Short-chain fatty acids and succinate in feces of healthy human volunteers and their correlation with anaerobe cultural counts

Fumarate, a central electron acceptor for Enterobacteriaceae beyond fumarate respiration and energy conservation

C4-dicarboxylates (C4-DCs) such as fumarate, l-malate and l-aspartate are key substrates for Enterobacteria such as Escherichia coli or Salmonella typhimurium during anaerobic growth. In general, C4-DCs are oxidants during biosynthesis, e.g., of pyrimidine or heme, acceptors for redox balancing, a high-quality nitrogen source (l-aspartate) and electron acceptor for fumarate respiration. Fumarate reduction is required for efficient colonization of the murine intestine, even though the colon contains only small amounts of C4-DCs. However, fumarate can be produced endogenously by central metabolism, allowing autonomous production of an electron acceptor for biosynthesis and redox balancing. Bacteria possess a complex set of transporters for the uptake (DctA), antiport (DcuA, DcuB, TtdT) and excretion (DcuC) of C4-DCs. DctA and DcuB exert regulatory functions and link transport to metabolic control through interaction with regulatory proteins. The sensor kinase DcuS of the C4-DC two-component system DcuS-DcuR forms complexes with DctA (aerobic) or DcuB (anaerobic), representing the functional state of the sensor. Moreover, EIIA^Glc from the glucose phospho-transferase system binds to DctA and presumably inhibits C4-DC uptake. Overall, the function of fumarate as an oxidant in biosynthesis and redox balancing explains the pivotal role of fumarate reductase for intestinal colonization, while the role of fumarate in energy conservation (fumarate respiration) is of minor importance.

Intertwined Relationship of Mitochondrial Metabolism, Gut Microbiome and Exercise Potential

The microbiome has emerged as a key player contributing significantly to the human physiology over the past decades. The potential microbial niche is largely unexplored in the context of exercise enhancing capacity and the related mitochondrial functions. Physical exercise can influence the gut microbiota composition and diversity, whereas a sedentary lifestyle in association with dysbiosis can lead to reduced well-being and diseases. Here, we have elucidated the importance of diverse microbiota, which is associated with an individual's fitness, and moreover, its connection with the organelle, the mitochondria, which is the hub of energy production, signaling, and cellular homeostasis. Microbial by-products, such as short-chain fatty acids, are produced during regular exercise that can enhance the mitochondrial capacity. Therefore, exercise can be employed as a therapeutic intervention to circumvent or subside various metabolic and mitochondria-related diseases. Alternatively, the microbiome-mitochondria axis can be targeted to enhance exercise performance. This review furthers our understanding about the influence of microbiome on the functional capacity of the mitochondria and exercise performance, and the interplay between them.

What We Know So Far about the Metabolite-Mediated Microbiota-Intestinal Immunity Dialogue and How to Hear the Sound of This Crosstalk

Trillions of microorganisms, termed the "microbiota", reside in the mammalian gastrointestinal tract, and collectively participate in regulating the host phenotype. It is now clear that the gut microbiota, metabolites, and intestinal immune function are correlated, and that alterations of the complex and dynamic host-microbiota interactions can have deep consequences for host health. However, the mechanisms by which the immune system regulates the microbiota and by which the microbiota shapes host immunity are still not fully understood. This article discusses the contribution of metabolites in the crosstalk between gut microbiota and immune cells. The identification of key metabolites having a causal effect on immune responses and of the mechanisms involved can contribute to a deeper insight into host-microorganism relationships. This will allow a better understanding of the correlation between dysbiosis, microbial-based dysmetabolism, and pathogenesis, thus creating opportunities to develop microbiota-based therapeutics to improve human health. In particular, we systematically review the role of soluble and membrane-bound microbial metabolites in modulating host immunity in the gut, and of immune cells-derived metabolites affecting the microbiota, while discussing evidence of the bidirectional impact of this crosstalk. Furthermore, we discuss the potential strategies to hear the sound of such metabolite-mediated crosstalk.

