Clostridium Butyricum ZJU-F1 Benefits the Intestinal Barrier Function and Immune Response Associated with Its Modulation of Gut Microbiota in Weaned Piglets

This study investigated the effects of dietary C. butyricum ZJU-F1 on the apparent digestibility of nutrients, intestinal barrier function, immune response, and microflora of weaned piglets, with the aim of providing a theoretical basis for the application of Clostridium butyricum as an alternative to antibiotics in weaned piglets. A total of 120 weanling piglets were randomly divided into four treatment groups, in which piglets were fed a basal diet supplemented with antibiotics (CON), Bacillus licheniformis (BL), Clostridium butyricum ZJU-F1 (CB), or Clostridium butyricum and Bacillus licheniformis (CB-BL), respectively. The results showed that CB and CB-BL treatment increased the intestinal digestibility of nutrients, decreased intestinal permeability, and increased intestinal tight junction protein and mucin expression, thus maintaining the integrity of the intestinal epithelial barrier. CB and CB-BL, as exogenous probiotics, were also found to stimulate the immune response of weaned piglets and improve the expression of antimicrobial peptides in the ileum. In addition, dietary CB and CB-BL increased the proportion of Lactobacillus. The levels of butyric acid, propionic acid, acetic acid, and total acid were significantly increased in the ceca of piglets fed CB and CB-BL. Furthermore, we validated the effects of C. butyricum ZJU-F1 on the intestinal barrier function and immune response in vitro and found C. butyricum ZJU-F1 improved intestinal function and enhanced the TLR-2-MyD88-NF-κB signaling.


Introduction
The intestinal tract, a very important organ of mammals, is not only the largest "gas station" and "sewage factory" in mammals, but also the largest "fortress" for disease prevention. Therefore, intestinal dysfunction will lead to a series of problems, such as diarrhea, which seriously threatens the health of humans, especially children, and even animals. Weaning is a common problem, which can lead to intestinal dysfunction in severe cases and can, in turn, induce severe diarrhea. Although antibiotics are usually prescribed for this problem, long-term use and abuse of subtherapeutic antibiotic doses can lead to dysfunction of the gut microbiota and the appearance of antimicrobial resistance, both of which have raised serious public health concerns [1,2]. For example, it was estimated that up to 14,000 Americans die each year from infection with Clostridium difficile, a drugresistant bacterial strain that causes life-threatening diarrhea [3]. As a result, the European Union, USA, and China, which together raise more than 90 percent of the world's pigs, The Clostridium butyricum ZJU-F1 strain was initially isolated from the feces of a healthy nongenetically modified pig and was conserved in the China General Microbiological Culture Collection Center (CGMCC No.8939, Shanghai, China). C. butyricum ZJU-F1 was grown anaerobically in reinforced clostridium medium (RCM) at 37 • C with shaking at 200 rpm for 48 h. After centrifugation at 3000× g for 10 min at 37 • C, the cells were harvested and dried with drying technology at 60 • C for 48 h. The number of viable bacterial spores was determined by serially diluting the suspension on RCM agar(Haibo Biology, Qindao, China)and incubating at 37 • C for 24 h. The final concentration of C. butyricum in the animal diets was adjusted to 1.0 × 10 8 viable spores/kg of diet. The Bacillus licheniformis (B.licheniformis) was a commercial product manufactured by Zhejiang Huijia Biological Technology Ltd. (Huzhou, China). This product is composed of spray-dried spore-forming B. licheniformis endospores and contains at least 1.0 × 10 9 viable spores/kg diet.

Animals and Experiment Design
The animal experimental protocols were approved by the Animal Care and Use Committee of Zhejiang University. A total of 120 weaned piglets of similar age (Duroc × Landrace × Yorkshire) were randomly allotted to four treatments based on initial body weight. There were three replicate pens in each treatment with 10 pigs per pen (5 sows and 5 boars) and the dietary treatments are shown in Table 1. The experiment lasted for 14 days with 6 piglets slaughtered in each group at the end of experiment. The pigs were fed a cornsoybean meal-based diet that contained no antibiotics or growth-promoting concentration of zinc. All basal diets met or exceeded the nutrient requirements as suggested by the National Research Council (NRC) and their compositions are shown in Table 2.

