On- and off-tumour bacterial diversity following iron therapy
A total of 7.9 million reads (109,777 reads/sample) and 2,367 features were obtained following quality control, with a sampling depth of 8,000 reads/sample. Comparison of α-diversity metrics shows that patients treated with oral iron have a significantly higher bacterial diversity (Shannon Diversity and Faith’s Phylogenetic Diversity), richness (Observed OTUs), and abundance (Chao1 and ACE) within their off-tumour microbiota, relative to those treated with intravenous iron therapy (Figure 1; p<0.05). Consistent with this Jaccard similarity and Bray-Curtis dissimilarity assessed β-diversity between treatment groups and show that within the off-tumour microbiota, oral and intravenous iron form significantly different bacterial community clusters (Figure 2; p<0.05).
The on-tumour microbiota of these patients shows no differences in α-diversity between iron treatment groups (Figure 1; ns). Along with this, no significant differences in β-diversity were observed (Figure 2: ns). Taken together this suggests that oral iron and intravenous iron-treated patients show significantly different bacterial diversity in their tumour-adjacent microbiota. However, the tumour-associated microbiota shows a consistent bacterial diversity between iron treatments.
Oral and intravenous iron-treated patients show differing bacterial communities
Differences in microbial populations between oral and intravenous iron-treated patients were assessed using LEfSe to determine bacterial taxa that are significantly enriched between treatments (Figure 3). The off-tumour microbiota of oral iron-treated patients shows a greater abundance of the Clostridia class, Clostridiales, Anaeroplasmatales and Cytophagales orders, Lactobacillaceae and Anaeroplasmataceae families and Lactobacillus, Agathobacter, Coprococcuss 3, Eubacterium eligens groups, Eubacterium xylanophilum group, Lachnospiraceae ND3007 group, Ruminococcaceae NK4A214 group, Ruminococcus 1, Ruminococcus 2 and Anaeroplasma genera. Whereas in the intravenous iron group there was higher abundance of Lachnospiraceae NK4A136 group genus (Figure 3a, b).
The on-tumour microbiota in oral iron-treated patients shows a greater abundance of the Bacilli class, Lactobacillales order and Prevotella 7, Butyrivibrio, Coprococcus 1, Hungatella, Lachnospiraceae ND3007 group, Eubacterium ventriosum group, Ruminococcaceae NK4A214 group and Pleomorphomonas genera. Whereas the on-tumour microbiota of intravenous iron-treated patients show higher abundance of the Alteromonadales order and Alloprevotella and Enhydrobacter genera (Figure 3c, d). These results suggest that the on- and off-tumour microbiota show differential responses to iron therapy. With oral iron leading to a more prominent change in bacterial taxa in the tumour-adjacent compared to the tumour-associated microbiota.
Differential predictive enzymatic pathways between iron treatment groups
In order to infer the metagenomic pathways associated with microbial profiles, we performed a PICRUSt2 metagenomic analysis using predicted enzyme classification abundances based on 16S rRNA bacterial populations. These show a large difference in predicted microbial enzymes between iron treatments in the off-tumour microbiota (Figure 4a), while fewer differences in the on-tumour microbiota were observed (Figure 4b). The off-tumour microbiota is associated with increased abundance of iron-related enzymes, Bacterial non-haem ferritin and Ferric-chelate reductase, in intravenous iron-treated patients compared to oral iron. Along with this, the off-tumour microbiota of intravenous iron-treated patients shows increased abundance of enzymes involved in the production of anti-inflammatory metabolites, including Lactaldehyde dehydrogenase and 2-iminobutanoate/2-iminopropanoate deaminase (Figure 4a).
The on-tumour microbiota is associated with increased abundance of enzymes involved in anti-inflammatory and colorectal cancer protective metabolite production, including Cellulose synthase and N-sulfoglucosamine sulfohydrolase, in intravenous iron-treated patients (Figure 4b). Overall, the predictive metagenomic results suggest that the on- and off-tumour microbiota profiles associated with intravenous iron-treated patients are involved in regulating iron metabolism and production of metabolites involved in inhibiting intestinal inflammatory and colorectal cancer, compared to oral iron-treated patients.
Paired on- and off-tumour microbiota show varying microbial communities following oral and intravenous iron therapy
As the tumour-associated and tumour-adjacent microbiota show differing outcomes following iron therapy in respect to bacterial taxa, we aimed to assess if the method of iron administration led to changes between paired on- and off-tumour microbiota (Figure 5a). Patients treated with oral iron show their off-tumour microbiota being enriched with the Bacteroidaceae family and Bacteroides genus, while their on-tumour microbiota shows a higher abundance of Nocardiodaceae, Intrasporangiaceae and Brevibacteriaceae families and Prevotella 9, Nocardioides, Kocuria, Brevibacterium, Veillonella and Catenibacterium genera (Figure 5c, d).
In contrast, patients treated with intravenous iron show their off-tumour microbiota being enriched with the Firmicutes phylum and Clostridia class, along with a greater abundance of the Clostridiales and Sphingomonadales orders, Sphingomonadaceae family and Paraprevotella genus. Whereas the on-tumour microbiota of patients treated with intravenous iron shows a higher abundance of the Epsilonbacteraeota phylum, Campylobacteria class, Campylobacterales order, Campylobacteraceae, Propionibacteriaceae and Porphyromonadaceae families and Campylobacter, Porphyromonas and Cutibacterium genera (Figure 5a, b). These results suggest that patients treated with oral iron have a more consistent tumour-associated and tumour-adjacent microbiota, showing only small changes at lower taxonomic levels. Whereas patients treated with intravenous iron show a much greater difference between their tumour-associated and tumour-adjacent microbiota, with major differences at the phylum, class, and order levels.