Next-generation therapeutic bacteria for treatment of obesity, diabetes, and other endocrine diseases

The human gut microbiota has appeared as an important factor affecting host health and intestinal bacteria have recently emerged as potential therapeutics to treat diabetes and other endocrine diseases. These mainly anaerobic bacteria have been identified either via comparative "omics" analysis of the intestinal microbiota in healthy and diseased subjects or of data collected by fecal microbiota transplantation studies. Both approaches require advanced and in-depth sequencing technologies to perform massive genomic screening to select bacteria with potential benefits. It has been shown that these potentially therapeutic bacteria can either produce bioactive products that directly influence the host patho-physiology and endocrine systems or produce specific signaling molecules that may do so. These bioactive compounds can be formed via degradation of dietary or host-derived components or the conversion of intermediate compounds produced by fermentation of intestinal bacteria. Several of these bacteria have shown causality in preclinical models and entered clinical phase studies, while their mode of action is being analyzed. In this review, we summarize the research on the most promising bacterial candidates with therapeutic properties with a specific focus on diabetes.

Compartmentalised acyl-CoA metabolism and roles in chromatin regulation

BACKGROUND

Many metabolites serve as important signalling molecules to adjust cellular activities and functions based on nutrient availability. Links between acetyl-CoA metabolism, histone lysine acetylation, and gene expression have been documented and studied over the past decade. In recent years, several additional acyl modifications to histone lysine residues have been identified, which depend on acyl-coenzyme A thioesters (acyl-CoAs) as acyl donors. Acyl-CoAs are intermediates of multiple distinct metabolic pathways, and substantial evidence has emerged that histone acylation is metabolically sensitive. Nevertheless, the metabolic sources of acyl-CoAs used for chromatin modification in most cases remain poorly understood. Elucidating how these diverse chemical modifications are coupled to and regulated by cellular metabolism is important in deciphering their functional significance.

SCOPE OF REVIEW

In this article, we review the metabolic pathways that produce acyl-CoAs, as well as emerging evidence for functional roles of diverse acyl-CoAs in chromatin regulation. Because acetyl-CoA has been extensively reviewed elsewhere, we will focus on four other acyl-CoA metabolites integral to major metabolic pathways that are also known to modify histones: succinyl-CoA, propionyl-CoA, crotonoyl-CoA, and butyryl-CoA. We also briefly mention several other acyl-CoA species, which present opportunities for further research; malonyl-CoA, glutaryl-CoA, 3-hydroxybutyryl-CoA, 2-hydroxyisobutyryl-CoA, and lactyl-CoA. Each acyl-CoA species has distinct roles in metabolism, indicating the potential to report shifts in the metabolic status of the cell. For each metabolite, we consider the metabolic pathways in which it participates and the nutrient sources from which it is derived, the compartmentalisation of its metabolism, and the factors reported to influence its abundance and potential nuclear availability. We also highlight reported biological functions of these metabolically-linked acylation marks. Finally, we aim to illuminate key questions in acyl-CoA metabolism as they relate to the control of chromatin modification.

MAJOR CONCLUSIONS

A majority of acyl-CoA species are annotated to mitochondrial metabolic processes. Since acyl-CoAs are not known to be directly transported across mitochondrial membranes, they must be synthesized outside of mitochondria and potentially within the nucleus to participate in chromatin regulation. Thus, subcellular metabolic compartmentalisation likely plays a key role in the regulation of histone acylation. Metabolite tracing in combination with targeting of relevant enzymes and transporters will help to map the metabolic pathways that connect acyl-CoA metabolism to chromatin modification. The specific function of each acyl-CoA may be determined in part by biochemical properties that affect its propensity for enzymatic versus non-enzymatic protein modification, as well as the various enzymes that can add, remove and bind each modification. Further, competitive and inhibitory effects of different acyl-CoA species on these enzymes make determining the relative abundance of acyl-CoA species in specific contexts important to understand the regulation of chromatin acylation. An improved and more nuanced understanding of metabolic regulation of chromatin and its roles in physiological and disease-related processes will emerge as these questions are answered.