Nutrient Digestibility, Digestive Enzyme Activity, and Intestinal Morphology Assessment
Nutrient digestibility was calculated as described by Wang et al. [22]. For the digestive enzyme activity of duodenum and jejunum determination, the mucosa of the duodenum and jejunum were collected and homogenized by adding sterile 4% saline solution to prepare 10% (W:V) homogenates. The homogenate was centrifuged at 1000× g for 10 min at 4 • C, then each digestive enzyme activity (protease, amylase, and lipase) in the supernatant was determined by spectrophotometry using a commercial kit according to the manufacturer's instruction (Sigma-Aldrich Co, Saint Louis, MO, USA). The middle duodenum was collected and fixed in 10% PBS-buffered formalin and embedded in paraffin. The 0.2 µm thick paraffin sections were cut and stained with hematoxylin and eosin (H&E). Images of the H&E-stained sections were acquired through a Leica DM3000 Microsystem (Leica, Germany). Then, the morphology of the duodenum was evaluated by scanning electron microscopy (SEM, TM-1000, Hitachi, Japan) and transmission electron microscopy (TEM, H-7650, Hitachi, Japan) as previously described [23]. SEM and TEM images were acquired and analyzed using a Hamamatsu ORCA-HR digital camera system operated with AMT software (Advanced Microscopy Techniques Corp., Danvers, MA, USA) and Semicaps 2000 software, respectively.

RNA Isolation and qRT-PCR
Isolation of total RNA and cDNA synthesis by reverse transcription was performed using the TRIzol reagent and a reverse transcriptase kit (Thermo Fisher Scientific, Boston, MA, USA). The cDNA was quantified using a Nanodrop ND-1000 Spectrophotometer (Nanodrop Technologies, Inc. Wilmington, DE, USA) and mRNA expression was quantitated using the SYBR Green quantitative PCR system. Primers for amplifying each target gene are listed in Table 3 and all data were analyzed using the 2 −∆∆Ct method.

Plasma D-Lactate and DAO Levels
After slaughter, 10 mL of venous blood was collected. The concentrations of D-lactic acid and the activity of diamine oxidase (DAO) were measured by enzymatic spectrophotometry using a commercial kit (Jiancheng Bioengineering Institute of Nanjing, China), according to the manufacturer's instructions.

Cell Damage and Adherence Assay
IPEC-J2 cells were removed with trypsin and were inoculated into a 48-well plate (Coring, NY, USA) for culture. When the cells reached 60%~70% confluence, they were stimulated with the pathogenic enterotoxigenic Escherichia coli (ETEC) K88 and C. butyricum ZJU-F1 for 1.5 h, separately, with four replicates per treatment. At the end of the stimulation, the culture medium was collected and lactate dehydrogenase (LDH) activity was detected with an LDH kit (Roche Applied Science). Assays for adherence, competition, exclusion, and displacement were performed as previously described [25].

Diversity Analysis of Cecal Microorganisms
Total bacterial DNA was extracted with a fecal genomic DNA extraction kit (Solarbio, Beijing, China). The extraction quality was determined by 1% agar-gel electrophoresis according to the instructions. The pure DNA was stored at −80 • C. Then, 16S rDNA was amplified by PCR with universal primers in the V3-V4 region (338F: ACTCCTACGGGAG-GCAGCA, 806R: GGACTACHVGGGTWTCTAAT). PCR was conducted as follows: predenaturation at 95 • C for 3 min, denaturation at 95 • C or 25 s, annealing at 55 • C for 25 s, stretching at 72 • C for 45 s, 25 cycles, and a final stretching at 72 • C for 10 min. The AxyPrep DNA gel extraction kit (Axygen Biosciences, Union City, CA, USA) and QuantiFluor-ST instrument (Promega, USA) were used to further extract, purify, and quantify the PCR products. The MiSeq platform (Shanghai Majorbio Biopharm Technology Co., Ltd.) was used to describe the bacterial community based on the gene segment from the V3-V4 portion of the 16S rRNA gene. PE reads obtained by MiSeq sequencing were splintered according to overlap relation to form a complete DNA sequence. The effective sequence was obtained by distinguishing the samples according to the primer sequence label and primer label, and the sequence direction was corrected. Meanwhile, Trimmomatic and Flash software were used for sequence quality control and filtration. OTUs were clustered with 97% similarity cutoff using UPARSE (version 7.1). Mothur 1.30.1 software was used to analyze the microbial diversity in the samples with outcome indexes Chao1, Shannon, and Coverage. Qiime1 software was used to calculate the community similarity of samples and principal component analysis (PCA) was conducted through the ecological distance between samples. The RDP classifier algorithm (http://rdp.cme.msu.edu/) against the Greengenes 16S rRNA database was used with a confidence threshold of 70%.

