Probiotics importance quotes,probiotic use in agriculture university,now foods probiotic 10 ebook,enzymes in the digestive tract and their optimum ph level - Review

Enter your email address to subscribe to this blog and receive notifications of new posts by email. Science, Technology and Medicine open access publisher.Publish, read and share novel research. Use of Yeast Probiotics in Ruminants: Effects and Mechanisms of Action on Rumen pH, Fibre Degradation, and Microbiota According to the DietFrederique Chaucheyras-Durand1, 2, Eric Chevaux1, Cecile Martin3 and Evelyne Forano2[1] Lallemand Animal Nutrition, Blagnac, France[2] INRA UR 454 Microbiologie, Saint-Genes Champanelle, France[3] INRA UMR 1213 Herbivores, Saint-Genes Champanelle, France1. Episode 83: Shawn Wells discusses the good and bad of the supplement industry, red flags when it comes to selecting supplements, markers of good products and the importance of gut health. Shawn Wells, MPH, RD, CISSN has a unique blend of knowledge in the field of performance nutrition and supplementation. As an accomplished author, formulator and clinician, in 2011, Shawn took his experience and passion to become Director of R&D at Dymatize Nutrition. They assist our body in producing vitamins, absorbing minerals and aiding in the elimination of toxins. Feel free to join the discussion by leaving comments, and stay updated by subscribing to the RSS feed. Ruminal pH pattern (solid line) during the 8 days of sampling as affected by live yeast supplementation.The dashed line depicts average ruminal pH.
IntroductionThe valorization of fibrous feed sources by ruminants is possible thanks to their unique digestive system involving an intensive preliminary ruminal fermentation step prior to a more classical enzymatic phase.
Wells has held the role of Chief Clinical Dietitian with over a decade in acute and skilled nursing care, grounding his ethics and practice of patient focused care.
Dymatize Nutrition, now owned by Post, has cemented its role the global leader in finished product research and innovation with over 200 products in more than 50 countries. Although it may take some taste buds a little time getting used to, once you love it you’ll be hooked. Effects of supplementation with a yeast probiotic (Saccharomyces cerevisiae CNCM I-1077) on ?bre (NDF) degradation of maize silages after 36h of incubation in the rumen of cows: open circles, high ?bre degradation group, full circles, low ?bre degradation group.
The reticulo-rumen hosts a highly specialized anaerobic microbial community responsible for fibre breakdown, which is influenced by biochemical and microbial characteristics of the rumen environment. Wells attended UNC-Chapel Hill, earning a Master’s degree in Nutrition and minor in Exercise Science. Fulfilling the position of CEO of Zone Halo Research, a consulting group for supplement formulations, he gained significant notoriety in the industry. Shawn was recently acquired by the top non-GMO & natural dietary supplement company in the industry, BioTRUST Nutrition, as their Vice President of Research and Development. In particular, the role of the different microbial species involved in pH regulation and the influence of feed management are presented in section 2. His education along with credentials of Registered Dietitian, Certified Sport Nutritionist (CISSN), Advisory Board member of the ISSN, presenting at numerous conferences, editing several scientific journals, and authoring textbooks has distinguished him as an expert in the field diet and nutrition. Within a year at BioTRUST, he took on the role of Chief Scientific Officer (CSO) responsible for R&D and Quality Control.
Indeed, intensive farming pratices may disturb the microbial balance due to an excessive high fermentable carbohydrate supply required to sustain high animal performance, and it can turn into metabolic disorders that are likely to impact animal health as reviewed in section 3.
This is one area where yeasts probiotics can help the ruminant and the feed nutritionist optimizing the cows nutrition owing to an increasingly well understood proper mode of action. My favorite brand is GT’s Kombucha that you can find at health food stores and actually most Vons now. Section 4 reports the positive effects these feed additives, under the form of active dry yeast, have on rumen fermentation, feeding behaviour and feed efficiency, as well as tips to properly assess these effects.Once the optimal rumen conditions are set up (section 6), fibre will be efficiently digested. It becomes then interesting to dive into the world of the fibrolytic microbiota in section 5 to truly percieve the unicity of the fibre rumen degradation process, bearing in mind that the nature of fibre will impact its digestibility and subsequent animal production response. In addition to its role on rumen pH stabilization that directly affects the fibrolytic microflora, yeast probiotics represent a valuable tool to optimize cow nutrition as detailed in section 7. These bacteria either have the ability to prevent or encourage the development of many diseases.
