LuxS influences Escherichia coli biofilm formation through autoinducer-2-dependent and autoinducer-2-independent modalities.

Escherichia coli produces biofilms in response to the small molecule autoinducer-2 (AI-2), a product of the LuxS enzyme. LuxS is part of the activated methyl cycle and could also affect biofilm development by AI-2-independent effects on metabolism. A luxS deletion mutant of E. coli W3110 and an inducible plasmid-luxS-complemented strain were used to identify AI-2-independent phenotypes. Differential interference contrast microscopy revealed distinct surface colonization patterns. Confocal microscopy followed by quantitative image analysis determined differences in biofilm topography correlating with luxS expression; deletion mutant biofilms had a 'spreading' phenotype, whereas the complement had a 'climbing' phenotype. Addition of exogenous 4,5-dihydroxy-2,3-pentanedione (DPD), an AI-2 precursor, to the deletion mutant increased biofilm height and biomass, whereas addition of the methyl donor S-adenosyl methionine or aspartate prevented the luxS-complemented strain from producing a thick biofilm. The luxS-complemented strain autoaggregated, indicating that fimbriae production was inhibited, which was confirmed by transmission electron microscopy. DPD could not induce autoaggregation in the deletion mutant, demonstrating that fimbriation was an AI-2-independent phenotype. Carbon utilization was affected by LuxS, potentially contributing to the observed phenotypic differences. Overall, the work demonstrated that LuxS affected E. coli biofilm formation independently of AI-2 and could assist in adapting to diverse conditions.