Impact of an alpha helix and a cysteine-cysteine disulfide bond on the resistance of bacterial adhesion pili to stress

Abstract

Escherichia coli express adhesion pili that mediate attachment to host cell surfaces that are exposed to body fluids in the urinary and gastrointestinal tracts. Pilin subunits are organized into helical polymers, with a tip adhesin for specific host binding. Pili can elastically unwind when exposed to fluid flow force, reducing the adhesin load, thereby facilitating sustained attachment. Here we investigate biophysical and structural differences of pili commonly expressed on bacteria that inhabit the urinary and intestinal tracts. Optical tweezers measurements reveal that Class 1 pili of uropathogenic E. coli (UPEC), as well as Class 1b of enterotoxigenic E. coli (ETEC), undergo an additional conformational change beyond pilus unwinding, providing significantly more elasticity to their structure than ETEC Class 5 pili. Looking comprehensively at structural and steered molecular dynamics simulation data, we find this difference in Class 1 pili subunit behavior originates from an α-helical motif that can unfold when exposed to force. A disulfide bond cross-linking β-strands in Class 1 pili stabilizes subunits, allowing them to tolerate higher forces than Class 5 pili that lack this covalent bond. We suggest that these extra contributions to pilus resiliency are relevant for the UPEC niche since resident bacteria are exposed to stronger, more transient shear forces compared to those experienced by ETEC bacteria in the mucosa of the intestinal tract. Interestingly, Class 1b ETEC pili include the same structural features seen in UPEC pili, while requiring lower unwinding forces that are more similar to those of Class 5 ETEC pili.Significance StatementAdhesion pili are often essential virulence factors for attachment of pathogenic bacteria in specific environmental niches. We provide mechanistic details of structural differences impacting the biophysical properties of pili found on bacteria in the urinary and intestinal tracts. We see that pili from urinary tract bacteria are composed of subunits optimized for their microenvironment. First, they can tolerate higher forces than intestinal pili due to a disulfide bond that limits subunit unfolding. Second, their greater flexibility is due to an α-helical motif that can unfold, absorbing force that could otherwise lead to bacteria detachment. Our work provides insight into the central role of pilus structural and biophysical properties for the sustained bacterial adherence necessary to initiate disease.