Determination of Short-Chain Fatty Acids (SCFAs)
SCFAs were determined as previously described [26]. Briefly, the cecal contents were suspended in PBS, then ultrasonicated and centrifuged at 12,000× g for 10 min at 4 • C. Then 25% metaphosphate and 210 mmol/L cortical acid were added to the supernatant, followed by 30 s of vortex and 10 min of centrifugation (5000 r/min at 4 • C). The supernatant was taken out and methanol was added (volume ratio, supernatant:methanol = 1:3), then vortexed for 30 s and centrifuged at 1000 r/min for 10 min. The supernatant was then filtered through filter paper for HPLC detection. A PrevailTM Organic Acid column (250 mm × 4.6 mm) was used with the following detection conditions: temperature, 40 • C; wavelength, 217 nm; pressure, 0.1-4000 psi.

Statistical Analysis
All statistical analyses were performed using SPSS software (Version 20.0, IBM Corp, Armonk, NY, USA) and all data were expressed as means ± standard error (SEM). Significant differences between the control and experimental groups were determined by a one-way ANOVA with a Duncan multiple. Statistically, a p-value of less than 0.05 was considered significant.

Effects of C. Butyricum ZJU-F1 on Diarrhea Rate, Apparent Digestibility, and Digestive Enzymes Activity
Our previous study [21] showed that dietary C. butyricum ZJU-F1 (CB) significantly increased the growth performance and decreased the diarrhea rate. These effects were more marked when in combination with B. licheniformis (CB-BL) ( Figure 1A-B). However, no significant difference was found between the BL group and the CON group. We found further differences in the piglets in the BL group, specifically, digestion of dry matter (DM), crude protein (CP), Ca, and P of piglets in the CB or CB-BL groups were significantly greater ( Figure 1C). The improvement of apparent digestibility is usually closely related to the activity of digestive enzymes in the intestine, so we examined the activity of amylase, lipase, and protease in the duodenum and jejunum. Similar to the results of apparent digestibility, the digestive enzyme activity of the CB group or CB-BL group was significantly higher than that of group BL and CON ( Figure 1D-E). These results indicated that dietary C. butyricum ZJU-F1 was beneficial to the intestinal digestive function of weaned piglets, especially when used in combination with B. licheniformis.

Effects of C. Butyricum ZJU-F1 on Intestinal Morphology
To explore the reasons for the improvement of digestive function in weaned piglets by C. butyricum ZJU-F1, we evaluated the effects of C. butyricum ZJU-F1 on the intestinal barrier function. The data showed that villous height in the jejunum of the CB and CB-BL groups increased significantly compared with the CON group ( Figure 1F, top); this result was confirmed by SEM at 150× magnification ( Figure 1F, middle). In addition, surface damage to villi in the jejunum was alleviated by B. licheniformis treatment ( Figure 1F, middle). Furthermore, the height of microvilli in the jejunum of piglets fed with C. butyricum ZJU-F1, B. licheniformis, and a combination of the two, was much higher than that in the piglets fed CON diets ( Figure 1F, bottom). These results suggest that C. butyricum ZJU-F1 improved intestinal morphology and integrity in weaned piglets with clinical diarrhea.

