Sampling information
Based on the inclusion and exclusion criteria, 50 male patients with SZ and 50 healthy male individuals were recruited. The demographic and clinical characteristics of both the groups are presented in Table 1. There were no significant differences in age (p = 0.1311), weight (p = 0.1811), and body mass index (BMI, p = 0.4817). However, the NC group had a significantly greater height than the SZ group (p = 0.0460). In terms of BMI classification, there were statistical differences between the thin (BMI < 18.5, p = 0.0412) and obese (BMI ≥ 28, p = 0.0412) groups. Finally, the distribution of antipsychotic use of patients with SZ mainly consisted of risperidone (28%), clozapine (20%), olanzapine (40%), and quetiapine (24%), and 26% of them took two types of antipsychotic drugs during clinical therapy.
Microbial diversity
A total of 8,911,266 reads (1,869,170,520 bases) were obtained from the healthy control group, while 50 microbiome samples of SZ subjects consisted of 5,816,276 reads (1,320,847,922 bases). After quality filtration, adapter reduction, and paired-read assembly, we obtained 4,376,659 raw tags, 4,321,992 effective tags, and 21,507 OTUs in healthy control group, ranging from 23,459 to 414,271, 23,392 to 409,960, and 115 to 844 for each sample. Additionally, there were 2,667,140 raw tags, 2,645,060 effective tags, and 12,230 OTUs in the SZ group, ranging from 19,503 to 68,895, 19,387 to 67,766, and 101 to 516 for each sample, respectively. Both the rarefaction curve (Figure S1 A) and rank abundance curve (Figure S1 B) confirmed the validity of the high-throughput sequencing data and revealed that the abundance of the microbial community varied depending on the sample of individuals.
To characterize the richness and diversity of the microbial community, we calculated alpha indices for each sample. There were significant changes in the observed species index between the SZ and NC groups (Figure 1A, p = 6.88e-07), Shannon index (Figure 1B, p = 5.94e-05), and Simpson index (Figure 1C, p = 6.85e-04) of alpha diversity. Conversely, OTU-based beta diversity is a comparative analysis of microbial community composition between samples. We observed that the PCoA (Figure 2A) and NMDS-based map (Figure 2B, stress = 0.093) of unweighted UniFrac metrics revealed that SZ subjects were tightly clustered when NC subjects formed distinct clusters (within-group distance comparison). In addition, some NC clusters were close to those of the SZ group. Moreover, the ANOSIM analysis (Figure 2C) indicated that the microbial community structure was significantly different (unweighted UniFrac, R = 0.152, p = 0.001) between the two groups.
Differences in taxonomic composition
To better understand OTU information and taxonomic annotation, tags and OTUs were calculated and summarized. As is shown in Figure 3A, the predominant bacteria in the NC group were Firmicutes (57.43%), Bacteroidetes (33.08%), Proteobacteria (4.55%), Actinobacteria (2.31%), and Fusobacteria (1.83%), whereas the SZ cohort was dominated by Firmicutes (42.93%), Bacteroidetes (42.03%), Proteobacteria (9.04%), Fusobacteria (2.55%), and Actinobacteria (1.72%). When the relative abundances of bacterial phyla were compared (Figure S2A), Bacteroidetes (p = 4.11e-03) and Proteobacteria (p = 0.0371) were found to be more abundant in the SZ than in the NC group. In terms of Firmicutes levels, the SZ group showed a significant decrease (p = 3.98e-05) compared to the NC group. At the genus level (Figure 3B), the NC group was mainly assigned to Bacteroides (24.26%), Faecalibacterium (12.59%), Roseburia (6.89%), Prevotella (4.43%), Megamonas (3.31%), Blautia (3.13%), Lachnospira (2.58%), Clostridium (2.30%), Ruminococcus (1.99%), and Coprococcus (1.81%). The most abundant genera in the SZ group were Bacteroides (25.66%), followed by Prevotella (10.24%), Faecalibacterium (7.95%), Roseburia (4.93%), Succinivibrio (3.68%), Megamonas (2.96%), Parabacteroides (2.49%), Dialister (2.00%), Sutterella (1.67%), and Clostridium (1.33%). Compared to healthy cohort (Figure S2B), the relative abundance of Prevotella (p = 0.0157), Parabacteroides (p = 0.0342), and Sutterella (p = 0.0365) was significantly higher in the SZ cohort. However, Faecalibacterium (p = 4.25e-03), Blautia (p = 3.04e-05), Lachnospira (p = 6.36e-03), Clostridium (p = 0.0287), Ruminococcus (p = 0.0380), and Coprococcus (p = 0.0258) levels were higher in the healthy cohort.
The Venn diagram illustrates the distribution of shared and specific genera to identify the candidate microbial biomarkers. As is shown in Figure 4A and 4B, all individuals, irrespective of SZ patients and healthy persons, had in common 73 genera of the total members that were consistently detected. In addition, we calculated the indicator value to identify candidate biomarkers for microbiological diagnosis. Succinivibrio (p = 0.001), Megasphaera (p = 0.001), and Nesterenkonia (p = 0.005) were more enriched in the SZ group, whereas Blautia (p = 0.001), Paracoccus (p = 0.001), Adlercreutzia (p = 0.001), Enhydrobacter (p = 0.001), Eggerthella (p = 0.002), Corynebacterium (p = 0.002), Oxalobacter (p = 0.002), and Finegoldia (p = 0.005) in healthy control subjects (Figure 4C).
Functional differences of microbiome
Tax4Fun analysis was performed to reveal and explore the differences in the function of the gut microbiome between the SZ and NC groups. As is shown in Figure 5, the metabolism of terpenoids/polyketides (p = 4.37e-07), excretory system (p = 9.50e-05), energy metabolism (p = 2.08e-04), cancers (p = 3.93e-04), circulatory system (p = 2.87e-03), nervous system (p = 4.44e-03), signal transduction (p = 5.49e-03), and xenobiotic biodegradation/metabolism (p = 0.0220) in the SZ group showed an upward trend compared to that in the NC group. However, there were significant decreases in transcription (p = 1.33e-03), nucleotide metabolism (p = 2.10e-03), immune diseases (p = 3.42e-03), replication/repair (p = 5.46e-03), membrane transport (p = 0.0131), and translation (p = 0.0139) in the SZ group. Next, we analyzed the correlation between the relative abundance of the altered genera and differentially functional pathways. Faecalibacterium, Ruminococcus, Coprococcus, Adlercreutzia, Blautia, and Paracoccus were positively associated with nucleotide metabolism, transcription, replication/repair, and translation (Figure 6). Meanwhile, Parabacteroides was positively correlated with the metabolism of terpenoids/polyketides, the nervous system, excretory system, and circulatory system when Prevotella had a positive effect on cancers. In contrast, Coprococcus, Corynebacterium, and Adlercreutzia were negatively associated with signal transduction, the nervous system, and the circulatory system. Moreover, Blautia was negatively correlated with energy metabolism and cancer.