The Controversial Role of Human Gut Lachnospiraceae.

The complex polymicrobial composition of human gut microbiota plays a key role in health and disease. Lachnospiraceae belong to the core of gut microbiota, colonizing the intestinal lumen from birth and increasing, in terms of species richness and their relative abundances during the host’s life. Although, members of Lachnospiraceae are among the main producers of short-chain fatty acids, different taxa of Lachnospiraceae are also associated with different intra- and extraintestinal diseases. Their impact on the host physiology is often inconsistent across different studies. Here, we discuss changes in Lachnospiraceae abundances according to health and disease. With the aim of harnessing Lachnospiraceae to promote human health, we also analyze how nutrients from the host diet can influence their growth and how their metabolites can, in turn, influence host physiology.


Introduction
The human gastrointestinal (GI) tract has an estimated surface of more than 200 square meters and represents the interface between the body and the external environment, hosting a complex polymicrobial ecology that includes bacteria, archaea, fungi, protists, and viruses. The population of human gut microorganisms is estimated at approximately 10 13 -10 14 , and thus, outnumber the somatic cells of the host by over 10 times. Therefore, intestinal microbiota and the relative microbiome directly affect human health and disease, and have been considered as a new "organ" [1]. In the gut, microbes are physically separated from the epithelium by the mucus. In fact, the microbiome colonizes the outer layer of the mucus, and microorganisms use nutrients from the mucus itself. In healthy conditions, bacteria will only exceptionally cross the mucus to specifically interact with epithelial cells [2]. Overall, the microbiome is able to crosstalk with the host via several metabolic products (postbiotic), including short-chain fatty acids (SCFAs) like propionate, butyrate, and acetate, which originate from dietary fiber degradation, vitamins, and immunomodulatory peptides. The close interaction between gut bacteria and the host generates many benefits through the control of nutrient uptake and metabolism, strengthening the gut integrity, preventing pathogen propagation, promoting immunological tolerance to antigens, and regulating host immunity [3][4][5]. GI microbes produce several bioactive compounds which can influence the physiology of the host [6,7]; some, like vitamins, are beneficial [8], whilst others are toxic [9]. The Human Microbiome Project and MetaHit have led Lachnospiraceae are currently described in the National Center for Biotechnology Information (NCBI) as comprising 58 genera and several unclassified strains [14]. Within Lachnospiraceae, Blautia, Coprococcus, Dorea, Lachnospira, Oribacterium, Roseburia, and L-Ruminococcus (Ruminococcus genus assigned to the Lachnospiraceae family) are the main genera that have been detected in the human intestine by metagenomics analyses. All members of Lachnospiraceae are anaerobic, fermentative, and chemoorganotrophic, and some display strong hydrolyzing activities, e.g., through the activity of pectin methyl-esterase, pectate lyase, xylanase, α-L-arabinofuranosidase, β-xylosidase αand β-galactosidase, αand β-glucosidase, N-acetyl-β-glucosaminidase, or α-amylase [15]. Lachnospiraceae are present in early infants, found even in the meconium [16][17][18]. However, increases in Lachnospiraceae abundances are associated with aging [19]. Lachnospiraceae abundance also increases in the intestinal lumen of subjects with different diseases, although the taxa of this family have repeatedly shown their ability to produce beneficial metabolites for the host.
The aim of this review is to unravel the physiological functions and a pathological supply of Lachnospiraceae, which are one of the core families of the human gut microbiota.