Effects of C. Butyricum ZJU-F1 on Intestinal Barrier Function
Plasma D-lactate and DAO levels are endogenous markers that indicate changes in permeability, which could directly reflect the degree of damage to the intestinal epithelial mucosa [27]. As a direct validation, we determined the levels of DAO and D-lactate in each group. As shown in Figure 2A,B, no matter whether the piglets were fed with C. butyricum ZJU-F1, B. licheniformis, or both, the serum level of DAO and D-lactate decreased significantly, which indicated that the intestinal integrity was enhanced. Compared with BL, CB-BL significantly reduced D-lactate, which indicated that this was the effect of C. butyricum ZJU-F1. These results suggest that supplementation of diets with C. butyricum ZJU-F1 can significantly decrease intestinal permeability. To further illustrate the effects of C. butyricum ZJU-F1 on intestinal barrier function, we analyzed the expression of mucins and tight junction proteins. Mucin, secreted by intestinal epithelial cells and goblet cells, can mix with water and electrolytes on the intestinal mucosal surface to form a chemical barrier on the intestinal epithelium to protect the intestinal mucosa from chemical and mechanical damage [28,29]. Interestingly, we found that mRNA expression of mucins (MUC1, MUC4, and MUC20) in the CB and CB-BL groups was significantly increased, but B. licheniformis treatment alone had barely any effect on the mucins' expression in the jejunum ( Figure 2C, left). Similar observations were also made in the ileum ( Figure 2C, right). In addition, we also evaluated the expression of the intestinal tight junction proteins ZO-1, claudin-1, and occludin. As expected, we found that the three tight junction proteins were significantly increased in both the jejunum and ileum of piglets treated with C. butyricum ZJU-F1, B. licheniformis, or both ( Figure 2D,E). In addition, compared with BL, CB-BL significantly increased the expression of mucin and tight junction proteins, which indicated that this was the effect of C. butyricum ZJU-F1.

Effects of C. Butyricum ZJU-F1 on the Intestinal Immune Response
As inflammation is one of the primary factors responsible for the destruction of the epithelial barrier [30], we analyzed the expression of proinflammatory cytokines. Interestingly, we observed that, after treatment with BL, CB, and CB-BL, the expression of the proinflammatory factors IL-1β, TNF-α, IL-6, and IL-8 was significantly increased, while the expression of the anti-inflammatory cytokine IL-10 was also upregulated compared to the control group ( Figure 3A). Additionally, we also detected the expression of endogenous antibiotic peptides. Piglets treated by CB and CB-BL upregulated the gene expression of pBDs and PR-39 compared to the control group, but BL treatment did not improve their expression ( Figure 3B). In addition, compared with BL, CB-BL significantly increased the expression of cytokines (IL-1β, TNF-α, IL-8, and IL-10) and antimicrobial peptides, which indicated that this was the effect of C. butyricum ZJU-F1.

Effects of C. Butyricum ZJU-F1 on Intestinal Microbial Diversity
Intestinal flora plays an important role in the digestion and absorption of nutrients in the body, and it is also closely related to the metabolism, physiological status, and health of the host. Here, we analyzed the intestinal microbial diversity in the ceca of each group. The PCA plot showed that the distribution between CON and CB-BL was relatively discrete, indicating that CB-BL treatment had a significant effect on the distribution of cecal microorganisms in weaned piglets. However, analysis of the phyla indicated that there were no significant differences in the dominant bacteria, including Firmicutes, Bacteroidetes, and Proteobacteria ( Figure 4B). We found that the BL, CB, and CB-BL treatments increased the percentage of Clostridia but decreased the percentage of Bacilli and Gammaproteobacterial at the class level ( Figure 4C). Furthermore, we observed that CB increased the percentage of Clostridiaceae_1 at the family level ( Figure 4D). As shown in Figure 4E and Table 4, we further found that the main bacteria in the cecal contents of weaned piglets were Ruminococcaceae_uncultured, Clostridium_sensu_stricto_1, Lactobacillus, Leeia, Faecalibacterium, Coprococcus, Pseudobutyrivibrio at the genus level. In contrast to the control group, the CB-BL diet increased the percentage of Ruminococcaceae_uncultured, Lactobacillus, Streptococcus in Firmicutes in which Lactobacillus was significantly increased. In addition, there were significant differences in cecal Lactobacillus between the BL and the CB-BL as an effect of CB (Table 4).

