Skatole Formation Research Articles

Magnolol-driven microbiota modulation elicits changes in tryptophan metabolism resulting in reduced skatole formation in pigs

Skatole of gut origin has garnered significant attention as a malodorous pollutant due to its escalating emissions, recalcitrance to biodegradation and harm to animal and human health. Magnolol is a health-promoting polyphenol with potential to considerably mitigate the skatole production in the intestines. To investigate the impact of magnolol and its underlying mechanism on the skatole formation, in vivo and in vitro experiments were conducted in pigs. Our results revealed that skatole concentrations in the cecum, colon, and faeces decreased by 58.24% (P = 0.088), 44.98% (P < 0.05) and 43.52% (P < 0.05), respectively, following magnolol supplementation. Magnolol supplementation significantly decreased the abundance of Lachnospira, Faecalibacterium, Paramuribaculum, Faecalimonas, Desulfovibrio, Bariatricus, and Mogibacterium within the colon (P < 0.05). Moreover, a strong positive correlation (P < 0.05) between skatole concentration and Desulfovibrio abundance was observed. Subsequent in silico studies showed that magnolol could dock well with indolepyruvate decarboxylase (IPDC) within Desulfovibrio. Further in vitro investigation unveiled that magnolol addition led to less indole-3-pyruvate diverted towards the oxidative skatole pathway by the potential docking of magnolol towards IPDC, thereby diminishing the conversion of substrate into skatole. Our findings offer novel targets and strategies for mitigating skatole emission from the source.

Mechanistic insight into the role of immunocastration on eliminating skatole in boars

The accumulation of skatole in fat tissue is one of the predominant factors, causing boar taint. The present study was aimed to understand the mechanism whereby active immunization against GnRH (immunocastration) eliminates skatole in boars. Thirty-six boars were assigned within litter into three groups (n = 12): control, surgically castrated, or immunized against GnRH at 10 wk of age (with a booster 8 wk later). Faecal and blood samples (for skatole and skatole-regulatory hormone profiles) were collected at 4-wk intervals until boars were slaughtered (26 weeks). Immunocastration reduced (P < 0.05) serum levels of androstenone, 17β-estradiol and IGF1 especially after the booster immunization, and down-regulated (P < 0.05) mRNA expressions of both IGF1 and IGF1receptor (IGF1R) in mucosa of ileum as well as colon at slaughter. Compared to intact controls, immunocastration substantially decreased (P < 0.05) faecal skatole contents subsequent to the decrease of serum IGF1 levels, which persisted in boars after surgical castration. In parallel with the decreased formation of skatole in the intestine, levels of skatole in serum and then in fat tissue were also decreased (P < 0.05). On the other hand, deprivation of testicular steroids, especially androstenone and 17β-estradiol accelerated skatole degradation metabolism in the liver by increasing (P < 0.05) hepatic CYP2E1, CYP2A, CYP2C49 and CYB5A expressions. Collectively, our results suggested that immunocastration decreased skatole formation in the intestine and meanwhile accelerated skatole degradation metabolism in the liver, resultantly eliminating skatole accumulation in male pigs. Decreased intestinal skatole formation by immunocastration appeared to be associated with the attenuated actions of IGF1 on the turnover of both ileal and colon mucosa.

The Mode of Action of Chicory Roots on Skatole Production in Entire Male Pigs Is neither via Reducing the Population of Skatole-Producing Bacteria nor via Increased Butyrate Production in the Hindgut.

