Utilization of amino acids by bacteria from the pig small intestine

Early microbial and metabolomic signatures predict later onset of necrotizing enterocolitis in preterm infants

BACKGROUND

Necrotizing enterocolitis (NEC) is a devastating intestinal disease that afflicts 10% of extremely preterm infants. The contribution of early intestinal colonization to NEC onset is not understood, and predictive biomarkers to guide prevention are lacking. We analyzed banked stool and urine samples collected prior to disease onset from infants <29 weeks gestational age, including 11 infants who developed NEC and 21 matched controls who survived free of NEC. Stool bacterial communities were profiled by 16S rRNA gene sequencing. Urinary metabolomic profiles were assessed by NMR.

RESULTS

During postnatal days 4 to 9, samples from infants who later developed NEC tended towards lower alpha diversity (Chao1 index, P = 0.086) and lacked Propionibacterium (P = 0.009) compared to controls. Furthermore, NEC was preceded by distinct forms of dysbiosis. During days 4 to 9, samples from four NEC cases were dominated by members of the Firmicutes (median relative abundance >99% versus <17% in the remaining NEC and controls, P < 0.001). During postnatal days 10 to 16, samples from the remaining NEC cases were dominated by Proteobacteria, specifically Enterobacteriaceae (median relative abundance >99% versus 38% in the other NEC cases and 84% in controls, P = 0.01). NEC preceded by Firmicutes dysbiosis occurred earlier (onset, days 7 to 21) than NEC preceded by Proteobacteria dysbiosis (onset, days 19 to 39). All NEC cases lacked Propionibacterium and were preceded by either Firmicutes (≥98% relative abundance, days 4 to 9) or Proteobacteria (≥90% relative abundance, days 10 to 16) dysbiosis, while only 25% of controls had this phenotype (predictive value 88%, P = 0.001). Analysis of days 4 to 9 urine samples found no metabolites associated with all NEC cases, but alanine was positively associated with NEC cases that were preceded by Firmicutes dysbiosis (P < 0.001) and histidine was inversely associated with NEC cases preceded by Proteobacteria dysbiosis (P = 0.013). A high urinary alanine:histidine ratio was associated with microbial characteristics (P < 0.001) and provided good prediction of overall NEC (predictive value 78%, P = 0.007).

CONCLUSIONS

Early dysbiosis is strongly involved in the pathobiology of NEC. These striking findings require validation in larger studies but indicate that early microbial and metabolomic signatures may provide highly predictive biomarkers of NEC.

Re-print of "Intestinal luminal nitrogen metabolism: role of the gut microbiota and consequences for the host"

Alimentary and endogenous proteins are mixed in the small intestinal lumen with the microbiota. Although experimental evidences suggest that the intestinal microbiota is able to incorporate and degrade some of the available amino acids, it appears that the microbiota is also able to synthesize amino acids raising the view that amino acid exchange between the microbiota and host can proceed in both directions. Although the net result of such exchanges remains to be determined, it is likely that a significant part of the amino acids recovered from the alimentary proteins are used by the microbiota. In the large intestine, where the density of bacteria is much higher than in the small intestine and the transit time much longer, the residual undigested luminal proteins and peptides can be degraded in amino acids by the microbiota. These amino acids cannot be absorbed to a significant extent by the colonic epithelium, but are precursors for the synthesis of numerous metabolic end products in reactions made by the microbiota. Among these products, some like short-chain fatty acids and organic acids are energy substrates for the colonic mucosa and several peripheral tissues while others like sulfide and ammonia can affect the energy metabolism of colonic epithelial cells. More work is needed to clarify the overall effects of the intestinal microbiota on nitrogenous compound metabolism and consequences on gut and more generally host health.

Dietary requirements of "nutritionally non-essential amino acids" by animals and humans

Amino acids are necessary for the survival, growth, development, reproduction and health of all organisms. They were traditionally classified as nutritionally essential or non-essential for mammals, birds and fish based on nitrogen balance or growth. It was assumed that all "non-essential amino acids (NEAA)" were synthesized sufficiently in the body to meet the needs for maximal growth and health. However, there has been no compelling experimental evidence to support this assumption over the past century. NEAA (e.g., glutamine, glutamate, proline, glycine and arginine) play important roles in regulating gene expression, cell signaling, antioxidative responses, neurotransmission, and immunity. Additionally, glutamate, glutamine and aspartate are major metabolic fuels for the small intestine to maintain its digestive function and protect its mucosal integrity. Therefore, based on new research findings, NEAA should be taken into consideration in revising the classical "ideal protein" concept and formulating balanced diets to improve protein accretion, food efficiency, and health in animals and humans.