Effects of C. Butyricum ZJU-F1 on Cecal SCFAs
SCFAs play important roles in the intestinal flora [31]. We, therefore, determined the SCFAs present in each group. As shown in Figure 4F, compared to the CON group, the secretions of acetic, propionic acid, butyric acid, and total acid in the cecal contents of weaned piglets treated with BL, CB, and CB-BL were significantly increased. However, the concentrations of acetic acid, propionic acid, and butyric acid in the CB and CB-BL groups were higher than those in the BL group. In addition, there were significant differences in acetic acid and butyric acid between the BL and the CB-BL as an effect of CB. These results showed that C. butyricum ZJU-F1 diets can increase the concentrations of acetic acid, propionic acid, and butyric acid in the cecal contents of weaning piglets, which may be related to the regulation of intestinal microflora.

C. Butyricum ZJU-F1 Enhances the Intestinal Barrier Function and TLR-2-MyD88-NF-κB Signaling Pathway
To better understand the precise mechanism underlying the regulation of the intestinal barrier function by C. butyricum ZJU-F1, we established an in vitro model and selected the pathogenic enterotoxigenic Escherichia coli (ETEC) K88 as a representative of the many harmful bacteria in the gut. We first noted that the damage caused by C. butyricum ZJU-F1 to IPEC-J2 cells was significantly lower than that caused by ETEC K88, which was assessed by the LDH release rate ( Figure 5A). The results of the competition test showed that C. butyricum ZJU-F1 could significantly inhibit the adhesion of ETEC K88 to IPEC-J2 cells, reducing the adhesion rate of ETEC K88 from 12.52% to 4.59%, when C. butyricum ZJU-F1 and ETEC K88 were incubated simultaneously with the cells ( Figure 5B). Additionally, exclusion and displacement tests also indicated a significant reduction in the adhesion of ETEC K88 to IPEC-J2cells, in which the effect of exclusion was more significant ( Figure 5B).