However, section 8 will emphasize the yeast strain effect and the need of a viable feed additive to be able to offer a comprehensive solution to ruminants’ diet formulation. Finally, besides the clearly established benefits on rumen management and fibre degradation, live yeast as probiotics are also currently being assessed in other promising fields of applications (section 9).2. Rumen pH : A key parameter linked to rumen functionDue to intense microbial activity, fermentation of feedstuffs in the reticulo-rumen produces a wide range of organic acids. Some of these acids can accumulate and reduce ruminal pH if rumen buffering systems are unable to counteract their impact.
Low rumen pH for prolonged periods can negatively affect feed intake, microbial metabolism, and nutrient degradation, and leads to acidosis, in?ammation, laminitis, diarrhea and milk fat depression. High yielding dairy cows and fattening beef cattle fed diets rich in readily fermentable starch or sugars at high feed intake levels are particularly susceptible to acidosis, and goats, sheep and other ruminants are also prone to the disease. It is now recognized that subacute ruminal acidosis (SARA) affects from 10% to 40% of dairy cattle in a herd, resulting in large ?nancial losses and major concern for animal welfare reasons.
Rumenocentesis has proven to be a more reliable technique for the determination of ruminal pH than oral stomach tubing because saliva contamination is often associated with the stomach tubing technique [3,4]. If rumenocentesis may be done with minimal disturbance [5], frequent sampling raises ethical issues and is not without risk for the animal health. A permanent surgical modification, such as rumen cannulation, and the use of an external data logger connected to a pH probe immerged into the rumen [3,6] have been successful in well controlled research studies to monitor rumen pH kinetics, which allow to better characterize microbial fermentations and predict acidosis situations.
Recently, telemetric boluses able to measure and record rumen pH in cattle continuously have been developed by different companies. When interrogated by wireless, the bolus transmits the recorded data to an operator standing beside the cow with a receiving station. These rumen pH boluses methods offer a simple, accurate and long lasting measurement of pH in intact cattle [7]. They have been successfully applied in controlled animal studies and offer the opportunity to link pH kinetics to measurements in field situations, but clarifications are still needed about the location of the probes (reticulum, rumen) and thereby the representativeness of the measure, their calibration, long-term measure accuracy, and life time.
Microbial mechanisms which lead to pH modulation and acidosis Rumen microbial populations hydrolyze and ferment dietary compounds into volatile fatty acids (VFAs), whose amounts drive pH evolution. Moreover, lactic acid is a common product of carbohydrate fermentation, produced by bacterial species such as Streptococcus bovis, Selenomonas ruminantium, Mitsuokella multiacidus, Lachnospira multipara or Lactobacillus sp. Indeed, it is able of very rapid growth, is acid-resistant and produces extracellular and intracellular amylases which hydrolyze raw starch and soluble starch, respectively [8]. Megasphaera elsdenii is considered as the predominant lactate-utilizing bacterial species in the rumen and can be found in large numbers in the rumen of cereal grain-fed cattle [10].
It exhibits also a lactate racemase activity which is involved in the conversion of D- into L-lactate, which is more easily metabolized. Nevertheless, with high amounts of readily fermentable carbohydrates, or during adaptation from forage to concentrate diets, acid overload of the rumen is possible and may lead to a strong decline in rumen pH, which may trigger acidosis in cattle [1]. Indeed, as rumen pH falls, lactate producers may outnumber lactate utilizers, leading to an accumulation of this metabolite in the rumen.
Effect of the diet on rumen microbiota, microbial fermentations and pH evolutionThe effect of a diet shift (from high forage to high concentrate) on the composition of the rumen microbiota has been extensively studied, in particular since the last 10 years because of the development of culture-independent techniques quantifying microbial abundance and assessing population dynamics. In lambs, the effect of hay vs concentrate diet fed at weaning was studied on abundance of different species of the rumen microbiota [15]. Whereas abundance of total bacteria, measured by qPCR, was significantly higher with concentrate diet than with hay diet, the relative abundance of the fibrolytic species F. The rumen microbiome of dairy cows in which subacute ruminal acidosis (SARA) had been induced with either grain or alfalfa pellets has also been analysed [16].