Dietary supplementation with monosodium glutamate is safe and improves growth performance in postweaning pigs

Dietary intake of glutamate by postweaning pigs is markedly reduced due to low feed consumption. This study was conducted to determine the safety and efficacy of dietary supplementation with monosodium glutamate (MSG) in postweaning pigs. Piglets were weaned at 21 days of age to a corn and soybean meal-based diet supplemented with 0, 0.5, 1, 2, and 4 % MSG (n = 25/group). MSG was added to the basal diet at the expense of cornstarch. At 42 days of age (21 days after weaning), blood samples (10 mL) were obtained from the jugular vein of 25 pigs/group at 1 and 4 h after feeding for hematological and clinical chemistry tests; thereafter, pigs (n = 6/group) were euthanized to obtain tissues for histopathological examinations. Feed intake was not affected by dietary supplementation with 0-2 % MSG and was 15 % lower in pigs supplemented with 4 % MSG compared with the 0 % MSG group. Compared with the control, dietary supplementation with 1, 2 and 4 % MSG dose-dependently increased plasma concentrations of glutamate, glutamine, and other amino acids (including lysine, methionine, phenylalanine and leucine), daily weight gain, and feed efficiency in postweaning pigs. At day 7 postweaning, dietary supplementation with 1-4 % MSG also increased jejunal villus height, DNA content, and antioxidative capacity. The MSG supplementation dose-dependently reduced the incidence of diarrhea during the first week after weaning. All variables in standard hematology and clinical chemistry tests, as well as gross and microscopic structures, did not differ among the five groups of pigs. These results indicate that dietary supplementation with up to 4 % MSG is safe and improves growth performance in postweaning pigs.

Effect of susceptibility to enterotoxigenic Escherichia coli F4 and of dietary tryptophan on gut microbiota diversity observed in healthy young pigs

Healthy weaned pigs susceptible to enterotoxigenic Escherichia coli F4 (ETEC) require more tryptophan (Trp) to maximize their performance. This may be related to an effect on intestinal microbiota. We studied the intestinal bacterial diversity of healthy pigs with different susceptibility to ETEC and fed different Trp levels. Thirty-six littermate weaned pigs were selected to obtain a set potentially formed of 50% ETEC-susceptible and 50% non-susceptible pigs, based on a Mucin 4 gene polymorphism. Pigs were fed a diet with 0.17 (TrpL) or 0.22 (TrpH) standardized ileal digestible Trp:Lys ratio for 21 days. Slaughtered pigs were classified into non-susceptible, mildly susceptible, and susceptible, by testing ETEC adhesion to intestinal villi. Bacterial diversity in jejunum content was assessed by the 16S rRNA gene-targeted denaturing gradient gel electrophoresis (DGGE) fingerprinting analysis and expressed by the Shannon index. Susceptible pigs had a reduced bacterial diversity, particularly with TrpL diet (p=0.003). The ETEC adhesion class affected the quantification of enterobacteria DNA (p=0.027). One DGGE band, which referred to Clostridium bartlettii, was not evidenced in all the susceptible pigs; less DNA from this microbe was quantified by RT-PCR in the jejunum from TrpH susceptible pigs (p=0.025) compared to TrpL. The gene expression for β-galactoside α-2,3-sialyltransferase 1 was higher in jejunal tissue of ETEC-susceptible pigs (p=0.019). In studies on pig gut microbiota, the presence of intestinal receptors for ETEC should be considered because of their contribution to a reduced bacterial diversity. This effect could be partially reversed by dietary Trp addition.

L-Glutamine regulates amino acid utilization by intestinal bacteria

Catabolism of amino acids (AA) by intestinal bacteria greatly affects their bioavailability in the systemic circulation and the health of animals and humans. This study tests the novel hypothesis that L-glutamine regulates AA utilization by luminal bacteria of the small intestine. Pure bacterial strains (Streptococcus sp., Escherichia coli and Klebsiella sp.) and mixed bacterial cultures derived from the jejunum or ileum of pigs were cultured in the presence of 0-5 mM L-glutamine under anaerobic conditions. After 3 h of incubation, samples were taken for the determination of AA utilization. Results showed concentration-dependent increases in the utilization of glutamine in parallel with the increased conversion of glutamine into glutamate in all the bacteria. Complete utilization of asparagine, aspartate and serine was observed in pure bacterial strains after the 3-h incubation. The addition of glutamine reduced the net utilization of asparagine by both jejunal and ileal mixed bacteria. Net utilization of lysine, leucine, valine, ornithine and serine by jejunal or ileal mixed bacteria decreased with the addition of glutamine in a concentration-dependent manner. Collectively, glutamine dynamically modulates the bacterial metabolism of the arginine family of AA as well as the serine and aspartate families of AA and reduced the catabolism of most AA (including nutritionally essential and nonessential AA) in jejunal or ileal mixed bacteria. The beneficial effects of glutamine on gut nutrition and health may involve initiation of the signaling pathways related to AA metabolism in the luminal bacteria of the small intestine.

