Intestinal Microbiota in Sepsis

Sepsis is defined as a life-threatening organ dysfunction, which is caused by a dysregulated host response to infection. The composition of the intestinal microbiota is significantly different between patients with sepsis and healthy individuals. Intestinal microbial imbalance plays an important role in the occurrence and development of sepsis. Our review mainly introduces the mechanism of intestinal microbiota involvement in sepsis, the effects of microbiota dysbiosis on the damage of multiple organs and concisely discusses the prospects for microbe-specific treatment of sepsis in the future.


Introduction
Sepsis is the major cause of morbidity and mortality and is a major public health problem worldwide [1]. Sepsis accounts for nearly 26% of all deaths worldwide, with more than 20 deaths per minute [2]. It is defined as a dysregulation of the host response to infection leading to life-threatening organ dysfunction [3].
The intestinal microbiota is a huge and complex ecosystem, accounting for approximately 80% of the total flora. The intestinal flora is mainly composed of Firmicutes and Bacteroidota [4][5][6] in healthy volunteers. It is also known as the second genome and has a significant impact on human health [7]. The intestinal microbiota is also considered an important part of the intestinal barrier [8]. An imbalance of the intestinal flora may lead to bacterial translocation by increasing intestinal permeability and inducing mucosal immune dysfunction [9]. Microbiota dysbiosis can lead to various diseases, including diabetes and obesity [10][11][12].
Recently, it has also been considered an important factor for increased susceptibility to sepsis [13]. In a single-center controlled case study, 20 classifications of gut microbiota were used as markers for the diagnosis of sepsis, and the receiver operating characteristic score reached 81.25% [14]. It has 1 3 also been reported that the gut microbiota may be an active participant in the development of sepsis [15,16]. Therefore, targeted therapy of gut microbiota may be a promising strategy for sepsis. Here, we discussed the role of intestinal flora in the development of sepsis, and summarized the latest progress in the field of targeted treatment of sepsis.

Microbiota Disorder Related to Sepsis
In sepsis, factors such as hypoxic injury, inflammation, intestinal motility dysfunction, destruction of epithelial cell integrity can change the composition and proportion of intestinal microbiota [15,17]. Some harmful bacteria such as Vibrio cholerae and Trichinella spiralis [18] could induce mucin degradation and inhibit mucus production. However, Lactobacillus spp could stimulate mucin3 and mucin2 production and secretion [19,20], Akkermansia muciniphila could restore mucus growth and increase the number of goblet cells [21]. The microbial metabolites of short-chain fatty acids (SCFAs) also could stimulate mucin2 expression, increasing mucus secretion [22]. Intestinal micro-ecological collapse may occur in sepsis, and the dominant genus changes from common symbiotic bacteria to pathogenic genus [23]. The function to maintain healthy bacteria, such as Faeculus, Prevotella, Verrucobacterium, and Lauterella, also decreases [24]. The intestinal flora of septic shock patients is characterized by low population diversity and high degree of individualization associated with excessive reproduction of single genus [25,26]. Significant changes in microbiota may be related to the progression of sepsis [27]. Studies have showed that the gut microbiota is involved in the pathogenesis of late onset sepsis (LOS) and is the main risk factor for LOS [28,29]. Du et al. found the microbiota dysbiosis was associated with increased mortality, and the gut microbiota can be used as a prognostic marker of sepsis [30].
Under stress conditions, the production of opioids can reduce intestinal peristalsis, enhance the virulence of Pseudomonas aeruginosa, and promote the overgrowth of intestinal pathogenic bacteria [31][32][33]. A study of 1,265 intensive care unit (ICU) patients [34] found that more than 75% patients received antibiotic treatment every day, which usually comprised two or more antibiotics. Many broadspectrum antibiotics inhibit or kill the dominant bacteria, which causes increased colonization by conditional pathogens and fungi, results in opportunistic or secondary infections, and aggravates intestinal microecological imbalance [35]. Broad-spectrum antibiotics also increase the risk of Clostridium difficile infection [36,37], as well as the incidence of antibiotic-related diarrhea [38].
Parenteral nutrition damages the inherent mucosal immune function, leads to changes in the intestinal flora, reduces the expression of antibacterial protein from the intestinal Paneth cells and the bactericidal activity of mucosal secretions, and increases the sensitivity of intestinal invasion of Escherichia coli [39]. It is related to the increase of Treg and CD8 + T cells, reducing the total number of CD4 + T cells [40]. It also affects the function of intestinal-associated lymphoid tissue, and increases the expression of interleukin (IL)-4, IL-10 and immunoglobulin (Ig) A mRNA [41]. Significant changes in intestinal metabolomic components and increased intestinal permeability were observed after parenteral nutrition [42]. Long-term parenteral nutrition has a significant adverse effect on the levels of Lactobacilli and Bifidobacterium in preterm infants [43]. A recent meta-analysis suggests that parenteral nutrition has a higher abundance of Proteobacteria, the lower Firmicutes, Bacteroides and microbial diversity than enteral nutrition [44].
Intravenous infusion resulted in a dose-dependent increase in Bacteroides, decreased recombinant sodium/glucose cotransporter 1 (SGLT1) and Caspase3 levels, and arrested a transient increase in opportunistic pathogens, suggesting that crystal resuscitation may be beneficial to intestinal health [45]. It also affected the intestinal microbiome beta diversity and reduced the impact of bleeding on intestinal ecological imbalance [46]. Yang et al. found that microbiota led to the development of inflammation and intestinal mucosal high permeability after resuscitation [47]. However, Muraoka et al. found that resuscitation aggravated the loss of microbiota diversity and the danger of intestinal flora imbalance, which may be due to the high permeability of blood vessels and supplying a growth matrix for bacteria [48].
A retrospective analysis of 831 patients with bloodstream infections showed that hemodialysis is an independent risk factor for bloodstream infections [49]. The microbial abundance and diversity in hemodialysis patients showed a downward trend [50], and the altered intestinal bacteria may impact the prognosis of patients [51].
In addition, alkaline phosphatase in the normal intestine can catalyze the dephosphorylation reaction of lipopolysaccharide and weaken the stimulatory effect of lipopolysaccharide on Toll-like receptor 4 (TLR4) [52]. In sepsis, the phosphate necessary for the proliferation and growth of bacteria is reduced, which changes the type and virulence of intestinal microorganisms [53]. Pseudomonas aeruginosa appears and proliferates, in addition to secreting agglutination of plasminogen activator inhibitors or adhesin [54], and destroying the intestinal epithelial barrier, while Candida albicans transforms into a lethal phenotype [55].