T-RFLP analysis indicated that the most predominant shift during SARA was a decline in Gram-negative Bacteroidetes organisms. However, the proportion of Bacteroidetes was greater in alfalfa pellet-induced SARA than in mild or severe grain-induced SARA. Indeed, the production of zoospores by Caecomyces have been sharply decreased in vitro at pH 5.5. Moreover, the presence of large amounts of soluble sugars, as with high concentrate diets, may induce saturation of the spore adhesion sites and reduce fungal colonization [22].Changes in the structure of the rumen microbiota are generally accompanied with modifications of fibrolytic activities.
Indeed, compared with a forage diet, cereal grain supplementation induces a decrease in specific and total polysaccharidase activities of the solid-associated microorganisms, whereas the response of glycosidase activities is more variable [19].
A relationship between the decrease in polysaccharidase activities (xylanase, avicelase) of these microorganisms and the decrease in ruminal fibre degradation rate has been found by several authors [23-25]. Indeed, rapidly degradable starch (as in barley or wheat) will more strongly impact rumen pH than slowly degradable starch (as in corn or sorghum).
Figure 1.Relationships between acidogenic potential of feedstuffs and their degradation in sacco.
The effect of 3 dietary challenges differing by the nature and degradation rate of their carbohydrates (wheat, corn or beet pulp) was investigated on rumen pH kinetics and fermentation profile in sheep [28]. Indeed, physically effective Neutral Detergent Fibre (peNDF) represents the physical characteristics of ?bre by accounting for particle length and NDF content, which promote chewing and the ?ow of salivary buffers to the rumen [29]. Yang and Beauchemin [30] compared rumen pH response when short (7.9 mm) or long (19 mm) cut alfalfa silage was included in either high or low concentrate diets.
Consequences of a low rumen pH: Acidosis, inflammation, rumen wall integrity and impact on animal health Acute acidosis occurs after the consumption of an excessive quantity of readily fermentable carbohydrates that rapidly alters ruminal function and can have irreversible metabolic consequences.
Rumen pH values can then drop under 5.0 and trigger metabolic acidosis with an accumulation of D-lactate in the bloodstream.
SARA is probably more difficult to characterize because biological parameters in the rumen fluctuate within physiological limits and are difficult to maintain [31].


This unstable state may reflect the oscillatory behavior of the ruminal microbial population in response to diet-based fermentative jolts. According to Kleen and Canizzo [32], the exact de?nition of SARA remains debatable, but it is certain that SARA is present in a large number of dairy herds.
In Europe, field studies data indicate that SARA prevalence would range between 10 and 30% in dairy herds [32]. The microbial dysbiosis occurring in the rumen during acidosis may trigger the release of potential harmful molecules which may impact the animal health. Indeed, due to an increase of the death and lysis of Gram-negative bacteria under low pH, free lipopolysaccharide (LPS) concentration is increased in the rumen fluid and translocation of this endotoxin can occur across the rumen mucosa [34].
Endotoxin release can trigger an inflammatory response, with an increase in acute phase protein concentrations in peripheral blood [34-37]. Endotoxin is suggested to be involved in metabolic disorders such as laminitis, abomasal displacement, fatty liver or sudden death syndrome [38].
Repeated aggressions by fermentation acids may cause papillar atrophy, diffuse areas of acute or chronic lesions, scars resulting from severe local rumenitis, perforations and mucormycosis which are at the origin of pain, discomfort, as well as erratic feed intake and alteration of rumen function [39]. Low ruminal pH is often associated with increased occurrence of bloat, which is characterized by an accumulation of gas in the rumen and reticulum.
Indeed, frothy bloat is caused by entrapment of gas produced from fermentation of readily digestible feeds (high digestible legumes or cereals). Bloat can impair both digestive and respiratory function, and can occur both in cattle raised on pasture or in confinement [40]. Abscessed livers are generally considered to be associated with both acute and subacute ruminal acidosis. Ulcerative lesions, hairs, and other foreign objects that become embedded in the ruminal epithelium can provide routes of entry into the portal blood for microbes that cause liver abscesses [41]. Diarrhea has been very frequently associated with ruminal acidosis and microbial dysbiosis [1]. Under low rumen pH conditions, erratic feed intake is generally observed but a decrease in intake, mostly on acidogenic feed, has also been reported [44].
In fattening bulls fed high concentrate diets, it has been observed that animals change their feeding behavior to counteract acidosis by spreading their meals over the day [45]. A 10-30% increase in water intake was observed in sheep submitted to acidotic challenges [46]. Water intake could represent a means to dilute acidity but also to reduce rumen fluid viscosity. An increase in salt licking has been also measured in the same study and in goats fed with high concentrate diets [47]. Animals under acidosis would also be able to modify their dietary choice to optimize their digestive comfort.