A phase conversion headspace technique for the determination of anti-anaerobic activity of drug candidate based on the metabolic acidity change in culture medium

Screening for anti-anaerobic drug candidates is still challenging although the anaerobic bacteria are important sources for human infections, because the method for anti-anaerobic activity testing is not readily available with low-cost and -expertise. We report a novel method for the determination of the anti-anaerobic activity of drug candidates by automated headspace-gas chromatography (HS-GC). Anaerobic bacteria were inoculated in an anaerobic atmosphere or rapidly using sterile syringe in an air-tight manner, and incubated with and without drugs for 48 h. The metabolic acidities of the cultured media were used as an indicator of cell activities and measured as end-products in place by HS-GC after being completely converted to CO₂ with sodium bicarbonate. The present method is precise (relative standard deviation is below 5%) and validated by excellent agreements with a reference method on the determinations of the inhibition rates (root-mean-square error = 10%, n = 48) and half maximal inhibitory concentrations (R² = 0.996, n = 8) of both pure drug compounds and plant extracts. Advantageously, the present method is sensitive in response to cell activity, safe with regard to cross contamination, and suitable for routine screening of diversified drug candidates for anti-anaerobic activity.

Import of Aspartate and Malate by DcuABC Drives H2/Fumarate Respiration to Promote Initial Salmonella Gut-Lumen Colonization in Mice

Initial enteropathogen growth in the microbiota-colonized gut is poorly understood. Salmonella Typhimurium is metabolically adaptable and can harvest energy by anaerobic respiration using microbiota-derived hydrogen (H2) as an electron donor and fumarate as an electron acceptor. As fumarate is scarce in the gut, the source of this electron acceptor is unclear. Here, transposon sequencing analysis along the colonization trajectory of S. Typhimurium implicates the C4-dicarboxylate antiporter DcuABC in early murine gut colonization. In competitive colonization assays, DcuABC and enzymes that convert the C4-dicarboxylates aspartate and malate into fumarate (AspA, FumABC), are required for fumarate/H2-dependent initial growth. Thus, S. Typhimurium obtains fumarate by DcuABC-mediated import and conversion of L-malate and L-aspartate. Fumarate reduction yields succinate, which is exported by DcuABC in exchange for L-aspartate and L-malate. This cycle allows S. Typhimurium to harvest energy by H2/fumarate respiration in the microbiota-colonized gut. This strategy may also be relevant for commensal E. coli diminishing the S. Typhimurium infection.

The Role of Succinate in the Regulation of Intestinal Inflammation

Succinate is a metabolic intermediate of the tricarboxylic acid (TCA) cycle within host cells. Succinate is also produced in large amounts during bacterial fermentation of dietary fiber. Elevated succinate levels within the gut lumen have been reported in association with microbiome disturbances (dysbiosis), as well as in patients with inflammatory bowel disease (IBD) and animal models of intestinal inflammation. Recent studies indicate that succinate can activate immune cells via its specific surface receptor, succinate receptor 1(SUCNR1), and enhance inflammation. However, the role of succinate in inflammatory processes within the gut mucosal immune system is unclear. This review includes current literature on the association of succinate with intestinal inflammation and the potential role of succinate–SUCNR1 signaling in gut immune functions.

Gut-brain signaling in energy homeostasis: the unexpected role of microbiota-derived succinate

In the context of the obesity epidemic, dietary fibers that are found essentially in fruit and vegetables attract more and more attention, since they exert numerous metabolic benefits resulting in the moderation of body weight. Short-chain fatty acids, such as propionate and butyrate, produced through their fermentation by the intestinal microbiota, have long been thought to be the mediators of these benefits. In fact, propionate and butyrate were recently shown to activate intestinal gluconeogenesis, a function exerting metabolic benefits via its capacity of signaling to the brain by gastrointestinal nerves. Recently, succinate, the precursor of propionate in the bacterial metabolism, has also been shown to exert signaling properties, including the activation of intestinal gluconeogenesis.

An Oxidative Central Metabolism Enables Salmonella to Utilize Microbiota-Derived Succinate

The mucosal inflammatory response induced by Salmonella serovar Typhimurium creates a favorable niche for this gut pathogen. Conventional wisdom holds that S. Typhimurium undergoes an incomplete tricarboxylic acid (TCA) cycle in the anaerobic mammalian gut. One change during S. Typhimurium-induced inflammation is the production of oxidized compounds by infiltrating neutrophils. We show that inflammation-derived electron acceptors induce a complete, oxidative TCA cycle in S. Typhimurium, allowing the bacteria to compete with the microbiota for colonization. A complete TCA cycle facilitates utilization of the microbiota-derived fermentation product succinate as a carbon source. S. Typhimurium succinate utilization genes contribute to efficient colonization in conventionally raised mice, but provide no growth advantage in germ-free mice. Mono-association of gnotobiotic mice with Bacteroides, a major succinate producer, restores succinate utilization in S. Typhimurium. Thus, oxidative central metabolism enables S. Typhimurium to utilize a variety of carbon sources, including microbiota-derived succinate.