Interaction Between Intestinal Microbiota and Systemic Immunity
The intestinal flora not only plays an important role in innate [56] and intestinal mucosal immunity [57,58], but also affects intestinal microcirculation through inflammation and vascular reaction [59]. Studies have observed that IgA-secreting cells [60] and CD4 + T cells [61] were significantly reduced in germ-free mice. The destruction of the balanced intestine could cause immune system responses [62], leading to inflammatory reactions and ultimately destroying the mucosal barrier [63]. The intestinal flora affects the systemic immune response by regulating several key pathways, including the production of SCFAs [64], oral immune tolerance [65], control of inflammatory T cell populations, and expansion and differentiation of extraintestinal T cells [66,67]. Disordered intestinal flora can induce a decrease in the number and differentiation of dendritic cells [68], suppress T cell immune responses [69], and increase susceptibility to pathogenic microorganisms in sepsis [70]. In addition, flora dysbiosis can induce the macrophages to differentiate into the M1 type [71] and release tumor necrosis factor-α (TNF-α), IL-1 [72], and IL-6 [73], which aggravate the inflammatory reaction in sepsis (Fig. 1).

Intestinal Microbiota is a Main Source of Respiratory Flora
Intestinal flora could provide extensive protection against respiratory infections to a certain extent. Mice with reduced intestinal flora demonstrate an impaired immune response to bacterial or viral infection in the respiratory tract [74]. A recent study confirmed that the imbalance in intestinal flora could activate the TLR4/NF-κB pathway, thereby upregulating lung tissue inflammation (Fig. 1). It could be a major reason to explain why the lung is often the first organ to be injured after bacterial translocation [75]. The prevalence of intestinal flora in the bronchoalveolar lavage fluid of patients with sepsis is high and is related to systematic inflammation. The lung microbes recovered in septic mice are mostly similar to intestinal flora, suggesting that the lower digestive tract could be the main source of lung flora [73]. Translocation of the intestinal microbiota seems to be related to the development of acute respiratory distress syndrome (ARDS). Dickson et al. [75] recently discovered that the lung microbial community is enriched by microbiota transferred from the intestine in patients with Fig. 1 Role of microbiota imbalance in lung and intestinal immunity. The imbalance of intestinal flora in sepsis shows that the diversity of dominant bacteria decreases along with an increase in pathogens, resulting in bacterial translocation. It affects the production of short chain fatty acids (SCFA) and alkaline phosphatase, which can decrease the stimulatory effect of lipopolysaccharide on Toll-like receptor 4 (TLR4). Neutrophils are increased and show delayed clear-ance; Treg cell apoptosis is blocked; the number and differentiation of dendritic cells decreases; macrophages differentiate into the M1 type; there is T and B cell apoptosis along with impaired function; tumor necrosis factor-α (TNF-α, Interleukin-1 (IL-1), and IL-6 are released; and the lung is the prime organ to be injured after bacterial translocation. Additionally, TLR4/NF-kB pathway is activated and upregulates lung tissue inflammation sepsis and ARDS. Interestingly, fecal microbiota transplantation (FMT) from normal mice can reduce acute lung injury by restoring intestinal microecology [76].