For example, sheep undergoing successive acidotic challenges were more active and more aggressive towards each other, spent more time standing, adopted alarm postures more often, and reacted more slowly to hot stimulus during the acidosis bouts [46].
The quality of the ingested feed directly matters too where pH turns out below 6 when the rumen digested starch accounts for more than 40% of the diet DM [26].
When the rumen acidity is alleviated with a buffer, total VFA production increases, and so does milk production and milk fat content, especially for high concentrate fed cows. Milk fatty acid profile gives also a good insight of what happened in the rumen and more trans 10-11 C18:1 is well correlated to a depressed milk fat due to its inhibitory effect on de novo fatty acids synthesis in the mammary gland [48]. In addition, the stage of lactation may modulate the animal sensitivity to high-concentrate diet with a better resistance to less optimal rumen fermentation conditions for late lactation cows [49]. However, not only the forage:concentrate ratio matters on rumen pH but the nature or technological process of the grains [50] and the frequency of distribution of the concentrate [51] also do. High fibre diets will not sustain an elevated production of propionate that will negatively impact the milk lactose synthesis and overall milk yield.
The cow will thus mobilize her body fat reserves (ketone bodies metabolized in the liver from butyrate) to compensate for this lack of energy.4. TargetspH evolution is the result of impaired microbial balance and animal compensation mechanisms.
Strategies aiming to induce beneficial effects on the balance of the rumen microbiota and thereby stabilize rumen pH can represent interesting means to reduce the risk of acidosis. How best measuring a probiotic effect on animal performance?Two types of experimental design are basically available to the scientist: contemporaneous or crossover. Parallel designs (i) can be completely randomized design with only one explanatory variable or (ii) randomized complete block design in presence of 2 factors where the experimenter divides animals into subgroups called blocks (eg. In crossover design, each experimental unit receives two or more treatments through time, and as the comparison of treatments is made within subjects, each subject acts as its own control which increases statistical power to detect a direct treatment effect [52] and makes it more efficient than the randomized complete block design. However, there are limitations important to bear in mind amongst with a carryover effect is likely to occur between periods, the latter being able to vary between treatments.The particular nature of probiotics as live microorganisms impacting the rumen flora balance and fermentations make their comparative assessment critical when using experimental design encompassing a carry-over effect.
The inclusion of a washout period between successive treatments is a good way of minimizing the remanent treatment effect over time, but there is good evidence suggesting that the 15-28 days usually applied are not long enough.Indeed, in a complete rumen content transfer study between two cows, Weimer et al. A measurement of methanogens population dynamics over time [54] indicated that 4 weeks were not enough to adapt from the dietary shift of grazing to concentrate. These recent microbial studies support questioning about the relevance of crossover type of designs in assessing probiotics effect on rumen parameters [55]. Experimental proofsStabilization of ruminal pH in the presence of yeast probiotics has been reported by several authors [56-59]. However, the authors admitted that the studies selected for the meta-analysis had used different strains of S. A recent study conducted in a commercial dairy herd [60] compared sodium bicarbonate and live yeast supplementation in 2 pens of 60 cows on milk production and feed efficiency and rumen pH was monitored every 5 min during 5 weeks in 4 cows equipped with a pH probe. Sodium bicarbonate is very often used as an efficient buffer to overcome pH fall in dairy cows.
Modes of action on rumen microbiota and lactate accumulationEffects of live yeasts have been studied on lactate-metabolizing bacteria.
The impact of yeast probiotics on ruminal lactate concentration has been confirmed in in vivo studies. In sheep receiving a live yeast product during their adaptation to a high-concentrate diet, ruminal lactate concentration was significantly lower compared to control animals. Consequently, rumen pH was maintained at values compatible with an efficient rumen function, as shown by higher fibrolytic activities in the rumen of the supplemented animals [24, 65]. In dairy cows, reductions in ruminal lactate concentrations have also been observed with yeast probiotics [66-67].
