Fig. 6

Alteration of specific metabolites in patient serum and association between altered metabolites and microbial changes. A,B The PLS-DA (A) and OPLS-DA (B) analyses suggest differences in metabolites of serum samples from different groups. C Average relative abundance of the top 49 Class II metabolites in serum from normal individuals, in-hospital patients, and their follow-up within 3 months after discharge. D The differential abundance of serum Class II metabolites (Log2 FC > 1.5 or <  − 1.5, VIP > 1.0, p < 0.05) in the follow-up and mild patient groups is presented as a lollipop chart. E Annotated Class II metabolites were analyzed and compared between the follow-up and normal groups (log2 FC > 1.5 or <  − 1.5, VIP > 1.0, p < 0.05). F Up- and downregulated Class II metabolites in the follow-up group compared with the normal group or the mild COVID-19 group. G Association analysis between the abundance of significantly altered Class II metabolites in serum and altered abundance of digestive tract microbiomes at the phylum, genus, and species levels in the follow-up group. “ns” represents no significance, *p < 0.05, ** p < 0.01, ***p < 0.001, and ****p < 0.0001 (Student’s t test)