Intestinal Microbiota and Organ Dysfunction
LPS destroys the integrity of the gastrointestinal (GI) barrier, partly due to the weakening of bile components and altering mucosal hydrophobic properties. It can also cause gastric bleeding, bile acid reflux into the gastric cavity, and reduce bile salts in the ileum [77]. Phosphatidylcholine (PC) containing hydrophobic layer covers and protects the surface of the GI tract, contributing to the barrier's integrity. The integrity of the intestinal barrier is impaired in sepsis because of PC degradation [78]. Heparin-binding protein levels were associated with GI dysfunction in critically ill patients and positively correlated with sepsis [79]. Sepsis also induces high intestinal permeability and epithelial cell apoptosis. Preventing intestinal cell apoptosis can prevent intestinal barrier dysfunction [80]. A prospective clinical trial in 54 sepsis patients found that early enteral malnutrition can improve intestinal barrier function and inflammatory status [81]. Klebsiella pneumoniae infection leads to profound changes in the intestinal microbiome and metabolome. However, the intestinal microbiota and SCFAs could have a protective effect on lung injury [82]. The disturbed intestinal microbiota can directly contribute to organ dysfunction other than lung during sepsis. SCFAs derived from the gut microbiota can prevent acute kidney injury (AKI) [83]. The intestinal microbiota reaches the liver through the portal vein, promoting the capture and killing of circulating pathogens by Kupffer cells. However, antibiotic driven death of microbiota can lead to a failure in pathogen clearance and diffuse infection [84]. And microbiota has been also considered a key player in liver diseases by interacting with bile acid or inflammatory signals [85]. Cecal ligation and puncture mice receiving septic feces showed more severe liver inflammation and injury than receiving healthy feces [86]. It is fascinating that the gut microbiota can not only cause delirium indirectly through inflammatory pathways [87], but can also lead to acute brain dysfunction through bacterial translocation to the brain during sepsis [88]. However, FMT can reduce the harmful neurological effects [89].

Microbial-Specific Therapy in Sepsis
Studies [90,91] have shown that probiotics may reduce the severity, duration, and incidence of respiratory infections in children, adults, and the elderly. Although the quality of the evidence in these meta-analyses was low, the heterogeneity of treatment effects between the studies was significant. However, the association between probiotic use in critically ill patients and infection risk remains to be verified [92].
FMT has been shown to have a clear or potential therapeutic effect in many diseases, such as in Clostridium difficile infection [93]. FMT enhances pathogen clearance by restoring host immunity and reversing the process of fatal sepsis, which is related to the proliferation of butyrateproducing Bacteroides [94]. A unified approach should be used in the selection of donors, preparation of fecal bacterial liquid, transplantation methods, and subsequent treatment. Standard clinical practice guidelines and high-quality clinical randomized controlled studies are needed to further verify its long-term safety.
Selective decontamination of the digestive tract (SDD) is the most controversial treatment method against pathogenic groups. SDD uses local antibiotics to suppress potential pathogens while maintaining colonization resistance [95]. A comprehensive network meta-analysis showed that SDD could prevent secondary infections and reduce overall mortality in critically ill patients [96]. However, in a clinical randomized controlled study conducted in 13 European ICUs, the use of SDD was not associated with a reduction in ICU-acquired bloodstream infections and mortality [97].
In recent years, the improvement of intestinal microbiota disorders by traditional Chinese medicine is also a hot spot in the treatment of sepsis. Jinzhi [98] and Xuan Bai Cheng Qi decoction (XBCQ) [99] can restore the damaged tight binding protein occludin of the intestinal epithelium and improve the intestinal barrier dysfunction. Studies have found that Chinese medicine, such as XBCQ [99], Qing Re Jie Du Fang Decoction (QRD) [100], Shenfu decoction (SFD) [101], and Jinzhi [98] could alleviate the intestinal flora disorders and have similar diversity and structure to the control group in sepsis. QRD [100], SFD [101], Jinzhi [98], and Sini decoction (SND) [102] also reduced the mortality rate and inflammatory cytokines in sepsis. As shown in supplementary material Table 1, we searched all articles related to sepsis and intestinal microbiota, and summarized the research progress of sepsis and intestinal microbiota.

Future Prospects
With the continuous in-depth study of the intestinal microecology in patients with sepsis, the role of intestinal microbiota in the development of sepsis will become clearer. Early identification of microbiota disorders as well as the restoration of the intestinal microbiota balance are expected to become important aspects for the treatment of sepsis.

Conclusion
Intestinal flora is essential for maintaining homeostasis in the host. Intestinal microbiota dysbiosis may be widespread and play a key role in the pathogenesis of sepsis, leading to organ dysfunction. Microbial targeted therapy has been shown to improve the prognosis of sepsis; however, large-scale studies are required to explore their safety and effectiveness.

Acknowledgements
We would like to thank the Master Yuqing Cui for consulting related literature and Master Mengxing Peng for the diagram.
Author Contributions All the authors contributed substantially to the work presented in this article. TWS conceived of the study. LXW and HBZ revised the article. All authors have approved the final and submitted version of the manuscript.

Conflict of Interest
The authors have no conflicts of interest to disclose.
Ethical Approval Not applicable.

Consent to Participate Not applicable.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visithttp:// creat iveco mmons. org/ licen ses/ by/4. 0/.