Investigating the Contribution of Sinorhizobium meliloti Flagella on Biofilm Formation and Symbiosis During Host Colonization

Abstract

Many bacteria transition from a motile to a sessile, aggregate state, known as a biofilm, ensuring bacterial survival in harsh environments. Biofilms pose serious threats to public health due to their association with bacterial resistance to antimicrobial treatments against infections. Sinorhizobium meliloti, a nitrogen‐fixing bacterium, forms biofilms on abiotic surfaces and on roots of leguminous plants. Our laboratory is currently investigating the contribution of biofilm to symbiosis between Sinorhizobium meliloti and the model host legume Medicago truncatula. Recently our laboratory detected a nonsense mutation in the podJ gene of the commonly used S. meliloti laboratory strain, Rm1021. By correcting this mutation, the new strain, podJ+, showed an ability to produce a polar adhesive called holdfast, and to build more robust biofilms. PodJ regulates flagellar motility and chemotaxis in S. meliloti, and the full‐length protein appears to be required for holdfast production, which in turn contributes to surface adhesion and biofilm formation. However, previous studies showed that holdfast alone is insufficient for biofilm formation; for example, holdfast and flagella together facilitate the surface attachment of cells in the related bacterium Caulobacter crescentus. The goal of this project is to investigate whether the flagella of S. meliloti contribute to holdfast‐dependent biofilm formation in S. meliloti. Additionally, I would like to determine whether the presence of flagella improves symbiosis of S. meliloti with the legume host. To achieve this goal, the presence of flagella was examined in strains with mutations in flagellar genes, in both the podJ+ background (JOE3759) and the original, uncorrected background (Rm1021). First, soft‐agar plates were used to assess the flagellar motility of the strains: this test indicates which strains synthesize or lack flagella. To confirm the presence and absence of flagella and to assess the formation of adhesive clusters in these strains, scanning electron microscopy and scanning transmission electron microscopy were utilized to examine the cells. Next, competitive infection assays were performed to assess the effect of flagella on symbiosis efficiency. This was performed by inoculating the Medicago host simultaneously with podJ+ strains that lack or produce flagella, thereby, determining which strain colonizes more root nodules. These results will help uncover the molecular pathways by which rhizobia form biofilms, thereby providing the basis for understanding and controlling biofilm formation in related pathogens.Support or Funding InformationNIH RISE programThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.