According to the composition of the diet, the fermentation pattern can be shifted to butyric orientated acidosis [28]. Authors suggested that this strain could act by stimulating ciliate Entodiniomorphid protozoa, which are known to engulf starch granules very rapidly and thus compete effectively with amylolytic bacteria for their substrate [68]. In addition, starch is fermented by protozoa at a slower rate than by amylolytic bacteria and the main end-products of fermentation are VFAs rather than lactate, which may explain why these ciliates had a stabilizing effect in the rumen by delaying fermentation. When ruminants encounter successive acidotic bouts, it is not well known whether live yeast supplementation could alter rumen microbiota and fermentations. Indeed, the severity of acidosis may change with repeated challenges, partly because of modifications in feeding behavior [69], and because of possible shifts in rumen microbial communities leading to selection of the most acid resistant species. Studies in sheep submitted to acidotic challenges showed that cellulolytic bacterial culturable population was greatly decreased after a first acidotic challenge but that after 3 challenges, the level of population came back to normal [70]. Beneficial consequences of yeast probiotics on rumen fermentations, feeding behavior, feed efficiency, and animal productionBach et al. In their multi-analysis reporting data collected from 14 dairy cow trials fed the same live yeast strain, De Ondarza et al. Live yeast supplementation seems to have an effect on intake pattern rather than on intake per se [56].
Milk fat and protein percentages have been found to be slightly but significantly lower in the presence of live yeast [73], but due to the increase in milk yield, yields of milk fat and true protein were higher than in control cows.5. Fibre digestion in the rumen: a key process in ruminant nutritionBy symbiosis with specific micro-organisms, ruminants possess a unique ability to use plant cell wall components as energy and nutrient sources and thereby convert plant biomass into milk, meat, wool and hides. A large proportion of energy intake of ruminant comes in the form of structural complex polysaccharides (cellulose, hemicelluloses, pectins), which are mainly present in the plant cell walls. Indeed, the rumen harbors an abundant and diversified community of bacteria, fungi and protozoa able to thoroughly hydrolyze plant cell wall polysaccharides. Effective degradation is the result of microbial adhesion to plant tissue and production of active enzymatic machinery well adapted to plant cell wall breakdown.
Reducing particle size will increase DMI but the effect on total digested fibre is also related to the quality of the roughage and its nature: legumes NDF is quicker digested than perennial grass NDF despite a higher lignification, but less resistance to breakdown [76].
Particle size also affects the reticulo-omasal passage kinetics along with the intrinsic fragility of the fibre, its density and shape. The importance of particle size on forage rumen degradation has been recently highlighted [77] as the adjustment parameter to increase the available surface area for attachment of ruminal fibrolytic bacteria and protozoa without negatively affecting cellulolytic activity and other fermentation processes in the rumen.Fibre occupies space and limit intake by filling the rumen as they are hollow and therefore fill a bigger volume than their mass indicates. In addition, a fraction of the dietary fibre will remain undigested or slowly degraded and will accelerate the rumen filling [78] reducing thus the entrance of other important ingredients to meet the animal nutritional requirements. Knowing that feed intake is the main predictive variable of milk yield [79], the increase of dietary forage will lead to a milk yield reduction besides isonitrogenous rations [80].
How to measure fibre digestionDifferent methods can be used to measure fibre digestion in the rumen. This compartment is mostly targeted because in general the proportion of fibre which is digested in the hindgut is small.


However, the contribution of the large intestine to plant cell wall digestion may increase with the proportion of cereal in the diet [82].Degradation of dry matter, and NDF fraction of raw materials or more complex mixture of ingredients can be assessed with various in vitro techniques requiring mixed rumen contents [83,84], in situ (nylon bags) kinetics [82,85] or rumen evacuation [86] in rumen cannulated animals, or in non cannulated ruminants (total fecal collection). The measurement of particle sizes in the fecal material using the Penn State forage and total mixed ration particle separator can be of interest to estimate fibre digestibility [60].Fibre degrading functional groups can be enumerated on complex culture media in which a source of polysaccharide is added as sole energy source. Measurement of fibrolytic activities can be performed on pure cultures as well as on rumen contents samples.
After extraction of ruminal microbial enzymes, activities are measured against various polysaccharides and the concentration of reducing sugars released after enzyme action is determined [19].
Microbial communities involved in fibre degradation in the rumenIn the rumen, degradation and fermentation of plant cell wall polysaccharides is achieved by bacteria, protozoa and fungi. The different fibrolytic species, or even strains, are specialized to a various extent in the degradation of specific substrates. The overall effective degradation is the result of these different capacities, related to substrate composition and to interactions existing between these communities and also between the fibrolytic and the non-fibrolytic microorganisms within the ecosystem.