The effect of high levels of dietary chicory roots (25%) and intracecal exogenous butyrate infusion on skatole formation and gut microbiota was investigated in order to clarify the mechanisms underlying the known reducing effect of chicory roots on skatole production in entire male pigs. A Latin square design with 3 treatments (control, chicory, and butyrate), 3 periods, and 6 animals was carried out. Chicory roots showed the lowest numerical levels of skatole in both feces and plasma and butyrate infusion the highest. In the chicory group, an increased abundance of the skatole-producing bacterium Olsenella scatoligenes compared to the control group (P = 0.06), and a numerically higher relative abundance of Olsenella than for the control and butyrate groups, was observed. Regarding butyrate-producing bacteria, the chicory group had lower abundance of Roseburia but a numerically higher abundance of Megasphaera than the control group. Lower species richness was found in the chicory group than in the butyrate group. Moreover, beta diversity revealed that the chicory group formed a distinct cluster, whereas the control and butyrate groups clustered more closely to each other. The current data indicated that the skatole-reducing effect of chicory roots is neither via inhibition of cell apoptosis by butyrate nor via suppression of skatole-producing bacteria in the pig hindgut. Thus, the mode of action is most likely through increased microbial activity with a corresponding high incorporation of amino acids into bacterial biomass, and thereby suppressed conversion of tryptophan into skatole, as indicated in the literature.IMPORTANCE Castration is practiced to avoid the development of boar taint, which negatively affects the taste and odor of pork, and undesirable aggressive behavior. Due to animal welfare issues, alternatives to surgical castration are sought, though. Boar taint is a result of high concentrations of skatole and androstenone in back fat. Skatole is produced by microbial fermentation in the large intestine, and therefore, its production can be influenced by manipulation of the microbiota. Highly fermentable dietary fiber reduces skatole production. However, various theories have been proposed to explain the mode of action. In order to search for other alternatives, more efficient or less expensive, to reduce skatole via feeding, it is important to elucidate the mechanism behind the observed effect of highly fermentable dietary fiber on skatole. Our results indicate that highly fermentable dietary fiber does not affect skatole production by reducing the number of skatole-producing bacteria or stimulating butyrate production in the large intestine.

Indoleacetate decarboxylase is a glycyl radical enzyme catalysing the formation of malodorant skatole

Skatole is a malodorous compound that contributes to the characteristic smell of animal faeces. Although skatole has long been known to originate from bacterial tryptophan fermentation, the enzyme catalysing its formation has so far remained elusive. Here we report the use of comparative genomics for the discovery of indoleacetate decarboxylase, an O2-sensitive glycyl radical enzyme catalysing the decarboxylation of indoleacetate to form skatole as the terminal step of tryptophan fermentation in certain anaerobic bacteria. We describe its biochemical characterization and compare it to other glycyl radical decarboxylases. Indoleacetate decarboxylase may serve as a genetic marker for the identification of skatole-producing environmental and human-associated bacteria, with impacts on human health and the livestock industry.

Dietary approaches reducing boar taint-Importance of Lawsonia intracellularis colonisation for interpreting results.

In the fattening of male pigs, boar odour is a major problem with regard to the acceptance of the meat by consumers. Skatole can be one cause. Tryptophan from non-digested feed ingredients and intestinal cell debris can be the precursor in skatole formation. Lawsonia intracellularis, one of the most widespread pathogens in swine, promotes the epithelial cell turnover and might favour the tryptophan influx into the hindgut. Therefore, the question arises how far the severity of a Lawsoniaintracellularis infection has an effect on results of dietary experiments with specific issues. Fifty finishing boars from a specific pathogen-free farm were randomly allotted to ten boxes in five feeding groups. Natural developing Lawsoniaintracellularis colonisation was monitored serologically (twice individually) and molecular biologically (weekly individually). Over 4weeks, animals were fed either a finely ground pelleted diet (FP), a coarsely ground meal diet (CM), a meal diet either with 22% cracked corn (CORN), 16.9% dried whey (WHEY) or 30% raw potato starch (RPS). Fifty % of animals showing lower differences in serological Lawsoniaintracellularis values between the start and the end of the trial were characterised by a higher dry matter content in faeces (256±29.4 vs. 239±23.6g/kg). Lawsonia intracellularis-negative caecal samples showed the highest butyrate concentrations (27.2±7.53mmol/kg). Lawsonia intracellularis-negative faecal samples of group FP showed the highest DM levels in faeces (neg: 290±46.1/pos: 250±52.2g/kg); negative samples from group RPS had the lowest values (217±24.4g/kg). Lawsonia intracellularis-negative faecal samples from the group CM were lower in skatole than positive samples (82.8±32.8 vs. 119±29.3μg/g DM). RPS group samples without pathogen detection had the lowest skatole concentrations (30.5±36.3μg/g DM). This study provides first evidence that clinically unremarkable colonisation with intestinal pathogens might influence the results of dietary approaches.

