What does the ratio between blood bacterial and fungal microbiome abundance tell us about fungal-bacterial interactions?

Ecological studies demonstrated a strong interaction between bacterial and fungal microbiomes (fungal-bacterial interaction) in various habitats (Wagg et al., 2019, Krüger et al. 2019). The consequences of bacterial-fungal interactions for the human host are largely unknown. Recently, we characterized the composition of whole-blood bacterial and fungal microbiomes in healthy individuals (Panaiotov et al., 2021). Using metagenomic sequence analysis we were able to identify a total of 24 bacterial orders (40 families and 50 genera) and 44 fungal orders (75 families and 94 genera). The aim of the present meta-analysis was to explore possible interactions between microbial and fungal communities. Blood group and gender data were included to assess the findings’ biological relevance.
Three ml of venous whole blood was collected from 28 subjects (14 females, 7 of each blood group – A, B, AB, O). Blood was lysed in d. water and the human DNA was treated with DNase. Microbial DNA was isolated by applying treatment with 4% SDS for microbial lysis. Isolated DNA was divided into two subsamples and 16S and ITS metagenomic analysis was applied for each subject. Microbial total and relative abundance were calculated. Then the bacterial vs. fungal (B/F) reads ratio was analyzed. Data were subjected to nonparametric statistical evaluation (Kruskal-Wallis) of gender and blood group effects.
The major findings were: (1) The overall fungal sequence number (median=8579) was higher than the bacterial (median = 1062; Related-Samples Wilcoxon Signed Rank Test, Z =383; P<0.001). (2) Individually, the B/F ratio varied significantly spanning from full fungal dominance to an almost complete lack of fungal sequences. (3) The mean B/F ratio was higher for males (mean B/F=0.95) as compared to females (mean BF = 0.18; P<0.001). (4) The blood group had an impact on the B/F ratio. For individuals of blood groups, A and B the ratio were around 1 and 0.2 (P<0.05) for individuals of blood groups AB and O.
In conclusion, despite the overall fungal dominance the B/F ratio showed high individual variability ranging from almost full fugal dominance to negligible fungal presence. The dependence of the B/F by gender and blood group suggests that it reflects the physiological status of the host. It could be hypothesized that B/F could serve as a health diagnostic index. It is worth testing the therapeutic correction of B/F in clinical practice.