Spore formation by the human gut pathogen Clostridium difficile, a strict anaerobe, provides protection from oxygen during transmission between hosts. However, whether anaerobic bacteria of the commensal gut microbiota can also form spores for transmission is not known. Browne et al. developed a method to archive pure cultures of bacterial species of the human gut microbiota, including many species that were previously thought to be unculturable. By combining the method with selection for ethanol-resistant cultures, 66 candidate spore-forming bacterial species were isolated. Genes that were conserved among these species were also present in C. difficile, consistent with a shared genetic basis of spore formation and germination. The prevalence of these conserved genes in metagenomic datasets suggests that 60% of the bacterial genera present in the human gut have genes for spore formation and that many commensal anaerobic species may therefore form spores for transmission between hosts.