Lachnospiraceae Metabolism
Human colonic microbiota can process a wide range of substrates, including proteins, oligopeptides, dietary polysaccharides, endogenous mucins, and glycoproteins that escape digestion by the host [20]. The metabolism of carbohydrates by the gut microbiota is a key process supplying nutrients and energy to the host. Among Firmicutes, the Lachnospiraceae, Lactobacillaceae, and Ruminococcaceae species hydrolyze starch and other sugars to produce butyrate and other SCFAs [21][22][23]. Genomic analysis of Lachnospiraceae revealed a considerable capacity to utilize diet-derived polysaccharides, including starch, inulin, and arabinoxylan, with substantial variability among species and strains ( Figure 2) [24]. The growth of Roseburia inulinivorans on starch induces the enzymatic activity of Amy13A [25], including a GH13 amylase and two or more carbohydrate-binding modules, allowing cleavage of the α-(1,4) linkages in amylose, amylopectin, and pullulan [26]. R. inulinivorans can also utilize fucose through the upregulation of three fucose-inducible genes [27]. Other Roseburia species (i.e., R. intestinalis) are able to degrade xylan [28]. On the other hand, Eubacterium eligens and Lachnospira pectinoschiza were identified as pectin-utilizing Lachnospiraceae species of the human gut [29]. revealed that Blautia hydrogenotrophica YIT 10080T is one of the four strains that produced a major amount of p-cresol (≥100 µM) [55]. In addition, in end-stage renal disease (ESRD) patients, seven days of vancomycin administration resulted in a significant decrease in fecal Blautia, IS, and pCS levels in the serum, followed by their rebound to baseline values after the suspension of treatment [56]. IS and pCS are the products of tryptophan and tyrosine metabolism by the gut bacteria, and their increase following vancomycin therapy in ESRD patients indicates the resilience of the taxa generating these toxins [57]. Within a cohort of 1018 middle-aged women from TwinsUK, Blautia was the most common taxon associated with lower levels of indole-propionic acid (IPA), whereas a positive correlation was observed between IPA and Coprococcus [58]. IPA is a deamination product of tryptophan metabolism that has an important effect on host gut barrier function and antioxidant activity [20].
Flavonoids undergo various chemical modifications via hydrolysis, reduction, and other less clearly defined reactions of human gut microbiota metabolism. Eubacterium limosum and Blautia sp. MRG-PMF1-appear to metabolize flavonoids with a methoxy group, such as isoxanthohumol and icaritin, respectively [59]. An in vitro study showed that Blautia sp. MRG-PMF1 bio-transformed polymethoxyflavones (PMFs) in chrysin, apigenin, galangin, kaempferol, luteolin, and quercetin [60]. This class of flavonoids is involved in many biological functions, among which it exerts important role in anticancer, anti-inflammation, antiallergic, antimutagenicity, and neuroprotection activities. Strengthen the intestinal barrier through up-regulation of tight junctions and mucin production by enterocytes [61].

MD LD
Anti-inflammatory effects by induction of regulatory T cells, downregulation of pro-inflammatory cytokines and the Toll-like receptor (TLR) 4 receptors [62].
Activation of G protein-coupled receptor (GPR) 43 involved in the modulation of inflammation and stimulation of glucagon-like peptides (GLP) 1 and gastric inhibitory polypeptide; modulate appetite, reinforce insulin sensitivity and glucose metabolism [63,64].
GPR 43 binding suppresses colon inflammation therefore protect liver and down-regulate insulin signal transduction in adipose tissue [66].
Elevated energy extraction in form of SCFAs related to a high intake of dietary carbohydrates [67].

MD LD
Lower expression of peroxisome proliferator-activated receptor-γ, and stimulation of uncoupling protein 2 and stimulate oxidative metabolism in liver and adipose tissue [75].
Intestinotrophic effects of SCFAs mediated by GLP-2 which contributes to the development or maintenance of obesity through elevated intestinal absorption of energy (kcal) intake [68].
MD Inhibition of Histone Deacetylases by altering the acetylation pattern of H3 and H4 histones and inducing beta-cell proliferation by inhibiting the p38/ERK apoptotic pathway [71,72].
Dyslipidemia due to elevation of cholesterol and triglycerides that increasing the levels of Acetyl-CoA in obese patient and metabolic disturbance [69].
MD Substantial amounts of propionate entering into the mitochondrial tricarboxylic acid (TCA) cycle bypass the first four TCA enzymes, causing a shift in the cycle with a potential toxic effect [78,79].  [81,82]. Trophic effect on the colonic epithelium by raising the mucosal blood flux [83]. MD Increased production leads to activation of the parasympathetic nervous system and stimulation of insulin secretion. The role of acetate in driving obesity depends on the gut microbiota and on dietary fiber intake [84]. Transported to the portal circulation across the colonic mucosa, acetate passes through the liver and is regained in peripheral blood, where it is adsorbed by tissues involved in the rise of cholesterol synthesis [85]. indole MD LD CKD IBD Activation of aryl-hydrocarbon receptor by microbially derived indoles, these molecules promotes tissue repair and homeostasis involving interleukin (IL)-22 [88].