Host impairments in patients with neoplastic diseases

Healthy individuals possess an immune system comprising physical barriers, innate and acquired immunity as well as the indigenous microflora that populate the body surfaces. The immune system maintains constant vigilance over the body at the cellular level as well as at the interface between the host integument and the resident microflora. However, neoplastic diseases and their treatment often lead to impaired immunity resulting in an increased risk of infections due to viruses, bacteria, fungi, and protozoa. This chapter explores the various aspects of host impairment focusing on the components of immunity and the interplay between them to explain why it is that these patients succumb to infections per se. In so doing, we hope that the reader will be better equipped to understand the risks patients face so as to anticipate potential infectious complications and implement appropriate measures to help attain successful remission of the neoplastic diseases and maintain the best quality of life for the patient.

Comparison of storage conditions for human vaginal microbiome studies

Background

The effect of storage conditions on the microbiome and metabolite composition of human biological samples has not been thoroughly investigated as a potential source of bias. We evaluated the effect of two common storage conditions used in clinical trials on the bacterial and metabolite composition of the vaginal microbiota using pyrosequencing of barcoded 16S rRNA gene sequencing and ¹H-NMR analyses.

Methodology/Principal Findings

Eight women were enrolled and four mid-vaginal swabs were collected by a physician from each woman. The samples were either processed immediately, stored at −80°C for 4 weeks or at −20°C for 1 week followed by transfer to −80°C for another 4 weeks prior to analysis. Statistical methods, including Kolmogorovo-Smirnov and Wilcoxon tests, were performed to evaluate the differences in vaginal bacterial community composition and metabolites between samples stored under different conditions. The results showed that there were no significant differences between samples processed immediately after collection or stored for varying durations. ¹H-NMR analysis of the small molecule metabolites in vaginal secretions indicated that high levels of lactic acid were associated with Lactobacillus-dominated communities. Relative abundance of lactic acid did not appear to correlate with relative abundance of individual Lactobacillus sp. in this limited sample, although lower levels of lactic acid were observed when L. gasseri was dominant, indicating differences in metabolic output of seemingly similar communities.

Conclusions/Significance

These findings benefit large-scale, field-based microbiome and metabolomic studies of the vaginal microbiota.

Comparative effects of very low-carbohydrate, high-fat and high-carbohydrate, low-fat weight-loss diets on bowel habit and faecal short-chain fatty acids and bacterial populations

Very low-carbohydrate diets are often used to promote weight loss, but their effects on bowel health and function are largely unknown. We compared the effects of a very low-carbohydrate, high-fat (LC) diet with a high-carbohydrate, high-fibre, low-fat (HC) diet on indices of bowel health and function. In a parallel study design, ninety-one overweight and obese participants (age 50.6 (sd 7.5) years; BMI 33.7 (sd 4.2) kg/m(2)) were randomly assigned to either an energy-restricted (about 6-7 MJ, 30 % deficit) planned isoenergetic LC or HC diet for 8 weeks. At baseline and week 8, 24 h urine and faecal collections were obtained and a bowel function questionnaire was completed. Compared with the HC group, there were significant reductions in the LC group for faecal output (21 (sd 145) v. - 61 (sd 147) g), defecation frequency, faecal excretion and concentrations of butyrate ( - 0.5 (sd 10.4) v. - 3.9 (sd 9.7) mmol/l) and total SCFA (1.4 (sd 40.5) v. - 15.8 (sd 43.6) mmol/l) and counts of bifidobacteria (P < 0.05 time x diet interaction, for all). Urinary phenols and p-cresol excretion decreased (P < or = 0.003 for time) with no difference between diets (P > or = 0.25). Faecal form, pH, ammonia concentration and numbers of coliforms and Escherichia coli did not change with either diet. No differences between the diets were evident for incidences of adverse gastrointestinal symptoms, which suggests that both diets were well tolerated. Under energy-restricted conditions, a short-term LC diet lowered stool weight and had detrimental effects on the concentration and excretion of faecal SCFA compared with an HC diet. This suggests that the long-term consumption of an LC diet may increase the risk of development of gastrointestinal disorders.

