Uncovering the Role of the scs Pilus Within the Complex Surface Architecture of Stenotrophomonas maltophilia

Stenotrophomonas maltophilia is an emerging multidrug-resistant pathogen that encodes numerous surface structures mediating attachment, biofilm formation, motility, and virulence. How individual adhesive systems function within this complex and potentially redundant network remains unclear. Here, we investigated the scs locus, a conserved chaperone-usher pilus system, to define its contribution to surface-associated behaviors and infection. Deletion of scs alone produced minimal effects on biofilm formation under standard laboratory conditions. However, significant and reproducible phenotypic differences emerged when scs loss was combined with mutations in other pilus systems or when bacteria were evaluated in infection-relevant environments. Across assays of biofilm formation, surface piliation, motility, flagellar gene expression, and virulence, scs contributed differentially to enhancing the adhesion and motility defects associated with the smf-1 pili, cbl pili, and fliC flagellar gene loci. Transmission electron microscopy corroborated these findings, revealing differential expression and distinct alterations in predominantly the pilus architecture among mutant strains. These pilus associated phenotypes also largely corroborated infection in a Galleria melonella model. Given the abundance of adhesion associated gene loci in the pan- S. maltophilia genome, our results demonstrate that the scs locus functions as a context-dependent determinant of attachment and virulence, revealing a potential crosstalk between components of the surface landscape of the bacteria. Importance Stenotrophomonas maltophilia is an emerging multidrug-resistant pathogen that relies on diverse surface structures, including chaperone-usher pili, to drive attachment, biofilm formation, and infection. Its rising clinical prevalence underscores the need to define how multiple chaperone-usher pili contribute to virulence and interact with other appendages such as flagella, which together shape pathogenic behaviors. Our work on the scs chaperone-usher pilus system advances this understanding by revealing its context-dependent role in virulence. We demonstrate how scs modulates defects in smf-1 and cbl pilin loci, and affects the fliC flagellar locus to reshape the cell surface pilus landscape, thereby illuminating how coordinated surface-structure interactions drive pathogenicity.