Discussion
In the study, we found that C. butyricum ZJU-F1 had beneficial effects on the physiological functions of the small intestine and significant effects on the expression of cytokines and AMPs and cecal Lactobacillus, acetic acid, and butyric acid. Moreover, we further confirmed in vitro that C. butyricum ZJU-F1 improves intestinal barrier function and found that it enhances the TLR-2-MyD88-NF-κB signaling pathway.
During weaning, the activity of intestinal digestive enzymes decreases, the mucosal morphology and structure change, and the permeability increases, resulting in intestinal barrier dysfunction together with decreased digestion and absorption, in turn leading to intestinal diseases and diarrhea, hindering piglet growth [32]. Probiotics can affect the activity of some endogenous digestive enzymes and nutrients digestibility in the host, but there are few reports about weaning piglets. Our results indicated that dietary C. butyricum ZJU-F1 was beneficial to the physiological functions of the small intestine, digestive enzymes activity, and digestive function of weaned piglets. This might be partly due to C. butyricum ZJU-F1 promoting the secretion of SCFAs in the small intestine and improving the gut microbiota, thus improving the activity of digestive enzymes, and finally resulting in the improvement of the digestion and absorption of nutrients.
Intestinal barrier function is formed by a single layer of intestinal epithelial cells bound by tight junction proteins, which separates the coelenterate from the internal environment and restricts the spread of pathogens, toxins, and allergens from the lumen to the circulatory system. Plasma D-lactate is a metabolic product fermented by bacteria in the gut and DAO is an enzyme in the intestinal mucosal upper villi cells. Under normal conditions, there are little or no D-lactate and DAO in the blood, which are released into the blood only when the intestinal mucosal barrier is damaged, so their levels could directly reflect the degree of damage to the intestinal epithelial mucosa and permeability [27]. In addition, the higher the permeability, the greater the loss of nutrients in the gut [33,34]. Tight junctions, closely related to nutrient absorption and resistance to disease-causing microorganisms, are the main type of connection in intestinal epithelial cells [35]. In recent years, the effect of probiotics on intestinal barrier function has attracted much attention. It has been reported that the growth performance improvement of C. butyricum was associated with reducing or preventing the barrier damage caused by ETEC K88 in piglets and broiler chickens [36,37]. Zhang et al. [37] found that oral administration of C. butyricum can significantly decrease intestinal permeability challenged with ETEC K88 in broiler chickens. Lactobacillus reuteri increased the expression of Claudin-1, Zo-1, and occludin in the intestinal epithelium of newborn piglets, and reversed the barrier function damage of IPEC-J2 cells in the intestinal epithelium of pigs caused by LPS. As expected, our results suggest that C. butyricum ZJU-F1 enhances the expression of intestinal tight junction proteins in vivo and in vitro, and improves intestinal barrier function in vivo, decreasing plasma D-lactate and DAO levels. Taken together, these findings show that the enhancement of nutrient digestion and absorption in weaned piglets treated with C. butyricum ZJU-F1 might be partly due to the reduction of intestinal permeability and the improvement of intestinal barrier function.
A large number of microbes live in the intestinal tracts of animals. The host provides sufficient nutrition and a stable environment for the intestinal microbial microbiota. In turn, microbes participate in the digestion, absorption, and energy supply of the host, while also regulating the host's physiological functions [38]. Many studies have shown that probiotics can regulate the balance of intestinal microbiota in piglets, contributing to the formation of intestinal microflora dominated by beneficial bacteria, and thus mediating physiological functions such as nutrient metabolism, immunity, and disease in animals. Zhang et al. [39] showed that dietary supplementation of C. butyricum dramatically increased Selenomonadales and decreased Clostridiales, and within Selenomonadales, Megasphaera became the dominant genus, increasing from 3.79 to 11.31%. Our results suggested that CB and CB-BL could slightly increase the percentage of Firmicutes, but with no significance, in the cecum of weaning piglets and slightly reduce the number of Bacteroidetes, but with no significance. Upon further analysis of the genus level, the results showed that CB and CB-BL can increase the proportion of Lactobacillus in the cecum contents. However, changes in gut microbes can significantly alter SCFAs in the intestine.
SCFAs, the primary end products of bacterial fermentation and the main energy source of intestinal epithelial cells, can bind with varying affinities to G protein receptors in the intestines and other cells to regulate energy metabolism, intestinal homeostasis, and immune responses [38,40,41]. Wang et al. found that supplementation of C. butyricum can increase the concentrations of acetic acid, propionic acid, and butyric acid in the colon of weaning piglets [42]. We found that there were significant differences in acetic acid and butyric acid between the BL and the CB-BL as an effect of CB. The difference in cecal Lactobacillus between the BL and the CB-BL was also significant. By balancing the intestinal microbiota, C. butyricum can accelerate the digestion and utilization of nutrients by intestinal microorganisms and thus ferment more SCFAs, thus positively regulating the metabolism of the body and thus covering the function of the small intestine. Therefore, we have a hypothesis that the improvement of physiological function in the small intestine of C. butyricum ZJU-F1 might be partly due to alterations in microbial-derived metabolites. However, the level of SCFAs is lower in the control group, and one possibility is that antibiotics reduce SCFAs by destroying intestinal microbiota. If antibiotics are not fed, the piglets will be infected with pathogenic bacteria, which will also cause the colonization of harmful bacteria in the intestinal tract, destroy the intestinal microbiota, and thus reduce SCFAs. Therefore, we believe that in order to evaluate this possibility, healthy piglets should be used to feed antibiotics or C. butyricum ZJU-F1 to in the future.
