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

Abstract

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. Bioinfor-matic analyses revealed that Tad pili are widely distributed and highly conserved in Bradyrhi-zobium. 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 ad-hesion to abiotic surfaces and loss of movement, indicating a physiological shift towards a bio-film 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 connec-tion 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 adaptabil-ity and potential applications in agriculture and biotechnology. Understanding these mecha-nisms 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. ImportanceBiofilm formation is critical for bacterial survival in the soil. In this study we analyzed the role of Tad pili in Bradyrhizobium diazoefficiens biofilm forming capacity and its connection with the second messenger c-di-GMP, which is involved in regulating the transition between plank-tonic and sessile states. Since bacteria applied in inoculants are in the planktonic state, where-as bacterial survival is optimal in the sessile (biofilm) state, our findings may contribute to im-prove the switch towards the sessile state in novel inoculant formulations, enhancing bacterial persistence in formulations and soil.