This study was a randomized controlled study to determine whether prophylactic synbiotics maintained gut microbiota and prevented the occurrence of enteritis and VAP in patients with sepsis. Sepsis treatment including massive infusion, inotropic therapy, antibiotics, and other therapies, can affect the human gut microbiota following sepsis. Shimizu et al. reported that the number of obligate anaerobes, especially Bifidobacterium and Bacteroides, were decreased and continued in patients with systemic inflammatory response syndrome (SIRS) [18, 19]. The number of obligate anaerobes were around 10 (log 10 colony-forming units (CFUs)/g of feces) on average in normal people and postoperative patients [20, 21]. In the present study, the number of total bacteria tended to decrease to about 6 (log 10 CFUs/g of feces) on average in the No-Synbiotics group, which was much less than that in normal people and postoperative patients. The numbers of pathogenic bacteria such as Enterococcus and Pseudomonas increased significantly within 1 week in both groups (p < 0.05), but they did not reach statistical significance with the interaction of group and week. The numbers of pathogenic bacteria, such as total facultative anaerobes, and also those of total obligate anaerobes were the significant prognostic factors in patients with SIRS [4]. These findings suggest that sepsis influenced the microbiota of the patients and might be related to the occurrence of subsequent complications.

Synbiotics, as a combination of probiotics and prebiotics, have been reported to promote immunity against severe injuries such as trauma and infection. Although the mechanisms of probiotics have not yet been clarified, one of the important factors is microorganism-host crosstalk such as microorganism-associated molecular patterns (MAMPs) of probiotics and pattern recognition receptors (PRRs) of the gastrointestinal mucosa [22]. The most well-known PRRs are Toll-like receptors (TLRs). MAMPs consist of flagellin, lipopolysaccharide, peptidoglycan, and other factors. For example, flagellins of the probiotic Escherichia. coli Nissle 1917 were shown to induce beta-defensin via TLR5 [23]. Asahara et al. reported that intraluminal acetate produced by B. breve strain Yakult could inhibit the toxin in a mouse model of toxin-producing E. coli infection [24]. The L. casei strain Shirota was effective against multidrug resistant Salmonella enterica serovar Typhimurium DT104 infection in a 5-FU treated mouse model [25]. In the present report, administered B. breve strain Yakult and L. casei strain Shirota might inhibit pathogenic bacteria and toxins through signal interaction and prevent septic complications. Also, the number of total bacteria was significantly higher in the Synbiotics group. The numbers of Bifidobacterium and total Lactobacillus in the Synbiotics group were especially increased over those in the No-Synbiotics group. Looking at total Lactobacillus, the L. gasseri subgroup and L. fermentum tended to increase more in the Synbiotics group than in the No-Synbiotics group. L. gasseri has a role in vaginal homeostasis and Helicobacter pylori infection and improvement of diarrhea [26]. L. fermentum is reported to enhance the immunologic response of influenza vaccination [27]. These findings suggest that synbiotics not only increase the number of administered bacteria but also increase their genus groups and other microbiota, which could lead to the maintenance of gut microbiota. In previous studies, synbiotics could maintain gut microbiota following SIRS [8] and major surgeries [21]. Synbiotics could have a supplemental effect to increase the number of microbiota.

Short-chain fatty acids (SCFAs) consist of acetic, propionic, and butyric acids with 2–4 carbon atoms. Anaerobic metabolism of peptides and proteins by the microbiota produces SCFAs that all have important functions in host physiology. SCFAs are utilized mainly by intestinal epithelial cells as energy substrates, and some are absorbed into the portal flow to the liver and utilized as systemic energy sources [28]. SCFAs bind to the G-protein-coupled-receptor 43 (GPR43). Maslowski et al. reported that GPR43-deficient mice showed exacerbation of inflammation in models of colitis, arthritis, and asthma [29]. Asahara et al. reported that intraluminal acetate produced by synbiotics could inhibit the toxin in a mouse model of toxin-producing E. coli infection [24]. In the present report, the synbiotic-treated group had significantly maintained gut microbiota and organic acids, especially acetate. Increased levels of acetate might attenuate inflammation to reduce septic complications. Butyric acid in the feces of the patients decreased in both groups from the normal values (16.6 ± 6.7 μmol/g (mean ± SD)) described in our previous paper [18], which could be due to the decreased numbers of bacteria and lactate levels. The differences in the values did not reach statistical significance between the groups in the present study.