CKD
Indole and indoxyl sulfate affect arterial blood pressure via peripheral and central mechanisms dependent on serotonin signaling and contribute do cardiovascular disease in renal insufficiency [89]. Lachnospiraceae

Coprococcus
Tryptophan indole-propionic acid MD Engage the pregnane X receptor, leading to the upregulation of genes that regulate intestinal permeability and to the downregulation of TNF-α expression by enterocytes [90].

Lachnospiraceae in Health
The gut microbiota is able to influence human health through the production of small molecules that accumulate in the colon and circulate systemically [20]. In the intestinal environment, some bacterial taxa degrade cellulose and hemicellulose components of indigestible plant material, and this step increases their bioavailability for host absorption. A high percentage of butyryl-CoA:acetate CoA-transferase was found in ten healthy volunteers, resulting from the presence of E. hallii and E. rectale, which were among the 10 most abundant species [32, 92,93]. Complementary studies showed that Blautia and Roseburia species, often associated with a healthy state, are some of the main SCFA producers [94][95][96][97][98]. Blautia and Roseburia represent the genera most involved in the control of gut inflammatory processes, atherosclerosis, and maturation of the immune system, demonstrating that the end products of bacterial metabolism (butyrate) mediate these effects [99,100]. SCFAs were reported to be the major source of nutrition for colonic epithelial cells [98,101], especially butyrate [102,103]. SCFA activity modulates the surrounding microbial environment and directly interacts with the host immune system [104]. In addition, SCFAs lead to improved host histone epigenetic states, a shift from glycolysis to fatty acid metabolism in colonic epithelial cells, and decreased levels of inflammatory markers [99]. In mouse studies, the levels of microbiota-derived SCFAs differed according to diet [105], with a reduction in the feces of germ-free (GF) and antibiotic-treated mice compared to the control [106]. Moreover, diminished colonic regulatory T cells (Tregs), which are essential for self-antigen tolerance and autoimmune disease prevention, can be restored with SCFA administration after vancomycin treatment [107]. Specifically, propionate and acetate promote Treg accumulation in the colon [105,106], whereas butyrate and propionate enhance Treg differentiation [104][105][106]. Butyrate can also stimulate colonic Treg differentiation, when locally administered [107], or in combination with dietary starch [105,107].
Furthermore, the Lachnospiraceae family has been associated with decreased lethality from graft-versus-host disease (GVHD) after allogenic blood/marrow transplantation in two clinical settings [108]. In particular, survival improvement was assessed in patients showing a higher amount of Blautia [108]. Evaluate of the expression of biomarkers for the inhibition of programmed cell death, e.g., protein 1 receptor (PD-1), programmed death ligand 1 (PD-L1), and cytotoxic T lymphocyte-associated protein 4 (CTLA-4) [109] has led to mounting evidence demonstrating how the intestinal microbiota can interact with and/or influence these proteins. By studying this relationship, the authors reported a positive correlation between PD-1 and the CTLA-4 blockade and increased levels of Dorea formicigenerans in humans [110].
Finally, Blautia showed a beneficial anti-inflammatory association with an improvement of the outcomes in other clinical settings, e.g., colorectal cancer, inflammatory pouchitis after ileal pouch-anal anastomosis, and liver cirrhosis [111,112]. Bajaj et al. [112] showed that the relative abundance of Lachnospiraceae in healthy subjects was ca. 22.4%. Moreover, a recent systematic review, characterizing the composition of the pediatric gut microbiome, fixed the relative abundance of this family at ca. 16.8% [113]. Since the aim of this review was to evaluate the changes referred to disease in comparison with healthy controls, any exclusion criteria (e.g., the sequencing methods, the selection of regions used for the taxonomic assignment, and the type of samples) were applied to assess the relative abundance in healthy subjects.

