Unraveling the Secrets of Bacterial Adhesion Organelles using Single Molecule Force Spectroscopy Ove Axner*, Oscar Björnham & , Mickaël Castelain*, Efstratios Koutris*, Staffan Schedin & , Erik Fällman* ,# , and Magnus Andersson* *Department of Physics, Umeå University, SE-901 87 Umeå, Sweden, and & Department of Applied Physics and Electronics, Umeå University, SE-901 87 Umeå, Sweden. # Deceased during the final preparation of the manuscript. Abstract Many types of bacterium express micrometer-long attachment organ- elles (so called pili) whose role is to mediate adhesion to host tissue. Until re- cently, little was known about their function in the adhesion process. Force- measuring optical tweezers (FMOT) have since then been used to unravel the biomechanical properties of various types of pili, primarily those from uropatho- genic E. coli, in particular their force-vs.-elongation response, but lately also some properties of the adhesin situated and the distal end of the pilus. This knowledge provides an understanding of how piliated bacteria can sustain external shear forces caused by rinsing processes, e.g. urine flow. It has been found that many types of pilus exhibit unique and complex force-vs.-elongation responses. It has been conjectured that their dissimilar properties impose significant differences in their ability to sustain external forces and that different types of pilus therefore have dissimilar predisposition to withstand different types of rinsing conditions. An understanding of these properties is of high importance since it can serve as a basis for finding new means to combat bacterial adhesion, including that caused by antibiotic-resistance bacteria. This work presents a review of the current status of the assessment of biophysical properties of individual pili on single bacteria ex- posed to strain/stress, primarily by the FMOT technique. It also addresses, for the first time, how the elongation and retraction properties of the rod couple to the ad- hesive properties of the tip adhesin.