Human gut strains of exhibit spontaneous induction of multiple prophages.

[UNLABELLED] Sulfate-reducing bacterium is a common member of the human gastrointestinal microbiome, associated with inflammatory conditions but also prevalent in healthy individuals. This suggests that lifestyle factors may shape its ecological role. We investigated prophage carriage and release in three new strains from healthy donors and strain FI11049 from a patient with ulcerative colitis. Sequencing revealed a larger genome in strain FI11455 (3.096 Mb) compared to FI11311 (2.985 Mb) and FI11458 (2.838 Mb), including a 154 kb megaplasmid which contained an 87 kb section with high similarity to the chromosome of strain FI11311, suggesting horizontal gene transfer between chromosomes and plasmids. This section encoded genes involved in DNA replication, transcription, and recombination, as well as protein folding and modification, defense, and phage proteins. Strain FI11049 showed less than 95% similarity to other strains but shared similar prophages with them. Each strain carried four to five predicted prophages, ranging from 30 to 60 kb, which clustered into four groups, with at least three groups per strain. Although the prophages had no nucleotide similarity to known phages, genes for lysis, integration, regulation, and structural proteins were identified, and three groups contained Mu-like proteins. Electron microscopy and PCR of mitomycin C-induced supernatants confirmed the release of tailed bacteriophage particles and capsids of multiple prophages. Similar results were demonstrated from uninduced samples, indicating spontaneous prophage release. Host defense systems were widespread, and cross-infections failed to identify suitable hosts in related strains and species. This is the first evidence of prophage release in gut-associated , with implications for gene transfer in the gut.

[IMPORTANCE] Gastrointestinal health has a significant impact on quality of life, and increasing profiling of the gut microbiome is identifying key players involved in disease states. However, evidence of the association of sulfate-reducing bacteria with pathologies, such as inflammatory bowel disease and colorectal cancer, conflicts with their prevalence in healthy subjects. Investigating the ecology of in the gut may be key to answering if and why it can be harmful and could inform future interventions. Here, we show that gut-associated strains carry multiple prophages, some of which are spontaneously released as bacteriophage particles in culture. Our results pave the way for future work to understand prophage release in gut conditions and its effects on populations.