Spermidine promotes biofilm formation by activating expression of the matrix regulator .

Ubiquitous polyamine spermidine is not required for normal planktonic growth of but is essential for robust biofilm formation. However, the structural features of spermidine required for biofilm formation are unknown and so are the molecular mechanisms of spermidine-stimulated biofilm development. We report here that in a spermidine-deficient mutant, the structural analogue norspermidine, but not homospermidine, restored biofilm formation. Intracellular biosynthesis of another spermidine analogue, aminopropylcadaverine, from exogenously supplied homoagmatine also restored biofilm formation. The differential ability of C-methylated spermidine analogues to functionally replace spermidine in biofilm formation indicated that the aminopropyl moiety of spermidine is more sensitive to -methylation, which it is essential for biofilm formation, but that the length and symmetry of the molecule is not critical. Transcriptomic analysis of a spermidine-depleted mutant uncovered a nitrogen-, methionine-, and -adenosylmethionine-sufficiency response, resulting in repression of gene expression related to purine catabolism, methionine and -adenosylmethionine biosynthesis and methionine salvage, and signs of altered membrane status. Consistent with the spermidine requirement in biofilm formation, single-cell analysis of this mutant indicated reduced expression of the operons for production of the exopolysaccharide and TasA protein biofilm matrix components and SinR antagonist Deletion of or ectopic expression of in the spermidine-deficient Δ background restored biofilm formation, indicating that spermidine is required for expression of the biofilm regulator Our results indicate that spermidine functions in biofilm development by activating transcription of the biofilm matrix exopolysaccharide and TasA operons through the regulator .