Nutritional Influences on Skatole Formation and Skatole Metabolism in the Pig.

SummarySkatole is a tryptophan metabolite with fecal odor. Skatole and the testicular steroid androstenone are regarded as the main compounds leading to ‘boar taint,’ a sex-specific odor from pork taken from entire males, as elevated concentrations of both substances may be found in adipose boars tissue. High skatole concentrations in adipose tissue are the result of a complex process, which includes microbial formation in the colon, absorption, metabolism and accretion in fat. Several of these steps leading to high skatole concentrations are influenced by feed components and additives. The present paper discusses the mechanisms by which effective feeding strategies and feed additives exert their influence in the prevention of high skatole concentrations in adipose pig tissue.

Easily fermentable carbohydrates reduce skatole formation in the distal intestine of entire male pigs

Two experiments were conducted to evaluate the effects of feeding different sources of easily fermentable carbohydrates to entire male pigs during the last weeks before slaughter. In Exp. 1, 48 entire male pigs (21.8 and 100.5kg initial and final body weight, respectively) were used to evaluate the effect of adding 0, 3, 6, or 9% chicory inulin (containing 70% inulin) to diets during the last 30days before slaughter on skatole concentration in the hind-gut and adipose tissue and on the composition of microbiota and mucosa morphology in the hind-gut. In Exp. 2, 66 entire male pigs (31.0 and 112.0kg initial and final body weight, respectively) were used to evaluate the effect of adding 20% raw potato starch to diets as a top dressing or adding increasing levels of potato starch (0, 5, 10, 15%, and 20%) to diets prior to pelleting during the last 14days before slaughter on skatole concentrations in hind-gut and adipose tissue and on the composition of microbiota in the hind-gut. Results from Exp. 1 showed a significant reduction in skatole levels in colon and rectum with increasing dietary levels of inulin (linear, P≤0.002). In these diets there was also a reduction of enterobacteria counts in colon descendens (linear, P=0.04) and rectum (linear, P=0.02), and a tendency towards reduced levels of Enterococcus spp. in colon descendens (linear, P=0.09) and rectum (linear, P=0.08). Results from Exp. 2 showed that adding raw potato starch as a top dressing reduced skatole levels in rectum and adipose tissue compared with the control, while adding increasing levels of potato starch to diets prior to pelleting did not affect skatole levels. Adding potato starch as a top dressing or to diets before pelleting did not affect the cultured bacteria in the hind-gut of pigs. Feeding inulin or potato starch did not affect mucosal epithelial cell counts in the cecum or colon in Exp. 1 and Exp. 2, respectively. In conclusion, feeding increasing levels of chicory inulin to entire male pigs during the last period before slaughter reduced the level of skatole in the distal intestine and in adipose tissue, feeding pelleted diets containing up to 20% potato starch had no effect, while feeding 20% raw potato starch as top dressing reduced the level of skatole in the distal intestine and the adipose tissue of the entire male pigs.

