating more favorable environment for growth of rumen microbes. Yeast Culture and Aspergillus oryzae stimulates rate of fiber degradation by F. succinogen S85 and R. flavefaciens FD1 (Callaway and Martin, 1997)
(5) Microbial protein synthesis:
There is about 10 % reduction in rumen ammonia concentration and 9.4 % more NAN at duodenum by YC supplementation (Erasmus et al., 1992). Supplementation of 10 g YC /day improves methionine from 13.5 to 14.5 % and lysine from 4.5 to 5.8% in total essential amino acids in duodenal digesta (Putnam et al., 1997).
Improvement in duodenal digesta can occur by two ways:
Overall effects of Yeast on performance of animals:
In ruminants the mode of action and beneficial effects of yeast as microbial feed additive on the performance of animals were summarized by Kamra and Pathak (2005) as:
The ban on antibiotic growth promoters in feed for production of animal foods has increased interest in evaluating the effect of yeast cultures (YC) on the GI ecosystem, rumen microbial populations and function. The effects of YC on animal productivity are strain-dependant. So, all YC preparations are not equivalent in efficiency. This aspect opened a new field of research for new strains, each being more specialized in its use. The goal of many of these research activities has been to define the application and production strategies that can optimize animal responses to YC supplements (Denev et al., 2007).
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