Lachnospiraceae in Disease
Different studies of GI characterization have been performed with the aim to investigate the host-disease-microbes interaction. To date, not all the published evidences are concordant in asserting an active or passive role of microbiota in pathologies onset. Whether an altered microbiota is a cause or consequence, several studies reported a positive or negative statistic correlations of Lachnospiraceae taxa with pathologic status. Based on this evidence, we have reported a list of pathologies for which the significant changes in Lachnospiraceae composition appeared to be more related than other co-factors, i.e., age, gender, genetics, geography or delivery mode. Moreover, we tried to clarify the impact of main metabolites related to Lachnospiraceae on different diseases.

Metabolic Diseases
Obesity and closely related metabolic disorders have become highly prevalent and are dramatically rising worldwide [114]. High-fat diets (in particular, industrially products characterized by trans-fatty acids) and refined carbohydrates are the main factors contributing to the onset of metabolic syndrome, determining central obesity and insulin resistance [115]. The components of metabolic syndrome, including arterial hypertension, insulin resistance, hypertriglyceridemia, and low serum HDL-cholesterol levels, dramatically increase the risk of developing diabetes and cardiovascular disease (CVD) [116] as well as liver steatosis [117], which has recently been renamed metabolic-associated fatty liver disease (MAFLD) [118]. Therefore, the human gut microbiota plays a significant role in metabolic syndrome etiology, interacting with the diet and host metabolism [119][120][121].
By evaluating the GI microbiota in relation to the body mass index (BMI), Ley et al. [122] found an increase in Firmicutes abundance (p-value = 0.002) and a corresponding decrease in Bacteroidetes (p < 0.001), associated with a high BMI. Additionally, weight loss gradually restored the ratio of Bacteroidetes/Firmicutes [122]. Within Firmicutes, high abundances of Lachnospiraceae were positively correlated with glucose and/or lipid metabolism, indicating metabolic disturbance [69,123,124] (Table 2). Zeng et al., administering 36 weeks of a high-fat diet to mice, found increased amounts of Firmicutes compared to mice fed with a low-fat diet, particularly Lachnospiraceae [125]. As a result of correlation analysis, Kostic et al. determined that triglycerides cluster with microbes; among these, there was a positive correlation between Blautia and long-chain triglycerides (p < 0.05 or Q < 0.05, cut-off of p < 0.001) [126]. They also determined that the alterations in microbiota may be related to the prediabetic stage of type 1 diabetes (T1D). Lachnospiraceae actively impaired glucose metabolism, leading to inflammation and promoting the onset of T1D [126]. According to this evidence, other metagenomics studies showed that Lachnospiraceae may also be specifically associated with type 2 diabetes (T2D) in both humans and mouse models [127,128].
A new member of the family Lachnospiraceae (Fusimonas intestini gen. nov. strain AJ110941P) was isolated from the feces of hyperglycemic obese mice, revealing its involvement in the development of obesity and diabetes in GF mice. Colonization by the abovementioned species, within GF mice, induced significant increases in fasting blood glucose levels, liver and mesenteric adipose tissue weights, associated with a decrease in plasma insulin levels and homeostasis model assessment-β (HOMA-β) values [128]. It was recently observed that treatment with S-allyl-cysteine sulfoxide, with hypoglycemic effects, determined a decrease of Lachnospiraceae in the microbiota of diet-induced obese mice [129].
In contrast with Ley et al. [122], using a real-time PCR analysis in overweight and obese individuals, Schwiertz et al. found a decrease in Firmicutes [130], due to different data collection or sample analysis. Moreover, some studies reported that obese individuals have higher fecal concentrations of SCFAs than normal weight controls [130,131], derived from a great fermentative activity. On the other hand, a recent metagenome-wide association study revealed a loss of several butyrate-producing bacteria in faecal samples from T2D patients [92], suggesting a potential protective role uniquely for butyrate. Under this light, the dietary carbohydrates intake seems to play a crucial role in patients with metabolic disturbances, revealing that only an adequate amount of butyrate should determine beneficial effects to the host.
It is important to remark that Lachnospiraceae (in particular Blautia) play a key role in the metabolism of undigested carbohydrates [24]. Despite the beneficial effects concerning SCFA production from saccharolytic metabolism [94], carbohydrate digestion by GI microbiota contributes to increasing the energy derived from the diet, and thus, affecting the above-reported fasting blood glucose levels ( Table 1).

