Fab Spagnolo (@fabspagnolo.bsky.social)

Species-specific prophage induction by ciprofloxacin in human gut metagenomes Antibiotics are known to trigger prophage induction in controlled laboratory settings, but it remains unclear whether this also occurs within microbiomes in nature. Current methods investigating the link between antibiotics and prophage induction within the human gut rely on in vitro culturing of human gut bacterial isolates. Using a metagenomic approach, we aimed to measure prophage induction and whether it is associated with antibiotic exposure. Across two independent human cohorts, we compared prophage to bacterial host read depth ratios (P:H) across known or measured antibiotic exposures. We found that induction is not broadly associated with antibiotic exposures at the level of the overall microbiome, but that ciprofloxacin increases P:H ratios in specific bacterial species. We documented heterogeneous trajectories of P:H ratios over the course of antibiotic exposure, sometimes increasing and remaining high, or returning to baseline. This study complements experimental models by providing in vivo evidence of induction in the human gut. Importance Bacteriophages are viruses that infect a bacterial host. The lytic and lysogenic cycles are the two classic outcomes of phage infection. In the lytic cycle, the phage immediately replicates and lyses its host to release new viral particles. In the lysogenic cycle, the phage, now called a prophage, integrates its genome into that of its host without killing it. Prophages can switch to the lytic cycle in a process called induction, in which the viral genome is replicated, the host cell is lysed, and viral particles are released. The most immediate consequence of induction is host cell death which can impact bacterial populations and communities. Since prophages are mobile genetic elements that can move between bacteria, they are also an important vehicle for horizontal gene transfer. While induction has been well studied in vitro , whether and how induction occurs within the complex microbial ecosystem in humans is less well characterized. Understanding prophage induction in vivo is therefore critical in corroborating in vitro observations. ### Competing Interest Statement The authors have declared no competing interest. NIH Common Fund, https://ror.org/001d55x84 Natural Sciences and Engineering Research Council https://www.biorxiv.org/content/10.64898/2026.03.11.711154v1