Regarding diarrhea in the ICU, Bleichner et al. reported that in 128 ICU patients the number of days with diarrhea was reduced in patients treated with Saccharomyces boulardii [30]. In our previous study of SIRS, the patients treated with synbiotics had a significantly reduced incidence of diarrhea compared with the controls [8]. In the present research, synbiotics showed beneficial effects against complications of enteritis in the patients with sepsis. Prophylactic synbiotics could maintain gut microbiota and reduce the incidence of enteritis. Further study is needed to determine the mechanisms of the prevention of diarrhea.

There are several reports on the effectiveness of probiotics and synbiotics on the incidence of VAP [31]. Morrow et al. [32] reported that the incidence of VAP in patients treated with L. rhamnosus GG was significantly lower than that in the controls (19.1% vs. 40.0%) in 138 ICU patients. Also, probiotic administration significantly reduced oropharyngeal and gastric colonization of pathogenic species. Fukuda et al. reported that Bifidobacteria continue to generate acetate through ATP-binding cassette-type carbohydrate transporter and prevent translocation in a mouse model [33]. Also, intraluminal acetate could increase the level of tight junction proteins including claudin-1, occludin, and ZO-1, which could prevent bacterial translocation in a mouse model of Acinetobacter baumannii infection [34]. In our report, the synbiotic-treated group had significantly maintained gut microbiota and organic acids, especially acetate. Increased levels of acetate and lactate might inhibit intraluminal toxins and maintain tight junctions. These changes indicated that synbiotic treatment could have beneficial effects on microbiota and reduce the development of VAP. However, other clinical reports showed no significant difference in the occurrence of VAP in the ICU [35]. One of the reasons is the difference in administered bacteria. In a mouse model, the antitoxic effects and organic acid concentration of probiotics such as Bifidobacterium and Lactobacillus are different with species [24, 25, 33]. In a rabbit infective endocarditis model, the incidence of infection was different with Lactobacillus species [36]. Besselink et al. [37] reported that mortality rates with six kinds of bacteria were significantly higher than those without these bacteria in the PROPATRIA study (16% vs. 6%). However, the incidence of infectious complications showed no significant differences and, in addition, the study has been criticized from multiple perspectives [38]. The effects of synbiotics for gut microbiota might be different with different species or combinations of bacteria and different diseases. Further analysis is needed to determine the appropriate probiotic species for preventing VAP and to elucidate the underlying mechanism of synbiotic treatment.

This study has some limitations. There is a source of potential bias in that gut microbiomes are different with different backgrounds such as geography, ethnicity, and lifestyle [39]. Thus, innate immunity via gut microbiota could be different against pathogenic bacteria. The modulation of gut microbiota by synbiotics might be different with these backgrounds. Second, there is limited generalizability. Because patients were collected through a tertiary center after being transferred directly from the emergency medical system in a limited area, the studied patients do not represent the national population, and the ethnicity of the subjects was only Asian. Therefore, caution is required in applying the findings to a larger worldwide population. Third, this is a quantitative research study of the main subset of microbiota, and changes in other whole bacteria were not evaluated. The bacteria influenced by synbiotics in the gut microbiome could be a target for further study. Fourth, the number of patients was lower than expected because of the small number of sepsis patients requiring mechanical ventilation with early enteral nutrition. Additional multicenter studies are needed to solve this problem. Fifth, cost-effectiveness analysis using measures such as the number needed to treat could be needed for further research to apply synbiotics clinically.