Genetic basis of biofilm formation and salt adaptation in the plant-beneficial strain Stutzerimonas stutzeri MJL19

Stutzerimonas stutzeri MJL19 represents a potential candidate for agrobiotechnological applications in regions affected by soil salinization, given its protective effects on plants under saline stress. This strain forms biofilms on some abiotic surfaces and on plant roots, a trait that influences the colonization and persistence capacities of bacteria in the rhizosphere. However, the mechanistic basis for the multicellular lifestyle of S. stutzeri MJL19 and its connection with the adaptation to saline conditions had not been explored. Analysis of the genome of MJL19 has allowed the identification of two gene clusters involved in the synthesis of exopolysaccharides (cellulose and a species-specific polymer). Deletion of either or both gene clusters exposed their differential roles on abiotic and biotic surfaces and phenotypic changes in response to increasing salt concentrations. Expression of both clusters is regulated by the two-component system GacS/GacA, as evidenced by analysis of a gacS mutant obtained by random transposon mutagenesis. This mutant also shows altered levels of the intracellular second messenger cyclic diguanylate (c-di-GMP), which is key in the transition between free-living and sessile lifestyles. Results also suggest the existence of regulatory interconnections between exopolysaccharide synthesis genes, and of these with c-di-GMP turnover, which is in turn modulated by the presence of NaCl. GacS is required for this response to varying salt concentrations. We also describe two additional elements that influence c-di-GMP levels and the response to salt: the gene katE, encoding catalase HP-II, and a gene that encodes a protein of the lipoteichoic acid synthases family. KEY POINTS: • GacS controls c-di-GMP levels and EPS synthesis in S. stutzeri MJL19 in response to salt. • Regulation of EPS genes is interconnected and linked to c-di-GMP turnover. • The catalase KatE influences c-di-GMP levels.