Meat odour and flavour and indoles concentration in ruminal fluid and adipose tissue of lambs fed green herbage or concentrates with or without tannins

A 2 × 2 factorial experiment was carried out to evaluate the effect of herbage or concentrate feeding system and tannin addition to diet on skatole and indole in ruminal fluid and adipose tissue and meat sensory properties. Twenty-eight male lambs aged 45 days were randomly assigned to one of two feeding systems (vetch green herbage or concentrates, n = 14) and within feeding system to one supplement (quebracho tannins added to the diet or none). Animals were kept in singular pens and slaughtered at the age of 105 days. Indole (P < 0.05) and skatole (P < 0.01) concentrations in ruminal fluid were higher in lambs fed herbage compared to those given concentrates. Skatole in ruminal fluid tended to be present at lower concentrations in animals that received the tannin supplementation (P = 0.07). Indole was also higher in the caudal fat of animals fed green vetch compared to those fed concentrate (P = 0.04). Skatole concentration was lower in the fat of lambs fed concentrates compared to those given herbage (P = 0.05) and was lower in the fat of animals supplemented with tannins compared to the animals not supplemented (P = 0.01). Sheep meat odour was lower in meat from animals supplemented with tannins compared to those not supplemented (P < 0.01). It is concluded that tannins are more effective in reducing skatole formation in ruminants when they are associated with concentrate diets than green herbages.

Concentration of indoles and other rumen metabolites in sheep after a meal of fresh white clover, perennial ryegrass or Lotus corniculatus and the appearance of indoles in the blood

AbstractThe objective of this study was to determine the effect of feeding three fresh forage diets, white clover (WC), perennial ryegrass (PRG) and Lotus corniculatus (LC), on the formation of indole and skatole in the rumen of sheep. The formation of indole and skatole in the rumen and their appearance in the blood were also compared. Peak rumen indole and skatole concentrations per kg crude protein intake (CPI) were significantly higher when feeding WC compared with PRG and LC (P &lt; 0.05) and this was associated with a significantly higher rumen concentration per kg CPI of ammonia, branched chain volatile fatty acids, total nitrogen and soluble nitrogen (P &lt; 0.05). Greater indole and skatole concentrations when feeding WC can be attributed to high solubility and rapid degradation of the forage protein. LC had a similar nutrient composition to WC, but the condensed tannins in LC slowed protein degradation and reduced indole and skatole formation. Indole and skatole concentrations peaked in the plasma 1–2 h after the end of feeding, indicating that skatole and indole are rapidly absorbed from the rumen into the blood. High indole and skatole formation with low intakes of WC indicates that the WC component of traditional New Zealand pastures may be the primary cause of undesirable pastoral flavours that result from the presence of indoles in meat. To ameliorate undesirable flavours, producers reliant on pastoral systems will need to consider using alternative forages such as LC to reduce protein solubility and degradation rate. Copyright © 2007 Society of Chemical Industry

Effect of different condensed tannin‐containing forages, forage maturity and nitrogen fertiliser application on the formation of indole and skatole in in vitro rumen fermentations

AbstractThe objectives of this work were to establish the effect of seven different forages with a varied condensed tannin (CT) content, plant maturity and nitrogen fertiliser application on the ruminal formation of skatole and indole using an in vitro method designed to mimic rumen fermentation conditions. After 10 h of incubation, the concentration of indole and skatole was highest when incubating white clover (P &lt; 0.05). Polyethylene glycol addition, to inhibit CT, showed that CT significantly reduced the formation of indole and skatole when incubating Lotus corniculatus, sulla, Dorynium rectum and Lotus pedunculatus (P &lt; 0.01). Mature forage growth resulted in a significantly lower concentration of indole and skatole being formed in vitro compared to the incubation of new spring growth (P &lt; 0.001). A higher application of nitrogen fertiliser to perennial ryegrass‐based pasture resulted in a higher crude protein concentration in the plant and a significantly higher concentration of skatole formed in vitro (P &lt; 0.001). Forages containing CT reduced the conversion of endogenous protein to indole and skatole and plants containing a higher CT concentration tended to be more effective, but compositional differences of CT between forages may also have had an influence. New forage growth or swards that had a high application of nitrogen fertiliser promoted the formation of indole and skatole. Copyright © 2007 Society of Chemical Industry

The effect of supplementation of a white clover or perennial ryegrass diet with grape seed extract on indole and skatole metabolism and the sensory characteristics of lamb