Simple gas chromatography analysis of faecal butyrate: application to patients at risk of pouchitis

In spite of the fact that pouchitis is the most frequently occurring and troublesome complication found in patients treated by ileo-anal anastomosis for ulcerative colitis, no biological marker currently exists to monitor the outcome of the disease. Since it has been noted faecal butyrate is reduced in patients with pouchitis, we developed a simple gas chromatography method to quantify butyrate in faecal water. This test is based on diethyl ether extraction with the use of methacrylic acid as an internal standard. We demonstrated that butyrate was effectively measured when this technique was applied to eleven patients with ileal-pouch anal anastomosis within the first year after the closure of their ileostomy. We also observed a noticeable reduction in the concentration of butyrate in patients who went on to develop a pouchitis.

The effect of FCE 22891, a new oral penem, on faecal flora anaerobes and their fermentation end products in patients with chronic obstructive pulmonary disease

The influence of FCE 22891 on the faecal flora was investigated in 11 patients with an acute exacerbation of chronic obstructive pulmonary disease (COPD). Colony counts of faecal (an)aerobes and concentrations of their short-chain fatty acids and organic acids were determined simultaneously in fresh faeces before, during and after administration of the drug, and compared to those of healthy human volunteers. The MIC of FCE 22891 for the 142 isolated anaerobes was determined, and compared to the biologically active concentration of the drug in faeces. The Bacteroides colony counts of the patients increased significantly by day 9 compared to day 3, but were significantly lower than those of the healthy controls before, during and after the study. Significant changes in short-chain fatty acids and organic acid concentrations occurred in faeces of the patients during and after treatment, and especially when compared to healthy controls. A strong increase in MICs during treatment was found in Clostridium species, other than Clostridium difficile. FCE 22891 had only a moderate effect on the composition of the anaerobic faecal flora, but bacterial fermentation was mainly disturbed before and early on in treatment, which could have been caused by the disease.

Anaerobes and their fermentation products in faeces of patients with familial adenomatous polyposis before and after subtotal colectomy and ileorectal anastomosis

In view of the importance of short-chain fatty acids (SCFA) for the colonic epithelial function and their possible relation with the reported spontaneous regression of rectal polyps after subtotal colectomy with ileorectal anastomosis, we compared the SCFA concentrations in faeces of five familial adenomatous polyposis (FAP) patients before, and in 10 FAP patients after operation to each other and to those of 10 healthy controls. Anaerobe cultural counts and concentrations of organic acids were also investigated in the same faecal samples from FAP patients and controls. The preoperative cultural counts were not significantly different from those of the controls. After colectomy, the Bacteroides (P < 0.002; P < 0.02), and bifidobacteria counts (P < 0.02; P < 0.005) were decreased compared to controls and untreated FAP patients. The ratio of acetic acid to total SCFA significantly increased, which coincided with a proportional decrease of the other SCFA. The frequently observed regression of rectal polyps after colectomy cannot be explained from these results. One would rather expect a possible stimulation of polyp growth and dedifferentiation of colonic epithelial cells.

Colonic fermentation of ispaghula, wheat bran, glucose, and albumin to short-chain fatty acids and ammonia evaluated in vitro in 50 subjects

The production of short-chain fatty acids and ammonia was measured in 16.6% fecal homogenates from 50 subjects incubated at 37 degrees C for 6 and 24 hours. All 50 homogenates produced ammonia and short-chain fatty acids of any chain length (C2-C5). Incubation for 24 hours with dietary fiber (ispaghula husk or wheat bran), albumin, or glucose (10 mg/mL) increased the short-chain fatty acid production (43.6 +/- 2.8, 45.4 +/- 2.0, 60.3 +/- 3.2, and 65.8 +/- 3.1 mmol/L, respectively) compared with controls (21.4 +/- 1.3 mmol/L). The degradation of different substrates was associated with the production of different amounts of ammonia and short-chain fatty acids. Ispaghula, wheat bran, albumin, and glucose were fermented to acetate (> 2 mmol/L; 24-hour incubations) in 86%, 96%, 98%, and 98% of the homogenates, to propionate in 80%, 76%, 100%, and 68%, and to butyrate in 32%, 94%, 88%, and 54% of the homogenates, respectively. Isobutyrate, valerate, and isovalerate were produced from albumin in all (100%) of the homogenates, but only in 2 to 4% of the homogenates incubated with ispaghula or glucose. Ammonia was always (100%) produced after the addition of albumin and always (98%) consumed (assimilated) when glucose was fermented. Surgery (sigmoid or right- or left-sided colonic resection) did not change the pattern of ammonia and short-chain fatty acid production from these substrates. This study suggests that the different colonic flora from a large number of subjects share general biochemical characteristics, which metabolize different substrates to specific patterns of ammonia and short-chain fatty acids.

