Campylobacter jejuni rotates a flagellum at each pole to swim through the viscous mucosa of its hosts' gastrointestinal tracts. Despite their importance for host colonization, however, how C. jejuni coordinates rotation of these two opposing flagella is unclear. As well as their polar placement, C. jejuni's flagella deviate from the norm of Enterobacteriaceae in other ways: their flagellar motors produce much higher torque and their flagellar filament is made of two different zones of two different flagellins. To understand how C. jejuni's opposed motors coordinate, and what contribution these factors play in C. jejuni motility, we developed strains with flagella that could be fluorescently labeled, and observed them by highspeed video microscopy. We found that C. jejuni coordinates its dual flagella by wrapping the leading filament around the cell body during swimming in high-viscosity media and that its differentiated flagellar filament and helical body have evolved to facilitate this wrappedmode swimming.

Mycoplasmas, a group of small parasitic bacteria, adhere to and move across host cell surfaces. The role of motility across host cell surfaces in pathogenesis remains unclear. Here, we used optical microscopy to visualize rheotactic behavior in three phylogenetically distant species of Mycoplasma using a microfluidic chamber that enabled the application of precisely controlled fluid flow. We show that directional movements against fluid flow occur synchronously with the polarized cell orienting itself to be parallel against the direction of flow. Analysis of depolarized cells revealed that morphology itself functions as a sensor to recognize rheological properties that mimic those found on host-cell surfaces. These results demonstrate the vital role of cell morphology and motility in responding to mechanical forces encountered in the native environment.

15Campylobacter jejuni rotates a flagellum at each pole to swim through the viscous 16 mucosa of its hosts' gastrointestinal tracts. Despite their importance for host colonization, 17 however, how C. jejuni coordinates rotation of these two opposing flagella is unclear. As 18 well as their polar placement, C. jejuni's flagella deviate from the Enterobacteriaceael 19 norm in other ways: their flagellar motors produce much higher torque and their flagellar 20 filament is made of two different zones of two different flagellins. To understand how C. 21 jejuni's opposed motors coordinate, and what contribution these factors play in C. jejuni 22 motility, we developed strains with flagella that could be fluorescently labeled, and 23 observed them by high-speed video microscopy. We found that C. jejuni coordinates its 24 dual flagella by wrapping the leading filament around the cell body during swimming in 25 high-viscosity media and that its differentiated flagellar filament has evolved to facilitate 26 this wrapped-mode swimming. Unexpectedly, C. jejuni's helical body is important for 27 facile unwrapping of the flagellar filament from the cell body during switching of 28 swimming trajectory. Our findings demonstrate how multiple facets of C. jejuni's flagella 29 and cell plan have co-evolved for optimal motility in high-viscosity environments. 30 31Campylobacter jejuni is a polar flagellate that causes several million cases of 50 gastroenteritis annually (Wallis 1994;Vliet and Ketley 2001) and requires its flagella for 51 gut colonization (Nachamkin, Yang and Stern 1993;Guerry 2007; Neal-McKinney and 52 Konkel 2012). C. jejuni constructs one flagellum at each pole, a pattern known as 53 amphitrichous flagellation, to swim at speeds approaching 100 µm/second, and 54 characteristically swims faster in moderately viscous fluids than it does at low viscosities 55 (Chaban, Coleman and Beeby 2018). Other factors contributing to C. jejuni's swimming 56 ability include a higher-torque flagellar motor, facilitating motility through viscous 57 environments such as the protective, viscous layer of mucous lining the digestive tract; a 58 helical cell shape important for colonization of hosts that may enable drilling through 59 viscous mucous; and a flagellar filament composed of two distinct regions that are 60 constructed from two nearly identical flagellin monomers. Together, these unique aspects 61 of C. jejuni's flagellation produce darting motility involving repetitive short runs and 62 reversals, although how the two flagella coordinate to facilitate these reversals remains 63unclear. 64Here we asked how C. jejuni swims productively despite its two apparently opposed 65 flagella. We constructed mutants of C. jejuni 81-176 whose flagella we could 66 fluorescently label, enabling us to visualize flagellar movement in different environments 67 and genetic backgrounds. We found that both filaments are always left handed, that C. 68 jejuni wraps one flagellum around its cell body, and that the cell body is a right-handed 69 ...