Among the diarrhea of newborn piglets caused by pathogenic bacteria, the diarrhea caused by ETEC K88 is the most serious. After ETEC invades the body, it secretes adhesin that helps it adhere to and colonize intestinal epithelial cells. Intestinal peristalsis or intestinal secretions make it difficult to remove ETEC K88, thus the ETEC K88 will absorb nutrients in the intestinal tract and rapidly reproduce, producing enterotoxin, causing diarrhea of piglets, and even leading to death of piglets [43]. Therefore, to better understand the precise mechanism underlying the regulation of the intestinal barrier function by C. butyricum ZJU-F1, we established an in vitro model and selected the ETEC K88 as a representative of the many harmful bacteria in the gut. Our results confirmed that C. butyricum ZJU-F1 could significantly reduce the adhesion of ETEC K88 to IPEC-J2. When C. butyricum ZJU-F1 adhered preferentially, the adhesion rate of ETEC K88 decreased to 3.89%. When the ETEC K88 adhered preferentially, C. butyricum ZJU-F1 could replace the adhesion of ETEC K88 with a decrease of adhesion rate from 12.52% to 7.44%. This may be because C. butyricum ZJU-F1 occupies the adhesion site of ETEC K88 or stimulates the secretion of mucin by IPEC-J2 cells to change the composition and characteristics of mucus, thereby reducing the adhesion and colonization of ETEC K88 to intestinal epithelial cells. In addition, acetic acid and butyrate secreted by C. butyricum ZJU-F1 can also kill or inhibit the growth and reproduction of pathogenic microorganisms.
C. butyricum not only has direct nutritional function, but also can stimulate intestinal mucosal immune response in animals [39]. Our results showed that the expression of proinflammatory cytokines TNF-α, IL-1β, IL-6, and IL-8, and anti-inflammatory cytokines IL-4 and IL-10 were significantly upregulated after C. butyricum ZJU-F1 treatment in IPEC-J2, consistent with the results of animal experiments. In general, animal or cell infection with pathogenic bacteria is detrimental, but the release of proinflammatory cytokines in the body after infection is a way to resist pathogenic bacteria. An appropriate amount of proinflammatory cytokines can regulate the immune response to some extent, resist or clean pathogen infection, and have the effects of promoting the repair of damaged tissues and causing tumor cell apoptosis, and so forth, but excessive amounts of proinflammatory cytokines can cause intestinal tissue damage and destroy the immune balance of the body [44]. In weaning piglets and weaning rex rabbits, studies have shown that C. butyricum weakly activates the body's immune response [19,45], and alleviates high-fat diet-induced steatohepatitis in mice via enterohepatic immunoregulation [46]. We believe that the CB group has a low level of immune response, which may be a self-protective mechanism of the immune system that has evolved, helping the body to clear the pathogen, thus contributing to the healthy growth of animals. Mechanistically, we found that C. butyricum ZJU-F1 can significantly increase the expression levels of TLR2, MyD88, and NF-κB, indicating that C. butyricum ZJU-F1 may activate the TLR-2-dependent Myd88/NF-κB signaling pathway. Consistently, the notion that the presence of TLR2 is required for C. butyricum to activate the immune response has been further confirmed by Gao et al. in HT-29 cells [18] and Sui et al. in Cajal cells [47]. Therefore, we speculated that C. butyricum ZJU-F1 may regulate the expression of proinflammatory and anti-inflammatory factors in intestinal epithelial cells by activating the TLR-2-dependent NF-KB signaling pathway of MyD88, thus playing a role in the immune regulation of intestinal mucosa.

Conclusions
In this study, we aimed to explore the mechanism underlying dietary C. butyricum ZJU-F1 s ability to decrease the rate of diarrhea and increase nutrient digestibility. The results showed that C. butyricum ZJU-F1 treatment increased the intestinal digestibility of nutrients, decreased intestinal permeability, and increased intestinal tight junction protein and mucin expression, thus maintaining the integrity of the intestinal epithelial barrier. These effects were more marked when in combination with B. licheniformis. In addition, dietary C. butyricum ZJU-F1 increased the proportion of Lactobacillus, which was accompanied by the increase of butyric acid and propionic acid. Mechanistically, we found C. butyricum ZJU-F1 promoted the improvement of intestinal function, and the enhancements of the TLR-2-MyD88-NF-κB signaling pathway and SCFA production in cecal may be involved.  Institutional Review Board Statement: The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Animal Care and Use Committee of Zhejiang University (protocol code ZJU20160396 and 2016-03-04).
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data of qRT-PCR, Western blot, SCFAs and microbiota changes used to support the findings of this study are available from the corresponding author upon request.