AbstractCondensed tannin in the form of a grape seed extract (GSE) was dosed to weaned wether lambs fed white clover (WC) or perennial ryegrass (PRG) over a 9‐week period to determine whether the ‘pastoral’ flavour and odour of meat could be altered. The concentrations of the pastoral flavour compounds indole and skatole were determined in the rumen fluid, blood plasma and intermuscular fat. The odour and flavour of fat and meat from the slaughtered lambs was assessed by a trained panel. The rumen fluid and blood plasma concentrations of indole and skatole were higher in those lambs fed WC compared to PRG (P &lt; 0.05) and the overall meat flavour intensity was greater when feeding WC (P &lt; 0.01). The observed concentration of indole and skatole in the fat between WC and PRG feeding treatments was not statistically different. Power analysis indicated that increasing the number of lambs per treatment group from 20 to 65 would result in a higher fat skatole concentration (P &lt; 0.05) being detected in lambs fed WC compared to PRG. Dosing with GSE gave a small reduction in skatole concentration in the rumen fluid and reduced plasma concentration of indole and skatole (P &lt; 0.001). Odour and flavour scores of the fat and meat samples were not particularly high however, dosing with GSE lowered the overall and sweet odour and the sheepy, camphor, faecal and barnyard flavour (P &lt; 0.05). Although the plasma concentration of indole and skatole suggests that GSE reduced indole and skatole formation, the intermittent supply of the GSE to the rumen environment was not sufficient to reduce their concentration in the fat. Hence, the small difference in the scores for pastoral odour and flavour attributes associated with GSE treatment may arise from other unknown factors. From a primary investigation, there was no difference in the concentration of indole and skatole in fat samples collected from carcasses before and after chilling. Further investigations into meat pastoral flavour are warranted through feeding condensed tannin‐containing forages. Copyright © 2007 Society of Chemical Industry

Controlling the formation of indole and skatole in in vitro rumen fermentations using condensed tannin

AbstractIndole and skatole are formed in the rumen from the fermentation of tryptophan and have been correlated to the presence of undesirable pastoral flavours in meat from grazing ruminants. A series of four in vitro rumen fermentation experiments was carried out to determine the effectiveness of condensed tannin (CT) for reducing the formation of indole and skatole. Experiment 1 utilised fresh white clover (WC; Trifolium repens) in ratios with the CT‐containing forage Lotus pedunculatus (LP; 97 g CT kg−1 dry matter (DM)). Increasing the ratio of LP to WC decreased the formation of indole and skatole. Experiments 2 and 3 used extracts of CT from LP and grape seed added to incubations of fresh non‐CT‐containing WC or perennial ryegrass (PRG; Lolium perenne). Including the CT extracts in incubations at 40 and 80 g kg−1 DM was more effective at reducing indole and skatole formation than lower concentrations of CT extract (P &lt; 0.05). Including fresh LP in ratios with WC gave a linear decrease in indole and skatole concentration (indicating dilution), while including a CT extract in the incubations gave an exponential decrease in indole and skatole concentration (suggesting binding). Experiment 4 elucidated the mechanisms behind the action of CT by delayed addition of tryptophan and polyethylene glycol (PEG) to incubations of LP. Rumen microbes that were exposed to CT of LP in planta for up to 6 h and then provided with tryptophan were still able to convert tryptophan to indole and skatole. Adding PEG to incubations of LP after 6 h inhibited the activity of plant CT and increased the availability of substrate for indole and skatole formation. These studies have shown that a higher concentration of CT is more efficient for reducing indole and skatole formation and that CT contained within plants acts differently in in vitro rumen fermentations than additions of extracted CT. Under the conditions of these experiments, there was no evidence that CT contained in LP affected the protein present in WC in a mixed fermentation. Copyright © 2007 Society of Chemical Industry

Skatole and indole concentration and the odour of fat from lambs that had grazed perennial ryegrass/white clover pasture or Lotus corniculatus

