The gene encoding ornithine decarboxylase for putrescine biosynthesis is essential for the viability of .

[UNLABELLED] is a Gram-negative anaerobe associated with periodontitis and colorectal cancer. It secretes putrescine, a polyamine that promotes biofilm formation by oral co-colonizers and enhances the proliferation of cancer cells. However, the physiological importance of putrescine for itself remains unexplored. Here, we show that putrescine biosynthesis, mediated by the ornithine decarboxylase gene , is essential for viability. Deletion of was only possible when a functional copy was provided in , and CRISPR interference of expression resulted in complete growth defects. The essentiality of was conserved across multiple subspecies. Supplementation with exogenous putrescine enabled the isolation of a conditional mutant whose growth was strictly putrescine-dependent. Putrescine depletion caused filamentation, membrane disruption, detergent hypersensitivity, and lysis in hypoosmotic conditions, indicating a critical role in maintaining cell envelope integrity. RNA sequencing revealed broad transcriptional remodeling under putrescine-limited conditions, including upregulation of genes involved in lipid metabolism, osmoprotection, and cell wall remodeling. Notably, transcript levels increased when putrescine was depleted, suggesting a negative feedback mechanism. These findings demonstrate that putrescine is not only an extracellular communal metabolite but is also vital for the cellular integrity and survival of under anaerobic conditions.

[IMPORTANCE] is a prominent member of the oral microbiota and has been linked to various human diseases, including periodontitis, preterm birth, and colorectal cancer. Despite its clinical significance, the metabolic requirements that support its growth and viability remain poorly understood. In this study, we identify the gene, which encodes ornithine decarboxylase, as essential for survival due to its role in putrescine biosynthesis. We demonstrate that depletion of putrescine leads to severe growth and morphological defects, accompanied by widespread transcriptional changes. These findings reveal an underappreciated metabolic vulnerability and highlight the critical role of polyamine homeostasis in maintaining cellular integrity in this notorious anaerobe.