Treatment outcome of the patients with COVID-19
We recruited 13 COVID-19 patients, 15 non-COVID-19 pneumonia controls, and 15 healthy controls. The median ages of the three groups were 66, 51, and 64 years old, respectively (Table S1). All COVID-19 patients displayed respiratory symptoms (Table S1) and several of them had gastrointestinal manifestations, including one individual with diarrhea. Two patients (COV06 and COV13) showed positive results for SARS-CoV-2 in stool based on viral RNA metagenomics.
The COVID-19 patients were treated with a probiotics-assisted therapy (see Materials and Methods). All patients were discharged after their COVID-19 tests turned negative. We monitored the alterations of some relevant clinical variables in 8 COVID-19 patients before and after the treatment that registered a minimal of 2-fold change (Table S2). Decreases of inflammatory indicators were common in the COVID-19 patients after the treatment, which included C-reactive protein (CRP; 3 patients), tumor necrosis factor (TNF)-α (7 patients), interleukin-1β (7 patients), interleukin-2 (3 patients), interleukin-4 (7 patients), interleukin-5 (6 patients), interleukin-6 (5 patients), interleukin-8 (3 patients), interleukin-12P70 (7 patients), interferon (IFN)-α (3 patients), and interferon IFN-γ (5 patients). The hematologic trait of white blood cell (WBC) was reduced in COV03 and COV12, whereas lymphocyte was elevated in COV06. In addition, alanine aminotransferase (ALT), lactate dehydrogenase (LDH), total bilirubin, and blood urea nitrogen were reduced in COV09, COV03, COV05, and COV05, respectively. These results suggested the overall reduced inflammation in the COVID-19 patients after the treatment.
Taxonomic changes of gut microbiota in the patients with COVID-19
Analysis of 16S rRNA gene sequencing revealed that in comparison with the healthy controls and non-COVID-19 pneumonia participants, the COVID-19 patients exhibited a reduction in microbial diversity (Shannon index, P <0.05) and a difference in microbial structure (ANOSIM, P <0.05, Fig. 1a). These alterations were accompanied by substantial taxonomic shift. Compared with the healthy controls, the COVID-19 patients exhibited the reduced relative abundances of Firmicutes at the phylum level and 21 genera including Faecalibacterium, Roseburia, Gemmiger, and Clostridium XlVa as well as increased proportions of Enterococcus, Lactobacillus, Rhodococcus, and Acinetobacter (Wilcoxon rank-sum test, FDR < 0.05; Table S4). Furthermore, there was prominent compositional heterogeneity in the COVID-19 patients, as the most dominant genera that often accounted for over half of the total abundance differed among the individuals. According to the most abundant genera at T0 (i.e., before the treatment), the COVID-19 patients were stratified into 5 clusters, with each being dominant with one or two genera (Fig. 1b). Specifically, the clusters 1 (COV02 and COV11), 2 (COV12), 3 (COV03, COV04, and COV10), 4 (COV07), and 5 (COV05, COV06, COV08, COV09, and COV13) were characterized by a high relative abundance (26%-97%) of Akkermansia (54%-61%) and Bacteroides (7%-29%); Lactobacillus (66%) and Bifidobacterium (20%); Bacteroides (59%-87%) and Alistipes (27%) or Enterococcus (11%); Enterococcus (97%); Escherichia/Shigella (26%-66%) and multiple taxa including Anaerostipes (48%-16%), Enterococcus (17%), Klebsiella (24%), Akkermansia (18%) or Phascolarctobacterium (33%). Of note, two patients (i.e., COV06, COV13) with detectable SARS-CoV-2 RNA in stool were dominated with Escherichia/Shigella. We also detected microbial differences in the upper respiratory tract between the non-COVID-19 controls and COVID-19 patients and that there were likewise apparent interpersonal variations among individual COVID-19 patients. For example, the dominant genera were Prevotella and Veillonella in the pneumonia controls and Staphylococcus (COV02), Enterococcus (COV03, COV04, COV06, COV07), Staphylococcus, Pseudomonas (COV05), Streptococccus (COV08), Ralstonia (COV09), Rothia (COV10), Prevotella (COV11), and Neisseria (COV12) in the COVID-19 patients, respectively (Fig. S1).
Analysis of the microbial transcriptional activity revealed that compared with the healthy controls, the COVID-19 patients were characterized by augmented presence of 322 species, including Escherichia coli, Lactobacillus delbrueckii,Salmonella enterica, Providencia alcalifaciens, Staphylococcus auricularis, Klebsiella pneumoniae, and Enterococcus faecium, and decreased presence of 279 species, including Bacteroides vulgatus, Faecalibacterium prausnitzii, Bacteroides cellulosilyticus, Bacteroides dorei, and Eubacterium eligens (Wilcoxon rank-sum test, FDR < 0.05; Fig. 1c and Table S3). There was some noticeable coincidence between the taxonomic and transcriptional shifts, as evidenced by the genera or affiliating species of Faecalibacterium, Enterococcus, Rhodococcus, Acinetobacter, Roseburia, Blautia,Parabacteroides, Dialister, and Ruminococcus. As such, the transcriptomics-based analysis further underscored the substantial changes in gut microbial community in the COVID-19 patients.
