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Background: Anemia in preterm infants is associated with gut dysbiosis and necrotizing enterocolitis. Our study aimed to identify the bacterial functions and metabolites that can explain the underlying mechanisms of anemia associated disease conditions., Methods: We conducted a case control study in preterm infants with cases having a hematocrit ≤ 25%. The control infants were matched by birth gestational age and weight. Fecal samples were collected before, at the onset, and after the onset of anemia in cases and with matched postnatal age in controls for metagenomics and metabolomics analyzes., Results: 18 anemic and 20 control infants with fecal samples collected at 17 days, 5 weeks, and 7 weeks postnatal age were included. Virulence factor potential, decrease in beta diversity evolution, and larger changes in metabolome were associated with severe anemia. Metabolite abundances of N-acetylneuraminate and butyrobetaine were associated with virulence factor potential. Anemic group had decreased prostaglandin and lactic acid levels., Conclusion: Fecal omics data showed that severe anemia is associated with a pro-inflammatory gut microbiota with more virulent and less commensal anaerobic bacterial activities. Future studies can examine the link between anemia-associated dysbiosis and clinical outcomes and predict an infant-specific hematocrit threshold that negatively affects clinical outcomes., Impact: Severe anemia in preterm infants contributes to a pro-inflammatory gut with greater bacterial virulence and less commensal bacterial activities. The multiomics approach using non-invasive fecal biospecimens identified functional and metabolic changes in the gut microbiota and these mechanistic changes are plausible explanations for anemia-associated disease conditions in preterm infants. Our findings identified biological changes of the gut environment in severely anemic preterm infants that can offer guidance for clinical management., (© 2024. The Author(s), under exclusive licence to the International Pediatric Research Foundation, Inc.)