L-Arginine stimulates the mTOR signaling pathway and protein synthesis in porcine trophectoderm cells

Impairment of placental growth is a major factor contributing to intrauterine growth retardation (IUGR) in both human pregnancy and animal production. Results of recent studies indicate that administration of L-arginine (Arg) to gestating pigs or sheep with IUGR fetuses can enhance fetal growth. However, the underlying mechanisms are largely unknown. The present study tested the hypothesis that Arg stimulates the mammalian target of rapamycin (mTOR) signaling pathway and protein synthesis in porcine conceptus trophectoderm (pTr2) cells. The cells were cultured for 4 days in Arg-free Dulbecco's modified Eagle's Ham medium containing 10, 50, 100, 200, 350 or 500 μM Arg. Cell numbers, protein synthesis and degradation, as well as total and phosphorylated levels of mTOR, ribosomal protein S6 kinase 1 (p70S6K) and eukaryotic initiation factor 4E-binding protein-1 (4EBP1), were determined. The pTr2 cells exhibited time (0-6 days)- and Arg concentration (10-350 μM)-dependent increases in proliferation. Addition of 100 and 350 μM Arg to culture medium dose-dependently increased (a) protein synthesis and decreased protein degradation and (b) the abundance of total and phosphorylated mTOR, p70S6K and 4EBP1 proteins. Effects of 350 μM Arg on intracellular protein turnover were only modestly affected when nitric oxide synthesis was inhibited. Collectively, these results indicate a novel and important role for Arg in promoting growth of porcine placental cells largely via a nitric-oxide-independent pathway. Additionally, these findings help to explain beneficial effects of Arg supplementation on improving survival and growth of embryos/fetuses in mammals.

Proline and hydroxyproline metabolism: implications for animal and human nutrition

Proline plays important roles in protein synthesis and structure, metabolism (particularly the synthesis of arginine, polyamines, and glutamate via pyrroline-5-carboxylate), and nutrition, as well as wound healing, antioxidative reactions, and immune responses. On a per-gram basis, proline plus hydroxyproline are most abundant in collagen and milk proteins, and requirements of proline for whole-body protein synthesis are the greatest among all amino acids. Therefore, physiological needs for proline are particularly high during the life cycle. While most mammals (including humans and pigs) can synthesize proline from arginine and glutamine/glutamate, rates of endogenous synthesis are inadequate for neonates, birds, and fish. Thus, work with young pigs (a widely used animal model for studying infant nutrition) has shown that supplementing 0.0, 0.35, 0.7, 1.05, 1.4, and 2.1% proline to a proline-free chemically defined diet containing 0.48% arginine and 2% glutamate dose dependently improved daily growth rate and feed efficiency while reducing concentrations of urea in plasma. Additionally, maximal growth performance of chickens depended on at least 0.8% proline in the diet. Likewise, dietary supplementation with 0.07, 0.14, and 0.28% hydroxyproline (a metabolite of proline) to a plant protein-based diet enhanced weight gains of salmon. Based on its regulatory roles in cellular biochemistry, proline can be considered as a functional amino acid for mammalian, avian, and aquatic species. Further research is warranted to develop effective strategies of dietary supplementation with proline or hydroxyproline to benefit health, growth, and development of animals and humans.

Metabolism of select amino acids in bacteria from the pig small intestine

This study investigated the metabolism of select amino acids (AA) in bacterial strains (Streptococcus sp., Escherichia coli and Klebsiella sp.) and mixed bacterial cultures derived from the jejunum and ileum of pigs. Cells were incubated at 37°C for 3 h in anaerobic media containing 0.5-5 mM select AA plus [U-14C]-labeled tracers to determine their decarboxylation and incorporation into bacterial protein. Results showed that all types of bacteria rapidly utilized glutamine, lysine, arginine and threonine. However, rates of the utilization of AA by pure cultures of E. coli and Klebsiella sp. were greater than those for mixed bacterial cultures or Streptococcus sp. The oxidation of lysine, threonine and arginine accounted for 10% of their utilization in these pure bacterial cultures, but values were either higher or lower in mixed bacterial cultures depending on AA, bacterial species and the gut segment (e.g., 15% for lysine in jejunal and ileal mixed bacteria; 5.5 and 0.3% for threonine in jejunal mixed bacteria and ileal mixed bacteria, respectively; and 20% for arginine in ileal mixed bacteria). Percentages of AA used for bacterial protein synthesis were 50-70% for leucine, 25% for threonine, proline and methionine, 15% for lysine and arginine and 10% for glutamine. These results indicate diverse metabolism of AA in small-intestinal bacteria in a species- and gut compartment-dependent manner. This diversity may contribute to AA homeostasis in the gut. The findings have important implications for both animal and human nutrition, as well as their health and well-beings.