An experiment was conducted for 111 days in the summer of 2002/2003 to compare the effect of grazing lambs on condensed tannin-containing Lotus corniculatus L. (cv. Grasslands Goldie; n = 12) and perennial ryegrass/white clover pasture (PRG/WC; Lolium perenne/ Trifolium repens; n = 12) on the concentration of indole and skatole in rumen fluid, blood plasma and body fat and upon the odour of the fat. Rumen fluid and blood samples were obtained on days 0, 22, 58, 87 and 111 of the experiment. Fat from inter-muscular and tail-stub depots was obtained at slaughter. During the experiment the mean concentration of indole and skatole in both the rumen fluid and blood plasma was lower for lambs grazing L. corniculatus compared to PRG/WC pastures. The concentration of skatole in the tail-stub fat was lower and less variable (P<0.05) in the lambs that had grazed L. corniculatus. No differences were observed in the odour of the inter-muscular fat from lambs that had grazed L. corniculatus compared to PRG/WC pasture, suggesting that the reductions in indole and skatole formation from grazing L. corniculatus were not large enough to produce a detectable difference in fat odour. The experiment was complicated by the on-set of drought conditions during the second half which resulted in a relatively lower level of crude protein in the L. corniculatus forage. It was concluded that CT-containing forages hold some potential to reduce the concentration of indole and skatole in the fat to provide consistently flavoured meat products from grazing systems.

Flavor Improvement in Pork from Barrows and Gilts via Inhibition of Intestinal Skatole Formation with Resistant Potato Starch

Skatole originates from microbial processing of tryptophan in the large intestine of pigs and accumulates in adipose tissue. Formation may be inhibited by the anti-apoptotic function of butyrate formed out of raw potato starch. Two groups of pigs (each consisting of gilts and barrows) were fed from 30 to 110 kg life weight either a conventional diet (controls; n = 35) or an isocaloric diet containing 300 g of raw potato starch/kg of body weight (RS; n = 34). Skatole concentrations were measured in colon content, blood, and adipose tissue. Odor of cooked meat samples was evaluated by a test panel. RS reduced concentrations in colon content and blood plasma (P < 0.001). Back fat concentrations were decreased significantly from 25 to 1.40 ng/g (barrows; P < 0.001) and from 40 to 9 ng/g (gilts; P < 0.001). Odor rating (scale of 1-5 from very unpleasant to very pleasant) was 3.07 for low skatole concentrations and 2.66 for both medium and high skatole concentrations (P < 0.05).

Dose‐dependent Effects of Resistant Potato Starch in the Diet on Intestinal Skatole Formation and Adipose Tissue Accumulation in the Pig

Resistant starch (RS) intake leads to a reduction of apoptosis in the pig colon and thus to a reduction of skatole in the carcass. To determine the minimal amount of RS which is required for the effect, diets with varying amounts (20, 30 and 40%) of RS were fed to barrows and gilts. Controls were fed instead with 55% pre-gelatinized starch. Skatole was determined after slaughter in distal colon content and in adipose tissue (flare fat and belly fat). Resistant starch led to a dose-dependent reduction of skatole in the gut content from 134 microg/g dry matter (controls) to 4.8 microg/g in the 40% group. Flare fat concentrations were decreased from 159 to 20 ng/g fat and belly fat concentrations from 64 to 16 ng/g fat. Strategies to improve sensorial quality by feeding RS continuously or for only 1 week prior to slaughter are discussed.