We next examined the strain-level variations in the COVID-19 patients. To this end, we applied StrainPhlAn to build the phylogenetic trees based on single nucleotide variants (SNVs) in species-specific marker genes, which revealed considerable strain-level heterogeneity of Escherichia coli in the patients with COVID-19 (Fig. S2). Specifically, the dominant strains were phylogenetically close to E. coli UMN026 in patient COV11, to E. coli UMEA_3212-1 in patient COV13, and to E. coli KTE196 in patients COV01, COV06, COV09 and COV12, respectively. Hence, our analyses revealed that COVID-19 was associated with taxonomic and transcriptional shifts in the gut and/or airway microbiotas and that there was great heterogeneity among individual patients.
Functional shift of gut microbiota in the patients with COVID-19
Having revealed the clear taxonomic alterations of gut microbiota in the COVID-19 patients, we next sought to examine the microbial functional characteristics using the transcriptomic data. Of the 5,125 genes functionally annotated to Kyoto Encyclopedia of Genes and Genomes (KEGG) orthologous groups (KOs), 1,982 exhibited differential transcriptional activities between the HCs and COVID-19 patients, including 866 decreased and 1,116 increased in COVID-19. At pathway level, the COVID-19 patients were characterized by enrichment of 11 MetaCyc pathways including beta-Lactam resistance, Biofilm formation-Escherichia coli, and Bacterial invasion of epithelial cells as well as depletion of 86 MetaCyc pathways, most of which belonged to the categories of amino acid metabolism, lipid metabolism, and carbohydrate metabolism (Table S5). In addition, the expression level of several virulence factors in the COVID-19 patients was significantly higher than both the healthy controls and pneumonia controls (Fig. 1d). The top 5 transcriptionally active virulence factors in the COVID-19 patients were Enterobactin (VF0228), ECP (VF0404), T2SS (VF0333), Type 1 fimbriae (VF0221), and Yersiniabactin (VF0136), collectively accounting for 61.9% of the total transcriptomic abundance (Fig. 1e, Table S6). In addition, the top 5 contributing species of virulence factors in the COVID-19 patients were Escherichia coli, Shigella dysenteries, Yersinia pestis, Yersinia enterocolitica, and Salmonella enterica, collectively accounting for 98.8% of the total transcriptomic abundance (Fig. 1f, Table S7). The prominent presence of virulence factors and their contributing pathogens or pathobionts illustrated the deteriorating gut flora in the COVID-19 patients.
We next investigated the profile of antibiotic resistance genes in gut microbiota of the participants. Genes potentially conferring resistance to 35 different antibiotics were identified in the COVID-19 patients, of which Macrolide, Tetracycline, Penam, Diaminopyrimidine, and Phenicol were the most transcriptionally active (Fig. S3a, Table S8). The expression of antibiotic resistance genes (ARGs) was significantly increased in COVID-19 (Fig. 1d), with the most abundant types being resistance-nodulation-cell division (RND) antibiotic efflux pump, Erm 23S ribosomal RNA methyltransferase, major facilitator superfamily (MFS) antibiotic efflux pump, tetracycline-resistant ribosomal protection protein, and pmr phosphoethanolamine transferase (Fig. S3b, Table S9). Similar to our observation in taxonomic analyses (Fig. 1b and Fig. S1), there were great interpersonal differences in ARGs. For example, the most transcriptionally active ARGs were Erm 23S ribosomal RNA methyltransferase in COV04, COV10, and COV11, RND antibiotic efflux pump in COV05, COV06, COV07, COV08, COV09, COV12, and COV13, tetracycline-resistant ribosomal protection protein in COV01, COV03, and COV13, and CfxA beta-lactamase in COV02, respectively. Overall, our findings revealed extensive alterations of gut microbiota in COVID-19 in both taxonomic and functional aspects, which featured prominent variations among individual patients.
Association between gut microbiome and disease severity of COVID-19
We next investigate the correlation of gut or upper airway microbes with a set of clinical indicators (Fig. 2a). Among upper airway taxa, Bacteroides, Akkermansia, and Enterococcus showed negative association with CD3 and CD4. Among gut taxa, Rhodococcus and Acinetobacter showed negative association with CD3, CD4, CD45, haemoglobin (Hb) concentrations, and red blood cells (RBC); Bacteroides and Veillonella showed positive association with haemoglobins, RBC, and CD3, respectively; Enterococcus showed positive association with plasma concentration of carbon dioxide.
We also detected significant correlations between microbial functional components and clinical indicators. The expression level of bacteria virulence factors (e.g., Yersiniabactin, Salmochelin, Shu, and SgrA) were negatively correlated with CD8 (Fig. 2b), whereas multiple antibiotic resistance genes (e.g.,beta lactamase, chloramphenicol acetyltransferase (CAT), undecaprenyl pyrophosphate related proteins, and multidrug and toxic compound extrusion (MATE) transporter) were negatively associated with CD3 (Fig. 2c). These findings indicated that gut and airway microbiota shifts were linked to the clinical manifestations of inflammation, possibly related to COVID-19.