Liver Diseases
Several studies have shown the main role of the gut microbiota in the pathogenesis and progression of the metabolic equivalent of liver steatosis, including non-alcoholic (simple) fatty liver and evolutive forms of non-alcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and even hepatocellular carcinoma [132][133][134][135][136].
The gut microbiota of patients with non-alcoholic fatty liver disease (NAFLD) was enriched with Lachnospiraceae, particularly Blautia and Lachnospiraceae incertae sedis [132] (Table 2). In the same study, Shen et al. also found that patients with NASH or significant liver fibrosis display a greater abundance of Lachnospiraceae, including Blautia (p < 0.01; false discovery rate (FDR) < 0.01) [132].
As previously described, high amounts of SCFAs do not ever determine a beneficial effect. The conflicting role of SCFAs on liver health could be intended as a consequence of modern lifestyle typically characterized by an imbalanced energy intake in terms of calories (Table 1). An altered liver functionality determined a failure in lipid metabolism, thus, implying a worsening of the liver disease and the host health [67].
Compared to healthy controls, Lachnospiraceae were significantly increased in patients with primary sclerosing cholangitis (PSC)-inflammatory bowel disease (IBD), but not among patients with IBD alone [137,138]. This dysbiosis could cause a dysregulation of mucosal immunity promoting lymphocyte activation and an increase in intestinal permeability [137].

Kidney Diseases
Intestinal dysbiosis also occurs in chronic kidney disease (CKD) [139,140] and might actively contribute to the progression of renal failure [141,142]. The main signature of CKD dysbiosis is increased Proteobacteria [143], although increased Lachnospiraceae is also observed [140,144] (Table 2).
Along this line, patients with minimal renal dysfunction display an increase of Blautia and Roseburia species and other unclassified Lachnospiraceae (linear mixed effects regression with p < 0.05 and a FDR < 5%) [145]. Similarly, in CKD rats, Blautia contributed to the divergence of CKD rats from sham rats (principal coordinate analysis based on unweighted UniFrac distances; linear discriminant analysis (LDA) score > 2.0 with p < 0.05) [57]. Among the biochemical parameters and changes in the gut microbiota of CKD rats, Blautia was positively correlated with increased proteinuria, independent of the creatinine clearance rates and systolic blood pressure (p < 0.05), showing a direct association with the disease (Pearson correlation analysis; p < 0.01) [57]. In the following studies on the pathological metabolome of CKD rats, Blautia was included among taxa that showed a positive correlation with trimethylamine-N-oxide (TMAO), propanal, spermine, spermidine, N1-acetylspermidine, glycine, cinnamoylglycine, phenylacetylglycine, phenylpropionylglycine, and putrescine [57]. TMAO is the final product of the intestinal microbial metabolism of dietary lecithin, L-carnitine, and choline, and contributes to the development of atherosclerotic plaques interacting with macrophages and foam cells [146]. All these findings have demonstrated how the gut microbiota seems to have a substantial influence on systemic cardiometabolic regulation, inflammatory activation, and CVD onset by modulating the levels of bioactive metabolites [147]. Notably, plasma TMAO levels decline following the suppression of intestinal microorganisms with oral broad-spectrum antibiotics, while they nearly return to prior levels after antibiotic retraction [148]. TMAO, pCS, and IS are all uremic toxins or their precursors, and their accumulation results in an increased risk of CKD progression [149].
Diabetes is considered the major etiological cause of CKD onset [150], affecting kidney failure progression and cardiovascular comorbidity. There is a close relationship between Lachnospiraceae and impaired glucose metabolism. Additionally, a vegan low-protein diet (daily intake of 0.7 g/kg, as characterized by plant-based proteins and an integration between cereal and legumes to provide essential amino acids) is the main conservative therapy used to prevent the progression of kidney failure to ESRD [151]. Therefore, all these factors may contribute to the detected Lachnospiraceae overgrowth; however, further studies are needed to completely understand if and how specific operational taxonomic units (OTUs) of Lachnospiraceae are directly implicated in CKD dysbiosis.

