2.1 Patient profiles
We collected fecal samples from healthy individuals (n=10)from the health check-up center of the hospital and sepsis patients (n=10) who sought treatment at the hospital, between July 2019 and December 2020 at the Fifth People’s Hospital associated with Fudan University. The hospital ethics committee granted the research protocol approval (ethics batch number: 2019118), and all patients provided informed consent signed by their legal representatives or themselves. The inclusion criteria for patients with sepsis were: (i) diagnosed with sepsis based on the Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3)[12]; (ii) between the ages of 18 and 80; and (iii) patients who were hospitalized during the first 12 hours after the onset of sepsis symptoms.Pregnancy, nursing, and active cancer were the exclusion criteria for the sepsis. The following healthy individuals were included in our analyses: (a) age and gender matched with septic patients; (b) with no abnormality in biochemical indexes, which was confirmed in the health examination.. Among the healthy individuals who were eliminated from this study were: those with (a) prior sepsis or other severe infections; (b) prior hematological malignancies or other solid tumors; and (c) complicated with inflammatory disease.
Stool samples were harvested from patients who were diagnosed with sepsis in 12 hours and were frozen and kept at -80 °C in polyethylene tubes for subsequent analysis.
2.2 Gut microbiota analysis
Fecal DNA was isolated from stool samples with a Standard DNA Extraction Kit (QIAGEN), and its quality was assessed via agarose gel electrophoresis. Following amplification and the quality assessment of the V3-V4 regions of the 16S rRNA genes, the amplicon underwent purification, then reamplification. The Illumina MiSeq platform was utilized for the V3-V4 gene amplicons sequencing [13]. Filtration was performed on the raw data, and clean tags were eliminated to acquire valid tags to prepare operational taxonomic units (OTUs), which were divided utilizing the Vsearch software (version 2.4.2) carrying a 97% sequence similarity cut-off. Using OTU sequence comparisons, a phylogenetic tree was built using the pynast (v0.1) software. A rarefied OTU table was employed to analyze the intestinal microbiota to assess its diversity and composition. A normalized OTU table and consistent depth were utilized when computing the alpha diversity indices for stool specimens. Beta diversity indices were produced by the Bray-Curtis method combined with the unweighted UniFrac distance to determine if substantial differences existed within the gut microbiota between the two cohorts. Principal component analysis (PCA) was also executed to compare gut microbiota between the groups [14].
2.3 Metabolomics analysis
60mg of each stool sample from the patients was placed in a 1.5 mL Eppendorf tube containing internal standard, with subsequent additions of ice-cold methanol and water (4:1 v/v), prior to a 5 min storage at 20°C, followed by a 2 min grinding at 60 HZ, a 10 min ultrasonication in ice water bath, and a 30 min storage at -20°C. Subsequently, the solution mixture centrifugation for 15 min at 13000 rpm, 4°C, and 300 μL of supernatant underwent drying via a freeze concentration centrifugal dryer in a brown glass vial. 400 μL of methanol and water (1:4, v/v) were introduced, prior to a 30 s vortex, and maintenance at -20°C for 2 h. The samples underwent centrifugation again for 10 min at 13000 rpm, 4°C, before 150μL of supernatants were filtered through 0.22μm microfilters, prior to transfer to LC vials, which were then maintained at -80℃ until Liquid Chromatography-Mass Spectrometry (LC-MS) analysis. QC samples were generated via combinations of aliquots from all 18 samples. A Dionex Ultimate 3000 RS UHPLC system accompanied with Q Exactive quadrupole Orbitrap mass spectrometer and a heated electrospray ionization (ESI) source (Thermo Fisher Scientific, Waltham, MA, USA) was then employed for metabolic profiling analysis under both ESI positive and negative ion modes, using an ACQUITY UPLC HSS T3 column (100 mm × 2.1mm, 1.8 μm)[15].
2.4 In vivo study
6–8-week-old C57BL/6 mice, with body weight 20-23 g, were purchased from East China Normal University's Animal Center (Shanghai, China). After a week of conditional feeding, the animals were separated into 3 categories: normal control, sepsis, and sepsis + PE(0:0/14:0). Mice in the sepsis group (n = 20) were intraperitoneally injected with 0.9% aqueous sodium chloride solution at a dosage of 0.2 mL daily for 5 days before surgery. Cecum ligation and perforation were performed on day 6, after which the previous dosing method was continued for 7 days post-surgery. Control mice (n = 20) were intraperitoneally administered with 0.2 mL 0.9% aqueous sodium chloride solution every day from 5 days before surgery and continued to the end of the experiment. mouse in the sepsis +PE(0:0/14:0) (n = 20) group were intraperitoneally injected with PE(0:0/14:0) at a dosage of 5mg/kg, 10 mg/kg and 20mg/kg respectively once daily from 5 days before surgery till end of experiment. Mice were sacrificed by cervical dislocation 24 h after surgery, and fresh stool samples were collected immediately.
2.5 Histomorphological analysis
Six colon tissues underwent fixation in 4% neutral formalin before dehydration in varying ethanol concentrations, paraffin-embedding, and 5 mm sectioning. Following mounting on slides, they underwent cleaning, hydration, and staining with hematoxylin and eosin (H&E). Two experienced pathologists performed histological examinations on all specimens. A modified scoring system based on the severity of tissue injury was used to assess histologic changes in a blinded manner. A semi-quantitative histopathology score was derived: I, epithelial dysregulation; II, goblet cell reduction: III, inflammatory cell invasion; IV, submucosal stiffness.
