Role of Tad Pili during the transition from Planktonic to Biofilm State in Bradyrhizobium diazoefficiens USDA 110

Free-living soil bacteria may exist in two states: planktonic as swimmer cells, and sessile within biofilms. In biofilms, bacterial cells are embedded into an extracellular matrix, which confers protection against environmental stresses and extends bacterial survival. The transition from planktonic to biofilm state involves surface sensing and attachment mediated by flagella and pili. Here, we explored the role of Tad pili (Type IVc) in surface sensing, adhesion, and biofilm formation in Bradyrhizobium diazoefficiens , a nitrogen-fixing symbiont of soybean. Bioinformatic analyses revealed that Tad pili are widely distributed and highly conserved in Bradyrhizobium. In other model bacteria, pili deletion leads to decrease in biofilm-forming capacity, but other functions are also affected depending on the microorganism. Surprisingly, deletion of the most conserved genomic cluster encoding Tad pili in B. diazoefficiens caused an increased adhesion to abiotic surfaces and loss of movement, indicating a physiological shift towards a biofilm state. These observations suggest a sensor or regulatory role for this apparatus that could affect cell-cell interactions or interactions with the extracellular matrix. Additionally, a connection between pili and c-di-GMP levels in the cell was established. These findings highlight the critical role of Tad pili in B. diazoefficiens physiology, providing insights into bacterial adaptability and potential applications in agriculture and biotechnology. Understanding these mechanisms is key to improving biofilm management strategies and generating novel approaches to enhance bacterial viability in soil and inoculants as well as to optimize the symbiosis.