Inflammatory Bowel Disease
Studies link IBD and other chronic GI illnesses to host-microbe pathways [152,153]. Children and adolescents with newly diagnosed Crohn's disease (CD) displayed a loss in taxa belonging to the order of Clostridiales, including Dorea, Blautia, and L-Ruminococcus [154]. Compared to healthy controls (HC), the ileal-mucosa samples from sufferers of ileal Crohn's disease (ICD) had significantly lower levels of L-Ruminococcus, Roseburia, Coprococcus, and other unclassified Clostridiales (ICD: 3.1%, HC: 15.5%; P = 0.017) [155]. Lower amounts of Lachnospiraceae were also previously reported in ulcerative colitis (UC) patients compared to HC (p < 0.001; two-tailed Student's t-test) [156]. A positive correlation between Lachnospiraceae and SCFA levels was observed in UC fecal samples (R 2 = 0.48) [157]; otherwise, it was shown that Lachnospiraceae were not affected by the UC. For these reasons, the authors concluded that the decreased abundance of Lachnospiraceae and the resulting low butyrogenesis may play a role in triggering the recurrence of UC.
The disruption of the mucus layer might promote bacterial translocation and has been associated with IBD and CD [158,159]. The mechanisms for deconstructing mucin glycan structures rely on the cooperative action of several proteases, sulfatases, and glycosidases encoded by mucin-degrading bacteria ( Table 1). Most bacteria are supplied with incomplete enzyme packages specific for host mucin degradation that is likely to be achieved by a consortium of bacteria [53]. Ruminococcus gnavus has been identified as the major mucolytic bacteria in CD [80]. A comparative genomics analyses highlighted the presence of strain-dependent glycoside hydrolases (GHs), which is responsible for the breakdown and utilization of mucin-derived glycans [52]. With respect to UC, an increased bacterial sulfatase activity allowed R. torques mucolytic activity [80].
Moreover, Toll-like receptor 5 (TLR5)-deficient mice genetically sensitive to induced adherent-invasive Escherichia coli (AIEC) infections developed intestinal inflammation associated with microbiota alterations, among which, increases in Lachnospiraceae were observed [160]. Interestingly, members of Lachnospiraceae sampled by CD patients were previously identified as a microbial source of flagellins [161]. Hence, Jellbauer and Raffatellu supposed that the pathobiont-like AIEC triggers of the inflammation could be treated, but the increase of Firmicutes (i.e., Lachnospiraceae) remains the microbial hallmark of the depleted AIEC-infection [160].

