A large number of studies have shown that disorders of the intestinal flora can cause STC, and that FMT has an effect on STC. In the present study, the clinical improvement rate was 62.5% (5/8), clinical remission rate was 75% (6/8), and the WCS significantly decreased in patients after the third FMT treatment. The treatment also improved the patient’s quality and relieved the depressive symptoms of patients based on the GIQLI scores and HAMD. On the whole, FMT not only improves the symptoms of constipation, but also relieves depression. Patients who received FMT treatment had several adverse events, too.
Then, we collected the feces and serum from the patients of baseline and post FMT to characterize the differences of microbiota and metabolite based on 16S rRNA sequencing and metabolomics. Compared to previous studies, we found that the ecological diversity and richness in the fecal microbiome of constipated patients were different from post-FMT, indicating that constipation was significantly associated with an altered gut microbiome. As a whole, our results showed that the fecal microbiome of constipated patients exhibited an increased level of Bacteroidetes and decreased level of Proteobacteria/Actinobacteria from the Phylum. Lactobacillus, Bifidobacterium/Bacteroides, et al, decreased on the level Genus. After treatment, the numbers of these flora reversed. Together, these results showed that positive alterations of microbial community richness and diversity were exerted by FMT, suggesting that this might be a potential therapeutic mechanism . The gut microbiota in patients with constipation are different from the healthy controls, manifesting as the increasing abundance of Bacteroides and the decreasing abundance of Proteobacteria . Khalif et al. found that Bifidobacteria and Lactobacillus significantly decreased in the 57 adult patients with functional constipation; this result is consistent with our study . Moreover, it has been reported that Lactobacillus and Bifidobacterium can be used to prevent and relieve constipation [28–30]. Colonization with the specific microbiota such as Lacidophilus and Bifidobacterium bifidum in pathogen-free rats also normalizes small-bowel migrating motor complexes and gut transit time . Fecal qRT-PCR showed a significant low value in Bifidobacterium and Bacteroides species in fecal specimens from constipated patients when compared to healthy controls . There are many pathological damages in the intestinal epithelial cells of constipated rats: the depth of colonic crypt in constipated rats decreased, goblet cells were lost, et al . However, Lactobacillus can enhance intestinal epithelial barrier function and promote intestinal growth. Lactobacillus paracasei can increase short-chain fatty acid levels and promote intestinal peristalsis to relieve constipation [34, 35]. Meanwhile, Lactobacillus can promote intestinal electrolyte absorption by stimulating butyrate absorption and increasing Cl−/HCO3 − and Na+/H + transport [36, 37].
The metabolomic analysis showed the metabolite both in stool and serum changed significantly after FMT. The present study found that N-Acetyl-L-glutamate, gamma-L-Glutamyl-L-glutamic acid, and Glycerophosphocholine were significantly increased in stool after treatment. L-Arginine, L-Threonine, Ser-Arg, Indoleacrylic acid, Phe-Tyr, 5-L-Glutamyl-L-alanine levels were significantly increased and the level of Erucamide decreased in serum compared to the baseline. Especially, we found that the N-Acetyl-L-glutamate detected in feces is involved in the synthesis of L-Arginine. The research has shown that L-Arginine significantly inhibited the contraction in the distal colon to relieve symptoms of constipation and that L-Threonine is also associated with gastrointestinal diseases [38, 39]. It has been elaborated that nitric oxide(NO) plays an important role in relaxation of the smooth muscle in the gastrointestinal tract and the reduced production of NO may be an important contributing factor to gastrointestinal motility dysfunction . L-Arginine and nitric oxide synthases (NOS) can synthesize NO to relieve symptoms of constipation [41–43]. L-Threonine is involved in the metabolism of vitamin B6, which is involved in the formation of the neurotransmitter serotonin (5-HT) . The 5-HT regulates the central nervous system and enteric nervous system (ENS), including gastrointestinal (GI) motility and mood. Meanwhile, a deficiency of 5-HT can cause brain and intestinal dysfunction. Administration of slow-release 5-HTP to mice model of depression reduced depressive-like behaviors and promoted the growth of intestinal epithelium to normalize GI transit . Indoleacrylic acid is one of the metabolites of tryptophan, and tryptophan catabolites are important contributors to intestinal homeostasiss . In the antidepressant experiment, Erucamide induces the immobility time in the forced swimming test and tail suspension test which are used to evaluate depressive-like behavior. So Erucamide regulates the central nervous system to antagonize depression . Erucamide also can improve memory deficit . Our research showed that the patients’ depression symptoms were reduced, and this may be related to Erucamide. At the same time, changes of these metabolites are related to the intestinal flora. For example, L‐Arginine was positively correlated with lactobacillus. Erucamide had significant negative correlations with Sediminibacterium and Sharpea, while being positively correlated with Phascolarctobacterium. The N-Acetyl-L-glutamate gradually had significant positive correlations with lactobacillus.
Further research shows that L-Arginine and L-Threonine are involved in the protein digestion and absorption pathways which gradually upregulated in the KEGG pathways analysis. Intestinal epithelial cells absorb large amounts of Na + from this pathway. The imbalance of ion secretory and ion absorptive process in intestinal electrolyte transport can result in constipation and all of this is dependent on the Na+/K + ATPase. The main mechanism of mucus secretion in the intestine is the movement of Cl- from the blood-side to the lumen through epithelial cells, the subsequent electrical gradient drives the passive movement of Na+, and the osmotic gradient causes the movement of water into the lumen . The liquid extraction in preparation of stool depends on electrogentic Na + absorption in the distal colon . Sufficient small intestinal fluid secreted into the colon can change the consistency of stool and accelerate colonic transit [50, 51]. Our research shows that the protein digestion and absorption pathway produces more Na + in the intestine and may stimulate the intestinal mucosal epithelium secreting more water to relieve constipation. The motility of the GI tract is inseparable from electrical excitation and excitation–contraction coupling. Ion channels, which are the diverse group of pore-forming proteins, participate in the electrical signals of most tissues and, thus, generate every movement and perception . Ion channels, particularly Na + channels, are the essential players in electrical excitability of GI smooth muscle; consequently, drug developers considered that voltage-gated ion channels could be used as molecular targets in GI motility disorders . The Na+ channel NaV1.5, which was encoded by the SCN5A mRNA, decreased in the STC samples. A novel miRNA regulator of NaV1.5, let-7f, one of the overexpressed miRNA in STC, significantly decreased Na + current density and reduced motility of human smooth muscle. At the same time, overexpression of let-7f produced reduced GI smooth muscle contraction in animal experimentation .
A large amount of SCFA is produced in the process of protein digestion and absorption. SCFA is critical for colonic function, regulating colonic growth and differentiation, motility, blood flow, barrier integrity, and is a major source of metabolic fuel in the colon . Furthermore, in the colon, the microbial metabolism of undigested carbohydrates produces SCFA which promotes fluid absorption by Na + and SCFA − transporters and by increasing colonic NHE3 and NHE8 activity and expression .
Therefore, FMT can improve the symptoms of STC and the possible mechanism is increasing the production of L-Arginine and other metabolites by intestinal probiotics. These metabolites participate in the protein digestion and absorption pathways to produce Na+, thereby alleviating STC.