Triennial Growth Symposium: important roles for L-glutamine in swine nutrition and production

L-Glutamine (Gln) has traditionally not been considered a nutrient needed in diets for livestock species or even mentioned in classic animal nutrition textbooks. This is due to previous technical difficulties in Gln analysis and the unsubstantiated assumption that animals can synthesize sufficient amounts of Gln to meet their needs. Consequently, the current (1998) version of NRC does not recommend dietary Gln requirements for swine. This lack of knowledge about Gln nutrition has contributed to suboptimal efficiency of global pig production. Because of recent advances in research, Gln is now known to be an abundant AA in physiological fluids and proteins and a key regulator of gene expression. Additionally, Gln can regulate cell signaling via the mammalian target of rapamycin pathway, adenosine monophosphate-activated protein kinase, extracellular signal-related kinase, Jun kinase, mitogen-activated protein kinase, and nitric oxide. The exquisite integration of Gln-dependent regulatory networks has profound effects on cell proliferation, differentiation, migration, metabolism, homeostasis, survival, and function. As a result of translating basic research into practice, dietary supplementation with 1% Gln maintains gut health and prevents intestinal dysfunction in low-birth-weight or early-weaned piglets while increasing their growth performance and survival. In addition, supplementing 1% Gln to a corn- and soybean-meal-based diet between d 90 and 114 of gestation ameliorates fetal growth retardation in gilts and reduces preweaning mortality of piglets. Furthermore, dietary supplementation with 1% Gln enhances milk production by lactating sows. Thus, adequate amounts of dietary Gln, a major nutrient, are necessary to support the maximum growth, development, and production performance of swine.

Functional amino acids in growth, reproduction, and health

Amino acids (AA) were traditionally classified as nutritionally essential or nonessential for animals and humans based on nitrogen balance or growth. A key element of this classification is that all nonessential AA (NEAA) were assumed to be synthesized adequately in the body as substrates to meet the needs for protein synthesis. Unfortunately, regulatory roles for AA in nutrition and metabolism have long been ignored. Such conceptual limitations were not recognized until recent seminal findings that dietary glutamine is necessary for intestinal mucosal integrity and dietary arginine is required for maximum neonatal growth and embryonic survival. Some of the traditionally classified NEAA (e.g. glutamine, glutamate, and arginine) play important roles in regulating gene expression, cell signaling, antioxidative responses, and immunity. Additionally, glutamate, glutamine, and aspartate are major metabolic fuels for the small intestine and they, along with glycine, regulate neurological function. Among essential AA (EAA), much emphasis has been placed on leucine (which activates mammalian target of rapamycin to stimulate protein synthesis and inhibit proteolysis) and tryptophan (which modulates neurological and immunological functions through multiple metabolites, including serotonin and melatonin). A growing body of literature leads to a new concept of functional AA, which are defined as those AA that regulate key metabolic pathways to improve health, survival, growth, development, lactation, and reproduction of organisms. Both NEAA and EAA should be considered in the classic "ideal protein" concept or formulation of balanced diets to maximize protein accretion and optimize health in animals and humans.

Amino acid metabolism in the portal-drained viscera of young pigs: effects of dietary supplementation with chitosan and pea hull

Recent studies indicate extensive catabolism of amino acids (AA) by the portal-drained viscera (PDV) of pigs and humans. Because of ethical concerns over invasive surgical procedures on infants or adults, in vivo investigations are often performed with the pig which is both an agriculturally important livestock species and a widely used animal model for nutritional and physiological studies in humans. Here, we described a new technique for implanting chronic catheters into the portal vein, ileal mesenteric vein, and carotid artery to study AA metabolism in the PDV of young pigs. This method allowed for the reduction of surgery time by 1 h and measurements of the entry of dietary AA into the portal circulation. Using such an approach, we found that dietary supplementation with 100 mg/kg chitosan (a prebiotic and a polysaccharide not digested by animal cells) reduced oxygen consumption, as well as the net absorption of dietary AA into the portal vein, thereby enhancing their bioavailability for extraintestinal tissues. In contrast, opposite results were obtained with dietary supplementation of 12% pea-hull (containing 95% of fermentable nonstarch polysaccharide). Thus, this improved technique is useful to quantify in vivo absorption and metabolism of dietary AA in young pigs.