2.6 Serum cytokine quantification
After collecting blood and Caco-2 monolayer specimens from mice, samples underwent a 10 min centrifugation at 3,000 rpm and 4 ºC to extract serum and cell supernatant samples, respectively. These samples were stored at -80 ºC. Murine ELISA kits (88-7064, Thermo Fisher, Austria; EK280/3-01, MuLTI SCIENCE, Shanghai) were used to measure IL-6, D-lactic acid, and IL-1β levels. All the tests were conducted following kit directions.
2.7 Evaluation of FITC-dextran leakage
The intestines’ permeability was evaluated by measuring the amount of FITC-dextran that leaked out. After an 8-hour of fasting, the FITC-dextran (4 kDa, Sigma-Aldrich, USA) was diluted in PBS and administered to the mouse at 25 mg/mL. FITC-dextran was administered orally at 0.6 mg/g body weight. Blood samples (400 µL) were obtained from a retro-orbital puncture 4 hours later. A similar volume of PBS was added to the supernatants after centrifugation and mixed. The fluorescence intensity of each diluted serum sample (100 mL) was then measured with a multimode reader (excitation: 485 nm, emission: 528 nm, bandwidth: 20 nm). The FITC amount was computed using a standard curve.
2.8 Cell culture and treatment
Human colorectal adenocarcinoma (Caco-2) cells were maintained in Eagle's Minimum Essential Medium with 10% heat-inactivated fetal bovine serum (FBS, Gemini Bioproducts) and 1% non-essential amino acids from the American Type Culture Collection (Invitrogen, Manassas, VA, USA). Caco-2 cells (1 ´ 106 cells/well) were grown in 6-well plates, prior to exposure to Salmonella enterica serotype Typhimurium lipopolysaccharide (LPS) (Sigma). Prior investigations were conducted to ascertain the optimum LPS dosage (1 mg/mL) and duration of exposure to LPS (48 h). Cell stimulation employed LPS with or without 5μM, 10μM and 15μM PE(0:0/14:0)(Selleck, USA) respectively.
2.9 Quantitative PCR (qPCR)
Samples from the Caco-2 cell were harvested, frozen in liquid nitrogen, prior to storage at -80℃ until further analyses. Total RNA isolation was conducted from Caco-2 cell using TRIzol (15596026, Invitrogen, Carlsbad, CA, USA) and quantified utilizing the Universal SYBR FAST qPCR Kit Master Mix (2x) (KAPA Biosystems, USA). The following conditions were set as the qPCR parameters: 10 min, 95 °C; 45 cycles, 10 s at 95 °C; and 60 s, 59 °C; 15 s, 95 °C; 15 s, 72 °C; and 15 s, 95 °C. Relative gene levels of Zonula occludens-1 (ZO-1), Occludin, and Gapdh were assessed via real-time qPCR (RT-qPCR). The following primer sequences were employed in the RT-qPCR reactions: ZO-1 forward (F) 5′-GAGCAAGCCTCC-5′-GAGCAAGCCTCC-5′-GAGCAAGCCTCC-5′-GAGCAATGCACATA-3′, reverse (R) 5′-TCAGTTTCGGGTTTCCTT-3′; Occludin F 5′-CAACGGCAAAGTGAATGGCA-3′, R 5′-CTTTCCTTCGTGGGAGTC-3′; Gapdh F 5′-TGTGAACGGATTTGGCCGTA-3′, R 5′-GATGGTGATG GGTTTCCCGT-3′.
2.11 Western blot assay
Cellular and mice intestinal lysis used specific lysis buffers, and protein quantification was done via a BCA kit (Beyotime, China), prior to separation on an SDS/PAGE in a Bio-Rad Mini-PROTEAN apparatus, then transfer to PVDF membranes (Bio-Rad, Marnes-la-Coquette, France), followed by a 1 h blocking at room temperature (RT) in 5% nonfat milk (w/v), and overnight (ON) treatment with primary antibodies at 4℃. The employed antibodies are listed as follows: anti-Occludin antibody (13409-1-AP); anti-ZO-1 antibody (21773-1-AP); anti-GAPDH antibody (60004-1-Ig). All aforementioned antibodies were prepared in a 1:1000 dilution, and were acquired from Proteintech, USA. The employed secondary antibodies were: HRP-goat anti-mouse IgG (115-035-003) and HRP-goat anti-rabbit IgG (111-035-144) from Jackson ImmunoResearch. Protein band visualization was done with ECL chemiluminescence imaging system and IntDen (target protein)/IntDen (GAPDH) ratio computation via ImageJ (Version 1.50i; National Institutes of Health, Bethesda, MD, USA).
2.12 Immunofluorescence and confocal microscopy
Caco-2 monolayers underwent a 15 min fixation in 4% paraformaldehyde, prior to a 10 min permeabilization in 0.2% Triton X-100, followed by a 1 h blocking in PBS with 3% BSA at RT, ON incubation at 4°C overnight with primary antibodies, namely, anti-Occludin (1:200, 13409-1-AP; Proteintech, USA) and anti-ZO-1 antibodies, with subsequent 1 h incubation with secondary antibodies (diluted 1:400) without light. Following rinse and mounting with ProLong Gold Antifade Reagent with DAPI (Invitrogen), the cells were next visualized under a laser scanning fluorescence microscope (Leica TCS SP5; Leica).
2.13 Statistical analyses
Data provided as mean ± standard error. The one-way analysis of variance (ANOVA) was employed for comparison of differences across multiple subgroups, while the t-test was employed for inter-group comparisons. * p< 0.05 was established as the significance threshold.