Intestinal Dysbiosis Associated with the Gut-Brain Axis
Mounting evidence suggests that dysbiosis might also be involved in depression-like behavior [162][163][164]. Studies have focused on the gut-brain axis by evaluating the interactions between the GI microbiome and extraintestinal diseases. Pathways might involve reciprocal influences, linked by the sympathetic and parasympathetic system, circulating hormones, and neuropeptides [165][166][167][168]. Additionally, the vagus nerve determines the interaction between the brain and the stomach, suggesting that hormonal, neuronal, and bacterial changes in the bowel can be promptly transmitted to the brain via the vagus nerve [169].
The gut microbiome might influence multiple sclerosis syndrome (MSS) disease [173,174], and pathways might involve the immune system [175]. Chen and co-workers compared the intestinal microbiota of MSS patients in remission with the microbiota of healthy controls. The study aimed to evaluate the active role of the microbiome in predisposition/modification of the disease. MSS patients had increased amounts of Blautia and Dorea (P for Wilcoxon rank-sum test < 4.38 × 10 −4 and < 2.05 × 10 −5 , respectively) [176]. Some studies have shown that certain species of Dorea might promote the inflammation by supporting IFNγ production, metabolizing sialic acids, and degrading mucin [52,177]. Recently Shahi et al. [178] hypothesized that Dorea might play either pro or anti-inflammatory roles in MSS, depending on surrounding gut bacteria and/or cross-feeding interaction. According to the authors, in MSS patients, the growth of Blautia might be promoted through the utilization of gases produced by Dorea. The increase of A. muciniphila, another mucin-degrading bacterium, has been reported among MSS patients [179,180]. Dorea spp. and A. muciniphila can utilize a common pathway for mucin degradation, to induce proinflammatory responses, resulting in predisposition/ chronic inflammation. Therefore, the gut microbiota could be a cofactor responsible for the disease in genetically susceptible individuals. Further studies in this field are required. Blautia and Lachnospiraceae incertae sedis ↑ MDD Active-MDD patients (29) and responding-MDD patients (17) [172] Blautia and Dorea ↑ MSS Patients (31) [176] Abbreviations: ↑, increase; ↓, decrease; MD, metabolic diseases; NAFLD, non-alcoholic fatty liver disease; NASH, non-alcoholic steatohepatitis; PSC, primary sclerosing cholangitis; IBD, inflammatory bowel disease; IgAN, immunoglobulin A nephropathy; CKD, chronic kidney disease; AIEC, adherent-invasive Escherichia coli; CD, Crohn's disease; ICD, ileal Crohn's disease; CCD, Crohn's disease restricted to the colon; UC, ulcerative colitis; MDD, major depressive disorder; MSS, multiple sclerosis. syndrome.

