Bacteria can adjust the structure of colonies and biofilms to enhance their survival rate under external stress. Here, we explore the link between bacterial interaction forces and colony structure. We show that the activity of extracellular pilus motors enhances local ordering and accelerates fusion dynamics of bacterial colonies. The radial distribution function of mature colonies shows local fluidlike order. The degree and dynamics of ordering are dependent on motor activity. At a larger scale, the fusion dynamics of two colonies shows liquidlike behavior whereby motor activity strongly affects surface tension and viscosity.

Cellular positioning towards the surface of bacterial colonies and biofilms can enhance dispersal, provide a selective advantage due to increased nutrient and space availability, or shield interior cells from external stresses. Little is known about the molecular mechanisms that govern bacterial positioning. Using the type IV pilus (T4P) of Neisseria gonorrhoeae, we tested the hypothesis that the processes of phase and antigenic variation govern positioning and thus enhance bacterial fitness in expanding gonococcal colonies. By independently tuning growth rate and T4P-mediated interaction forces, we show that the loss of T4P and the subsequent segregation to the front confers a strong selective advantage. Sequencing of the major pilin gene of the spatially segregated sub-populations and an investigation of the spatio-temporal population dynamics was carried out. Our findings indicate that pilin phase and antigenic variation generate a standing variation of pilin sequences within the inoculation zone, while variants associated with a non-piliated phenotype segregate to the front of the growing colony. We conclude that tuning of attractive forces by phase and antigenic variation is a powerful mechanism for governing the dynamics of bacterial colonies.

Bacterial type 4 pili (T4P) are extracellular polymers that initiate the formation of microcolonies and biofilms. T4P continuously elongate and retract. These pilus dynamics crucially affect the local order, shape, and fluidity of microcolonies. The major pilin subunit of the T4P bears multiple post-translational modifications. By interfering with different steps of the pilin glycosylation and phosphoform modification pathways, we investigated the effect of pilin post-translational modification on the shape and dynamics of microcolonies formed by Neisseria gonorrhoeae. Deleting the phosphotransferase responsible for phosphoethanolamine modification at residue serine 68 inhibits shape relaxations of microcolonies after perturbation and causes bacteria carrying the phosphoform modification to segregate to the surface of mixed colonies. We relate these mesoscopic phenotypes to increased attractive forces generated by T4P between cells. Moreover, by deleting genes responsible for the pilin glycan structure, we show that the number of saccharides attached at residue serine 63 affects the ratio between surface tension and viscosity and cause sorting between bacteria carrying different pilin glycoforms. We conclude that different pilin post-translational modifications moderately affect the attractive forces between bacteria but have severe effects on the material properties of microcolonies.

10Bacterial type 4 pili (T4P) belong to the strongest molecular machines. The gonococcal T4P 11 retraction ATPase PilT supports forces exceeding 100 pN during T4P retraction. Here, we 12 address the question whether gonococcal T4P retract in the absence of PilT. We show that pilT 13 deletion strains indeed retract their T4P but the maximum force is reduced to 5 pN. Similarly, 14 the speed of T4P retraction is lower by orders of magnitude compared to T4P retraction driven 15 by PilT. Deleting the pilT paralogues pilU and pilT2 in the pilT background did not inhibit 16 T4P retraction, indicating that the PilT-like proteins do not compensate for PilT. Furthermore, 17 we show that depletion of proton motive force did not inhibit pilT-independent T4P retraction. 18 We conclude that the retraction ATPase is not essential for gonococcal T4P retraction. 19However, the force generated in the absence of PilT is too low to support important functions 20 of T4P including twitching motility, fluidization of colonies, or induction of host cell response. 21 22 IMPORTANCE 23 Bacterial type 4 pili (T4P) have been termed the "swiss army knive" of bacteria because they 24 perform numerous functions including host cell interaction, twitching motility, colony 25 formation, DNA uptake, protein secretion, and surface sensing. The pilus fibre continuously 26 elongates or retracts and these dynamics are functionally important. Curiously, only a subset of 27 T4P systems employs T4P retraction ATPases to power T4P retraction. Here we show that one 28 of the strongest T4P machines, the gonococcal T4P, retracts without a retraction ATPase. 29 Biophysical characterization reveal strongly reduced force and speed compared to retraction 30 with ATPase. We propose that bacteria encode for retraction ATPases when T4P have to 31 generate high force supporting functions like twitching motility, triggering host cell response, 32 or fluidizing colonies. 33 34 KEYWORDS 35 Pilus, molecular motor, twitching motility, Neisseria gonorrhoeae 36 37Bacterial type 4 pili (T4P) are among the strongest molecular machines known to date. In some 38 species they generate forces exceeding 100 pN (1-3), i.e. twenty-fold higher than the force 39 generated by muscle myosin. Force generation has been linked to diverse functions including 40 twitching motility (4-7), host cell interaction (8-11), and regulation of biofilm structure and 41 dynamics (12)(13)(14)(15)(16)(17). For all of these functions, the retraction ATPase PilT is required. 42 Interestingly, some T4P systems involved in protein secretion, DNA uptake during 43 transformation, or surface sensing bear no pilT-like gene. Very recently, it has been shown that 44 T4P can retract in the absence of a retraction ATPase (18-20). The forces generated by these 45 pili, however, are by an order of magnitude lower than the force observed for Neisseria 46 gonorrhoeae T4P retraction. It remains unclear, whether gonococci can retract T4P in the 47 126 127PilT-independent T4P retraction generates lower force compare...

Bacterial type 4 pili (T4P) belong to the strongest molecular machines. The gonococcal T4P retraction ATPase PilT supports forces exceeding 100 pN during T4P retraction. Here, we address the question of whether gonococcal T4P retract in the absence of PilT. We show that pilT deletion strains indeed retract their T4P, but the maximum force is reduced to 5 pN. Similarly, the speed of T4P retraction is lower by orders of magnitude compared to that of T4P retraction driven by PilT. Deleting the pilT paralogue pilT2 further reduces the speed of T4P retraction, yet T4P retraction is detectable in the absence of all three pilT paralogues. Furthermore, we show that depletion of proton motive force (PMF) slows but does not inhibit pilTindependent T4P retraction. We conclude that the retraction ATPase is not essential for gonococcal T4P retraction. However, the force generated in the absence of PilT is too low to support important functions of T4P, including twitching motility, fluidization of colonies, and induction of host cell response. IMPORTANCE Bacterial type 4 pili (T4P) have been termed the "Swiss Army knives" of bacteria because they perform numerous functions, including host cell interaction, twitching motility, colony formation, DNA uptake, protein secretion, and surface sensing. The pilus fiber continuously elongates or retracts, and these dynamics are functionally important. Curiously, only a subset of T4P systems employ T4P retraction ATPases to power T4P retraction. Here, we show that one of the strongest T4P machines, the gonococcal T4P, retracts without a retraction ATPase. Biophysical characterization reveals strongly reduced force and speed compared to retraction with ATPase. We propose that bacteria encode retraction ATPases when T4P have to generate high-force-supporting functions like twitching motility, triggering host cell response, or fluidizing colonies.