In the Bacteria domain, the cellulolytic function is covered by a very limited number of cultivated species. These species are established a few days after birth in the newborn ruminant, although no solid feed penetrates into the rumen [90]. Indeed, from one week of age, the size of the cellulolytic bacterial community is close to that found in adult animals.
Cellulolytic bacteria are unable to properly colonize the rumen in absence of a complex and diversified bacterial fermentative community [91,92]. In young lambs kept without contact with their dams or other adults, cellulolytic bacteria were not detected in the rumen during three months after birth, which suggests the essential role of newborn-dam contacts in the transmission of rumen microbiota and rumen maturation [92]. Quantitative PCR studies have shown that the main cellulolytic species Fibrobacter succinogenes, Ruminococcus flavefaciens and Ruminococcus albus represent 1-5% of the total bacteria [14, 93] but recent data suggest that these bacteria account for about 50% of the total active cellulolytic bacteria [94].
Other species are considered as secondary fibrolytic species such as Butyrivibrio fibrisolvens and P. However, they possess high carboxymethylcellulose-, xylan- and pectin-degrading activities and probably play an important role in overall fibre digestion [95,96]. The enzymatic equipment of the three main cellulolytic species has been well studied since the last 20 years. The recent whole genome sequencing programs confirm that a huge number of genes is involved in fibre breakdown in each bacterial cell, demonstrating great functional redondancy, which is essential for the good functionning of the ecosystem. Bacteria use different strategies to colonize plant material: for example, Ruminococci exhibit several structures on their cell surface, such as type IV pili and components of glycocalyx.
Moreover, they produce an elaborate cellulosomal enzyme complex that is anchored to the bacterial cell wall [99,100].
Characterization of their ability to directly process plant material have been addressed by diverse strategies, such as direct, biochemical detection of specific fibrolytic enzymes in extracts derived from individual protozoan species [102], or by molecular cloning studies to directly identify protozoal genes encoding enzymes capable of degrading cellulose or hemicellulose [103]. Among protozoa, only Entodiniomorphs (Polyplastron, Eudiplodinium, Epidinium) are considered as cellulolytic.
Ciliates are able to engulf whole plant particles, and digest plant polymers in digestive vacuoles.
They synthesize a well adapted enzymatic equipment composed of cellulases and hemicellulases [104,105]. Up to now, about a dozen of fibrolytic genes have been identified in the various protozoa species. An activity-based metagenomic study of a bovine ruminal protozoan-enriched cDNA expression library identified four novel genes possibly involved in cellulose and xylan degradation [106].
They represent a very homogenous phylogenetic group (phylum Neocallimasticota) and a very specialized functional group as all species are fibrolytic [109].
The fungal biomass is estimated to represent between 5 and 10% of the total microbial mass.
During their life cycle, flagelatted zoospores alternate with filamentous sporangia which are tightly attached to plant tissues, thanks to their cellulosome-like complexes [110]. Rumen fungi produce a very efficient set of cellulases and hemicellulases, whose specific activities are higher than that of bacteria [111]. They also possess esterase activities which contribute to the cleavage of ester bridges which link phenolic compounds of lignin to structural carbohydrates [112,113].
Moreover, thanks to the development of a rhizoidal network they are able to weaken and even disrupt plant tissue which enhances accessibility to digestible structures [114].
Studies carried out with gnotoxenic lambs harbouring or not fungi confirmed their important role in fibre breakdown in the rumen [115].6.
Animal characteristics A cow chews during eating and rumination to reduce feed (forage) particle sizes and allow the best fermentation process possible via a better distribution of feedstuff and bacteria in the rumen as well through rumen pH maintenance (high buffer capacity of the saliva). These results suggest that increased dietary physically effective NDF may affect chewing activity either through prolonging chewing time or increasing chewing rate. Composition of the diet and structure of fibre Many biotic and abiotic factors may limit the efficacy of fibre degradation in the rumen which may be driven by changes in fibre colonization efficacy. For example, the chemical composition of the plant material modulates the rate and extent of fibre digestion [120]. Digestibility of forage fibre (cell walls) has long been known to be negatively associated with lignin concentration. This relationship between lignin and fibre digestibility is very strong for a same forage compared according to different maturity stages, but it is less clear when comparing different forages harvested at a similar maturity stage, so with similar lignin concentrations [121]. To explain the observed variation in fibre digestibility of forages with similar lignin concentrations, composition of lignin and chemical cross-linking of lignin to cell wall polysaccharides have been suggested as involved additional factors.