Diet Modulates Lachnospiraceae Diversity
Although, the evidence suggests that the microbial composition can be clustered into enterotypes, the diet primarily modulates the gut microbiota composition [181][182][183][184][185]. In order to define the optimal diet for a healthy gut microbiota, a recent review unravelled the impact of single food components (macronutrients and micronutrients), salt, food additives, and different dietary habits on gut microbiota composition in term of richness and diversity [186]. Therefore, complex diets can provide a great range of growth-promoting and growth-inhibiting factors for specific phylotypes [187]. Human genomes are unable to encode most of the enzymatic patterns needed to metabolize dietary polysaccharides.
Otherwise, bacterial genomes codify several enzymes involved in saccharolytic degradation, including complex carbohydrates. Plant-derived polysaccharides enter the human large intestine in the form of insoluble structures. The presence of undigested nutrients in the large intestine determined the symbiotic interaction between humans and their GI microbiota [188]. However, it is important to underline that the transit time of digesta through the colon strongly influences the activities of gut microbiota [189].
Carbohydrates are mainly fermented in the proximal colon. The intestinal fermentation of carbohydrates determined the production of hydrogen and lactate, both as final and partial metabolites. In fact, metabolic cross-feeding represents a central process within anaerobic microbial communities [190,191]. Overall, the primary activity of the caecum and colon microbiota is in the decomposition of undigested carbohydrates. Certain species are responsive to particular dietary switches of carbohydrates, mainly bacteria that are specialized to use resistant starch or non-starch polysaccharides (NSP). Some members of the Roseburia/Eubacterium rectale group were the main responders to diets enriched in resistant starch [93,192]. Other Lachnospiraceae were strongly influenced by high-NSP diets [124]. Martinez et al. tested the influence of whole grains, barley, and rice administration on the gut microbiota composition. Compared to the baseline values, whole grain consumption increased the microbial diversity (alpha diversity) and abundance of Firmicutes. This change at the phylum level was primarily derived from an increased abundance of Blautia and Roseburia [193]. By including data from dietary intake and intestinal OTUs, Di Iorio found that several species of Lachnospiraceae (specifically Blautia wexlerae, B. obeum, B. coccoides, B. hydrogenotrophica, Coprococcus eutactus, Lachnospira pectinoschiza, Pseudobutyrivibrio xylanivorans, and Roseburia faecis) were positively correlated with vegetable proteins, fiber intake, and potassium (FDR < 0.05) [194]. In fact, their ability to use complex plant material and transport degradation products of various sizes and compositions was confirmed by metagenomics studies [21]. This was probably achieved through the byproducts of ATP binding cassette (ABC) transporter proteins codified by the genomes of several Lachnospiraceae species. Furthermore, it was observed that Roseburia and Lachnospira were strongly associated with vegetable diets (vegetarian and vegan diets), and also displayed a negative association (p < 0.01) with the omnivore diet. On the same line, a recent study positively correlated Lachnospira to the intake of beta-carotene, vitamin E and vegetable fat whereas a negative correlation was found with meat, total proteins, and cholesterol (FDR <0.05) [185]. Oppositely, L-Ruminococcus, Blautia, and Lachnobacterium were included in the cluster of bacterial taxa that positively correlated with animal-derived nutrients and negatively correlated with vegetable-based diet patterns [195].
High fat and sugar levels are the mainstay of the Western diet. As mentioned above, different OTUs of Lachnospiraceae were related to altered lipid metabolism and, thus, to obesity [69,124,126], or specific nutrients, such as saturated and total fats [195]. By analyzing the chemical composition at a single-cell level in C57BL/6NCrl mice, studies found that the microbiota composition, particularly amounts of Lachnospiraceae, was altered by high-fat feeding [196]. This microbial imbalance may originate from phylotype dynamic shifts, but also from altered Lachnospiraceae metabolic activity [196]. It is important to empathize that some dietary fats, particularly omega-3 polyunsaturated fatty acids (omega-3-PUFA), may improve human health reducing the risk of the coronary heart disease death and the develop of breast cancer [197][198][199]. Some Lachnospiraceae taxa showed that two weeks of diet implemented with omega-3-PUFA determined an increase of their abundances [200]. Specifically, at genus level Blautia and Coprococcus significantly increased, while at the species level, Roseburia spp./Eubacterium rectale became the predominant species [200]. Menni et al. [201] found a positive association between 36 OTUs and the serum levels of docosahexaenoic acid (DHA); 21 out of 36 OTUs belong to the Lachnospiraceae. DHA is one of the main structural lipids in the mammalian brain [202], positively linked to the prevention of numerous human pathologies including cancer and heart disease [203].
The involvement of Lachnospiraceae species in protein metabolism is less clear. In a previous trial, species of this family showed a marked negative correlation with the protein intake, especially animal proteins (FDR < 0.05) [194]. Additionally, in a study performed using a murine model, the relative abundance of Lachnospiraceae decreased after the consumption of a high-protein/low-carbohydrate diet, compared to a normal diet [204]. Although Lachnospiraceae appear to be less involved in proteolytic metabolism, the evidence provided could be the starting point for specific studies to link Lachnospiraceae to dietary digestion.

Conclusions
The evidence from different studies shows that Lachnospiraceae might influence healthy functions, although different genera and species of this family are increased in diseases. To the best of our knowledge, metabolic syndrome, obesity, diabetes, liver diseases, IBD, and CKD are all inflammatory conditions involving the Lachnospiraceae family or specific taxa of Lachnospiraceae. Furthermore, they appear to be involved in depressive syndromes and multiple sclerosis syndrome.
A deeper understanding of the mechanisms involved in interactions with the host will represent the main future challenge, with a specific focus on the immunological details and especially the diet interactions stimulating or restricting the presence of microbial pathways or the production of specific metabolites. The ultimate aim is to improve intestinal epithelial integrity and health. Further studies are needed to understand the potential impact of microbial-targeted therapies, including the modulation of Lachnospiraceae, with the end goal of their utilization in the prevention and treatment of both intestinal and extraintestinal diseases.