Effect of white clover (Trifolium repens), perennial ryegrass (Lolium perenne) and Lotus corniculatus on in vitro skatole and indole formation

Consumers accustomed to eating meat and dairy products from animals raised on grain often describe products from New Zealand pasture-raised animals as having an undesirable flavour. This could adversely affect the acceptability and returns for meat and dairy products in certain oversea markets. This flavour, often described as pastoral, grassy or faecal, has been attributed in part to the high concentration of indole and skatole present in the meat of animals raised on fresh perennial ryegrass/white clover pasture. Skatole and indole are formed in the rumen from the microbial degradation of tryptophan. A pasture diet is high in protein that is both soluble and rapidly degraded, resulting in an abundance of free peptides and amino acids in the rumen. Condensed tannins (CT) in Lotus corniculatus have been shown to reduce the solubility and degradability of plant protein in the rumen. This study investigated the in vitro rumen fermentation characteristics of white clover (Trifolium repens; WC), perennial ryegrass (Lolium perenne; PRG) and Lotus corniculatus (LC) to determine their effects on skatole and indole formation. All incubations were carried out in the presence and absence of polyethylene glycol (PEG), which inactivates CT. In the absence of PEG, maximal skatole and ammonia concentrations were greater when incubating WC compared with PRG and LC (P<0.05). A similar trend was evident for indole concentrations, but statistical significance was not reached. Adding PEG to the incubations with LC increased the concentration of indole, skatole and ammonia (P<0.05), but did not affect those incubations with WC or PRG. Differences in peak concentrations by the in vitro method were comparable with those found in an in vivo study using the same forages. Thus, the in vitro method can be used to screen a range of forages to determine their effect on skatole and indole formation in ruminants, and to identify candidate forages for managing the flavour of products from forage-fed ruminants.

Skatole and indole formation in comparison to rumen ammonia and volatile fatty acids in sheep fed fresh forages

Skatole and indole concentrations in the rumen were compared with the rumen concentrations of ammonia and volatile fatty acids (VFA) in sheep fed fresh white clover (WC; Trifolium repens), perennial ryegrass (PRG; Lolium perenne) or the condensed tannin-containing forage, Lotus corniculatus (LC). Six rumen fistulated Romney wethers were fed the cut forages over two 2-hour meal periods in the morning and afternoon, with rumen contents sampled at intervals after the start of the morning meal. Maximum concentrations of skatole, indole and ammonia were highest when feeding WC (P<0.05). Ammonia accumulates in the rumen when there is rapid protein degradation with unsynchronised energy supply. Condensed tannins in LC slowed protein degradation and consequently maximal ammonia, indole and skatole concentration were lower when feeding this forage (P<0.05). This indicates that skatole and indole formation will be greater when there is high protein degradability. Ammonia, indole and skatole concentrations in sheep fed PRG were intermediate, and not significantly different from either WC or LC. Maximum concentration of straight-chain volatile fatty acids (VFA) were not significantly different for the forages, but branchedchain VFA concentrations tended to be higher for WC (P=0.103) compared with LC. Straight-chain VFAs are a by-product of microbial carbohydrate fermentation in the rumen, but branched-chain VFAs are formed from deamination of amino acids. This suggests that skatole and indole production are not related to carbohydrate digestion in the rumen. In order for meat and milk producers in pastoral systems to ameliorate undesirable flavours and increase demand in certain markets, they will need to consider using alternative forages to reduce protein solubility and degradability.

Effects of butyrate on apoptosis in the pig colon and its consequences for skatole formation and tissue accumulation.

