Gut microbiota have both beneficial and adverse effects on human health. Recently, associations between the gut microbiota and CKD have gained attentions. Gut-derived metabolites and toxins affect CKD progression, and uremic toxins affect the microbiota[10, 11]. However, the relationship between gut microbiota and CKD is unclear, and further studies are necessary to explore the causal relationships between gut dysbiosis and kidney disease. Our study verified that TMAO level was correlated with both kidney damage and intestinal microflora disorder in patients with CKD, and providing a new perspective for better understanding the relationship between intestinal microflora and CKD. Here, we report the changes in gut microbiota and intestinal metabolite TMAO production in CKD patients.
TMAO is a gut microbiota-derived metabolite of choline, betaine, and carnitine. The plasma level of TMAO is determined by several factors including diet, gut microbial flora, drug administration, and liver flavin monooxygenase activity. Our data demonstrated that the plasma TMAO levels of the CKD patients were 15.4-fold higher than that of the control group, which was consistent with previous reports. Several studies reported that TMAO levels in CKD patients were elevated, and that renal function was also a major determinant of TMAO levels[12, 13]. Many studies have shown that circulating TMAO levels are linked to a wide range of health conditions including all-cause mortality, hypertension, cardiovascular disease, diabetes, cancer, and kidney function[14, 15]. Moreover, in mice choline and TMAO feeding is associated with increased renal fibrosis and impaired renal function, and inhibition of TMAO production attenuates CKD, suggesting that TMAO is not merely a marker of reduced renal function, but a potential cause of CKD[9]. In our study, CKD patients with low GFRs had significantly elevated TMAO levels, and those with high TMAO levels showed relatively higher levels of BUN and creatinine. Our results confirmed that renal function is a key contributor to TMAO concentration, and a high TMAO level might be an important risk factor affecting renal function in CKD patients.
Numerous studies have indicated that gut dysbiosis or toxic metabolite accumulation contribute to the onset and progression of CKD[16, 17]. The two most important bacterial phyla in the gastrointestinal tract, Firmicutes and Bacteroidetes, have gained considerable attention in recent years. The phyla Firmicutes and Bacteroidetes have been associated with metabolic diseases in humans[18]. Our data indicates that the relative abundance of Firmicutes phyla increased, while that of Bacteroidetes phyla decreased in CKD patients. The Firmicutes/Bacteroidetes (F/B) ratio is widely accepted to have an important influence in maintaining normal intestinal homeostasis. An increased or decreased F/B ratio is regarded as dysbiosis, whereby the former is usually observed with obesity and the latter is associated with inflammatory bowel disease (IBD)[19]. However, their association with disease is not always consistent between studies[20]. Accordingly, the F/B ratio was significantly increased in patients with CKD. Owing to the diversity within and across phyla, the mechanisms by which specific members of the microbiota can affect human health remains to be elucidated. Intestinibacter, Clostridium, and Romboutsia, belonging to Firmicutes, were negatively correlated with hypoglycemic effects[21]. Blautia and Intestinibacter increased significantly in Crohn's disease[22]. In our study, the abundance of genera Blautia, Romboutsia, Dorea, Lactobacillus, Intestinibacter, Holdemanella, Anaerovorax, Clostridium_XVIII, and Clostridium_sensu_stricto of the phylum Firmicutes was significantly exhausted in CKD patients. Bacteria-associated metabolites, such as butyrate, are the main energy sources for colon cells and play an important role in maintaining mucosal barrier function[23, 24]. Herein, we observed that the genera Roseburia and Megamonas, butyrate-producing bacteria from the phylum Firmicutes, were significantly reduced in CKD patients. Bacteroidetes are the largest phylum of gram-negative bacteria inhabiting the gastrointestinal tract and are considered the leading players in the healthy state and sophisticated homeostasis safeguarded by gut microbiota[25]. Our data indicated that the relative abundance of Bacteroidetes phyla decreased in CKD patients. Bacteroides and Prevotella are the two main Bacteroidetes genera. In our study, the genus Bacteroidetes was more abundant in the healthy controls, and the abundance of the genera Alloprevotella and Streptococcus of the phylum Bacteroidetes was significantly increased in CKD patients. The most frequently reported changes in the gut microbiome in stage 3–5 CKD, especially those undergoing dialysis, are related to lower levels of Bifidobacteriaceae and Lactobacillaceae and higher levels of Enterobacteriaceae[26]. Recently, a study investigated the microbial communities in the feces of CKD patients at different disease stages and healthy controls, suggested that the taxonomic composition of microbial communities differed between CKD patients and healthy controls, with the observed changes in the gut microbiota related to disease severity[27]. The patients in our study had higher GFRs and were not taking many phosphorus-lowering drugs or applying dietary restrictions. Data is lacking on how to fully characterize gut dysbiosis in CKD and understanding its physiological impact.
CKD is a global health problem leading to high rates of morbidity and mortality. One of the major causes of death in CKD patients is cardiovascular disease. Elevated levels of gut microbiome-derived TMAO are associated with tubulointerstitial fibrosis, atherosclerosis, and increased risk of major cardiovascular events. A study with 19,256 patients found that high concentrations of TMAO and its precursors were associated with an increased risk of major adverse cardiovascular events and all-cause mortality, independent of traditional risk factors[28]. Our data demonstrated that the plasma TMAO level of CKD patients was significantly higher than that of the control group. Pearson correlation analysis was used to study the correlation between TMAO and the gut microbiome. Primarily, two different bacterial species (Firmicutes and Proteobacteria) have been identified as responsible for the metabolism of choline to produce TMAO[29]. Our data showed that Firmicutes and Actinobacteria were positively associated with TMAO production. At the genus level, Collinsella, Alloprevotella, Streptococcus, Lactobacillus, and Clostridiumsensustricto were positively associated with TMAO production, however, Bacteroidetes, ClostridiumXIVa, Parasutterella, ClostridiumXIVb, Roseburia, Fusobacterium, and Bilophila were negatively associated with TMAO production. Recent studies have reported that the genus Lactobacillus contributes to TMAO production, however, the genus Bacteroides shows a significant negative correlation with plasma TMAO levels[30], which is consistent with our results. Further research is needed to explore the relationship between TMAO levels and gut microbiota.
Our study had certain limitations. First, this was a single-center study, and a relatively small number of individuals were included. The research findings need further validation in a larger cohort to confirm them. Second, there is a lack of in vitro studies to validate the effect of bacterial flora alteration on increased TMAO levels and CKD progression. Furthermore, TMAO levels and gut microbiota were measured only once, which might not capture the long-term levels of the gut microbiota and its metabolites. Despite these limitations, our results provide information regarding altered microbial diversity and increased TMAO levels in Chinese CKD patients. The influence of intestinal flora and increased TMAO levels require further investigation.