Fermentation to short-chain fatty acids and lactate in human faecal batch cultures. Intra- and inter-individual variations versus variations caused by changes in fermented saccharides

The fermentation to short-chain fatty acids, lactate, and ammonia from several non-starch polysaccharides, glucose, and albumin was investigated in 16.6% faecal homogenates. Increasing concentrations (0-30 mg/ml) of glucose, wheat bran, pectin, ispaghula, cellulose, or albumin incubated for 24 h in homogenates pooled from three individuals increased short-chain fatty acid production linearly. Amounts and ratios of short-chain fatty acids formed were highly dependent on the type of substrate fermented. Fermentable saccharides increased ammonia assimilation, in contrast to the metabolic inert cellulose. Nine faecal homogenates sampled from three individuals at three occasions were incubated for 6 and 24 h. The production of total short-chain fatty acids, acetate, propionate, and butyrate and the accumulation of D- and L-lactate changed considerably in relation to the type of substrate added (cellulose, ispaghula, wheat bran, pectin, gum arabic, and glucose; p less than 10(-4)-10(-7). In contrast, there were no significant (p greater than 0.05) differences in organic acid formation between the nine homogenates, and the intra- and inter-individual variations were of the same magnitude. Variations in fermentation, when measured as organic acid formation, were therefore related to the type of substrate fermented rather than the individual tested.

Short-chain fatty acid and organic acid concentrations in feces of healthy human volunteers and their correlations with anaerobe cultural counts during systemic ceftriaxone administration

Short-chain fatty acid (SCFA) and organic acid concentrations and (an)aerobe cultural counts were determined simultaneously in fresh feces from 10 human volunteers before, during, and after 5 days of single-dose ceftriaxone administration. The changes and the correlation coefficients between these variables were tested to be significant (P less than 0.05). to investigate whether significant changes in the intestinal microflora could be predicted by significant changes in SCFA/organic acid concentrations. Multivariate analysis was used with the same objective. Significant decreases in most SCFA, total SCFA (0.01 less than P less than 0.02), and pyruvic acid (0.02 less than P less than 0.05) concentrations in feces were found on day 3 of treatment and also on day 5, with the exception of the pyruvic acid concentrations. Lactic acid concentrations, however, were significantly increased (0.01 less than P less than 0.02) on day 5. All anaerobic cultural counts were significantly decreased (P less than 0.05) on days 3 and 5 of treatment except those for Clostridium difficile agar, which were only decreased on day 5. Highly significant correlations were found on day 3 between the total anaerobe counts (P less than 0.001), the Bacteroides counts (P less than 0.001), and propionic acid, of which the latter had a predictive value during treatment. The results of multivariate analysis were significant but did not have a predictive value. From this study we can conclude that propionic acid concentrations during ceftriaxone treatment in healthy volunteers have a predictive value for the Bacteroides cultural counts. The results of this study must be confirmed by those from patients treated with ceftriaxone.

Short-chain fatty acid and organic acid concentrations in feces of 10 human volunteers and their correlation with anaerobe cultural over a 15-month period

Short-chain fatty acid (SCFA) and organic acid concentrations were determined with gas-liquid chromatography (GLC) in fresh feces from 10 healthy volunteers, simultaneously with their fecal cultural counts. To find a simple and reliable method to detect disturbances in fecal flora, we calculated the intraindividual correlations between the concentrations determined by GLC and the fecal cultural counts. Twenty-three (4.7%) significant (P less than 0.05) linear correlations were thus found in nine of the volunteers. Using multivariate analysis, we attempted to assign fecal samples to individuals in accordance with their SCFA, cultural count, and organic acid profiles. Percentages of samples thus classified correctly were 49, 39, and 29, respectively. We conclude from this study that these percentages represent the normal situation in a group of healthy humans and that the low percentage of significant correlation coefficients shows that the relation between fecal SCFA/organic acid concentrations and fecal cultural counts has not been established.