For example, cross-linking of lignin and arabinoxylans may limit cell wall digestibility by placing lignin in very close proximity to the polysaccharides and preventing physical access by hydrolytic microbial enzymes [120].
The slow entrance of microbial cells into some plant cell tissues such as sclerenchyma and also their slow diffusion capacities down the lumina represent also an important limitation factor for totally efficient fibre digestion [122].Several studies have shown that the feed particle size may in?uence the degradation rate of ?bre fractions as well as the bacterial colonization of the feed particles. They found that Ruminococcus albus was more abundant on short particle size of forage, whereas the xylanolytic Roseburia sp.
From [124].It is generally admitted that most of fibre-degrading microorganisms are sensitive to oxygen because most of them lack detoxification enzymes necessary for removal of reactive oxygen species.
The presence of dissolved oxygen in the rumen ecosystem has been demonstrated [125,126] and oxygen regularly enters the rumen due to feed and water uptake and mastication, which can be illustrated by a greater post-feeding redox potential as measured in dairy cows by Marden et al. Oxygen concentration significantly influenced cellulolytic bacteria, whose numbers were increased by almost 15-fold when low O2 concentrations were applied in the fermenters. Adhesion of cellulolytic bacteria to cellulose has been shown to be inhibited in the presence of oxygen in vitro [129]. Physiology of fibrolytic microorganisms and microbial interactions Among biotic factors, the existence of a complex set of interactions between fibrolytic microbes and the other actors of feed digestion does impact fibre degradation. For example, synergistic cross feeding interactions have been described between cellulolytic and non cellulolytic species which lead to a global improvement in degradation [130]. A relevant example is the interaction between proteolytic bacteria and cellulolytic bacteria, the former releasing ammonia, used as preferential nitrogen source for the latter, and the latter releasing soluble sugars from cellulolysis, which will be metabolized by proteolytic bacteria. Moreover, hydrogen transfer between fibre degrading organisms and hydrogen consuming methanogens is necessary for an optimal functioning of fibre degradation mechanisms. Indeed, methanogens help to reduce the hydrogen partial pressure and thereby avoid the inhibition of ferredoxine oxidoreductase which has an essential role on NADH re-oxidation [130].
The result of this interaction is a gain in energy for both partners and an increase in fibre digestion. On the opposite, competition mechanisms have been described between cellulolytic bacterial species for adhesion on cellulose [131,132]. Secretion of inhibitory peptides by Ruminococcus strains have been shown in vitro to impact growth of rumen fungi [133]. Finally, the physiology of the microorganisms plays also an essential role on overall fibre digestion.
Indeed, there are great differences between species regarding their preference and affinity for substrates, their energy requirements, or their capacity to resist to environmental stresses.7.
TargetsTo optimize fibre digestion, there is a need to minimize the indigestible fibre fraction, maximize rate of fibre digestion, and maintain a ruminal environment that promotes the population of fibre-digesting bacteria. The indigestible fibre in forages (iNDF) is related to lignin concentration, but also contains structural carbohydrates (cellulose and hemicellulose) which are ‘trapped’ with lignin. Whereas lignin, of which biochemical degradation process involves oxidative pathways, is considered not digested in the animal gastro-intestinal tract, the release of the carbohydrates bound to lignin would be interesting in terms of increasing feed value of the forage.To achieve these goals with probiotics, several strategies may be developed depending on the dietary conditions of the animals.
Indirect benefits could be mediated through pH stabilization effects (see section 4), or modification of the environment of the microbiota which will definitely sustain or promote fibre-degrading microbiota and their action on plant cell walls. Modes of action on rumen microbiotaIn vitro, the potential of probiotic yeasts to enhance growth and activity of fibre-degrading rumen microorganisms has been established. Fungal zoospore germination and cellulose degradation were increased in the presence of a strain of S. Callaway and Martin [141] showed that the same yeast could accelerate the rate, but not the extent, of cellulose filter paper degradation by F. Ciliate protozoa, which are not able to establish unless bacterial communities have previously colonized the rumen [142], appeared more rapidly in the rumen of conventional lambs in the presence of live yeasts [143].This supports the hypothesis that live yeast supplementation accelerates maturation of the rumen microbial ecosystem.



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