Evidence exists that butyrate inhibits apoptosis of colon crypt cells in vivo so that less tryptophan from cell debris is available for skatole formation by microbes in the pig colon. In this study, potato starch containing a high proportion of resistant starch was fed to test the hypothesis that increased butyrate formation will occur in the colon and contribute to reduced epithelial cell apoptosis, thus leading to reduced skatole formation and absorption. Two groups of six barrows were provided with catheters in the jugular vein and fed either a ration with pregelatinized starch (high ileal digestibility; controls) or potato starch (low ileal digestibility; PS) as the main carbohydrate. All pigs were fed 31 MJ of metabolizable energy and 381 g of crude protein per day. The controls were fed for 19 d. The PS group received the same control ration for 10 d, and then changed to the PS ration. The total feeding period of PS consisted of a 5 d adaptation period followed by another 19 d. In the continously sampled feces, pH, short chain fatty acids, and skatole were determined. Skatole was additionally measured in blood plasma that was sampled daily. After killing barrows at the end of the feeding period, fat tissue for skatole measurement and colon tissue for histological quantification of mitosis and apoptosis were obtained. Feeding potato starch led to a rapid 2.2 fold increase of fecal butyrate when compared both with the control period of the PS group and the control group (P < 0.001). PS feeding resulted in a decrease in pH from 7.3 to 5.3 (P < 0.001) and apoptosis from 2.06 cells/crypt to 0.90 cells (P < 0.01), whereas there was no change in mitosis. Consequently, skatole decreased both in feces (controls vs PS group: 120.0 vs 1.9 microg/g; P < 0.001) and in blood plasma (1.6 vs 0.2 ng/mL; P < 0.001). The mean concentration of skatole in fat tissue was 167 ng/g tissue in controls, and below the detection limit (0.8 ng/g) in the PS group (P < 0.001). It is concluded that butyrate-dependent inhibition of apoptosis in the colon due to potato starch feeding efficiently inhibits skatole production in barrows. Because of the depressed skatole levels, improved sensory quality of pork is possible.

Review of microbiological and biochemical effects of skatole on animal production

Skatole, a compound formed by the bacterial degradation of tryptophan, has harmful consequences for the beef and pork industries. Skatole causes acute bovine pulmonary edema and emphysema (ABPE) in cattle and is responsible for boar taint in pigs. Many ruminants are susceptible to ABPE, a disease in which skatole acts as a very selective pneumotoxin that causes the degeneration of certain lung tissues. Skatole passes through the rumen, is absorbed into the blood, and is then bioactivated by cytochrome p450s of the MFO (mixed-function oxidase) system or the PHS (prostaglandin H synthetase) system to form a toxic intermediate. Skatole also affects the production of serotonin and, in high concentrations, can hemolyze bovine erythrocytes. Pigs are not afflicted by skatole, but the off-odor (boar taint) given off by their meat on cooking is offensive to consumers. Skatole is produced in the intestine by bacterial degradation and absorbed into the blood. If it passes through the liver without being metabolized, it accumulates in adipose tissue, the liver and the kidneys. Several factors are known to influence skatole deposition rates in fat, but one of the most important is believed to be genetic. Skatole is also toxic to many microorganisms. It has a fairly broad bacteriostatic effect and is toxic to rumen ciliated protozoa and to Tetrahymena. The fermentation of tryptophan may produce various compounds, including indole, indoleacetic acid and 3-methylindole (skatole), and may be influenced by chemical factors and the physiological and psychological status of the host. The bacterium responsible for the production of skatole is Lactobacillus sp. strain 11201. It is not pathogenic and is an integral part of the normal intestinal flora. Skatole production is not a constitutive function of this bacterial strain. The enzyme responsible for skatole formation is not thought to be cytosolic; it may be bound to the cell wall, exist as an aggregate enzyme or be a particulate enzyme. Skatole acts as a non-competitive inhibitor of the enzyme responsible for its formation.

Influences on skatole formation from tryptophan in the pig colon.

Variable amounts of skatole (3-methyl-indole) are formed by microbes out of tryptophan in the colon of pigs. It is resorbed and accumulated in fat, leading to a fecal odor of the meat. We investigated the mechanisms by which differences in diet composition lead to variations in skatole concentrations in blood plasma and fat. The experiments were based on the hypothesis that tryptophan is derived from mucosa cell debris from the small intestine. It was found, that gut cell mitosis is stimulated by the growth factor IGF-I. This factor increases when energy in the diet is high. In addition, the mitotic rate is elevated when high amounts of purines are available in the diet, allowing a more rapid DNA- and RNA-synthesis. Thus, high energy combined with high purines in the diet lead to a remarkable increase of gut cell mitosis which is accompanied by an increase of apoptosis. These apoptotic cells ultimately provide the substrate for skatole formation. In consequence, a dramatically rise of skatole in blood plasma and fat was measurable.