Convalescence coincided with the recovery of dysbiosis in the COVID-19 patients
To investigate the dynamics of the gut microbiome in the COVID-19 patients during the process of treatment, we compared the taxonomic data among the baseline (T0, first sampling date prior to the probiotics-assisted therapy), 7-day posttreatment (T1), and 14-day posttreatment (T2) for each patient. Among the 12 recruited COVID-19 patients, stool samples both before and after the therapy could only be collected from 8 individuals (Fig. 1b). At the end of the treatment, the 8 COVID-19 patients showed a partial rise in the microbial diversity (Wilcoxon rank-sum test, P < 0.05) and 6 out of the 8 COVID-19 patients (75%) showed a partial microbial compositional “recovery”, evidenced by a decrease of BC (Bray-Curtis) distance to the healthy group (Fig. 3a). This alteration was accompanied by substantial taxonomic shift, such as COVID-19-increase genera Enterococcus and Rhodococcus registered at least 2-fold reduction in 3 patients (COV03, COV05, and COV12) and 4 patients (COV05, COV08, COV09, and COV11), respectively, and COVID-19-depleted genera Faecalibacterium and Roseburia exhibited at least 2-fold increase in 3 patients (COV11, and COV12) and 4 patients (COV03, COV06, COV09, and COV12), respectively (Table S10). This partial compositional “recovery” was accompanied by a similar trend in the microbial transcriptome. After treatment, 23%-63% of the species that had COVID-19-associated transcriptional elevation exhibited at least 2-fold reduction, whereas 7%-67% of the species that had COVID-19-associated transcriptional reduction exhibited at least 2-fold increase (Table S11). For example, the major gut commensal bacterium Faecalibacterium prausnitzii was increased in patients COV05, COV08, COV09, COV11, and COV12; butyrate-producing anaerobic bacterium Roseburia hominis was increased in patient COV05, COV09, COV11, and COV12. On the contrary, opportunistic pathogen Escherichia coli was decreased in all patients except COV03; Salmonella enterica was decreased in COV05, COV06, COV08, COV09, and COV12; Providencia alcalifaciens was decreased in all patients except COV03 and COV11; Staphylococcus auricularis was decreased in COV03, COV07, COV11, and COV12; Klebsiella pneumoniae was decreased in all patients except COV03 and COV05 (Fig. 3b). The findings suggested that the treatment correlated with a noticeable recovery of the gut microbiota perturbations in the COVID-19 patients.
An important finding of the longitudinal 16S data was that there were great intertimepoint compositional variations of gut microbiome in most COVID-19 patients (Fig. 1b). In patients COV03, COV05, COV06, and COV08, the most dominant genus at T0 exhibited a great reduction in relative abundance at the end of treatment, whereas the most dominant genus at the end of treatment was barely detectable at T0. For example, the top taxa of patient COV03 were Bacteroides and Lactobacillus before and after the treatment, respectively; the top taxa of patient COV05, COV06, and COV08 was Escherichia/Shigella prior to the treatment, as were Bacteroides, Enterococcus, and Veillonella at the end of treatment. The prominent microbiota changes were also evident in patient COV11, whose shares of Akkermansia and Bacteroides were 54% and 29%, 75% and 5% or 19% and 44% at T0, T1 or T2. For the airway microbes, the drastic intertimepoint changes were present in only 3 patients. Specifically, the top genera of patients COV01, COV03, and COV03 were Ralstonia, Clostridium sensu stricto, and Pseudomonas at T0 and Lactobacillus, Lactobacillus, and Corynebacterium at T2, respectively (Fig. S1).
The drastic compositional shift over the course of treatment was mirrored by the bacterial transcriptinal activities. In patients COV03, COV05, COV06, COV09, and COV12, the most transcriptionally active species differed between the first and last timepoints. For example, the top species of patient COV03 were Bacteroides fragilis before the treatment and Yersinia enterocolitica afterwards. Of note, there was apparent divergence between the most abundant taxa and most transcriptionally active microbes. For example, in patient COV01, the most abundant taxon was Enterococcus, whereas the most transcriptionally active species was Bacillus cereus; in patient COV12, Fecaelibacterium prausnitzii was transcriptionally active, although the genus was barely detected in 16S rRNA gene-based analysis. However, noticeable coincidence can be found for Escherichia/Shigella and E. coli in patients COV05, COV06, and COV09, Akkermansia and A. muciniphila in patient COV11, Klebsiella and K. pneumoniae in patient COV06, and Veillonella and V. parvula in patient COV08. Overall, our longitudinal analyses revealed that over the course of treatment, the COVID-19 patients exhibited apparent recovery in the disease-associated microbial abnormalities and that the gut microbiota of the COVID-19 patients showed drastic intertimepoint variations.