Abstract
Pseudomonas aeruginosa move across surfaces by using multiple Type IV Pili (TFP), motorized appendages capable of force generation via linear extension/retraction cycles, to generate surface motions collectively known as twitching motility. Pseudomonas cells arrive at a surface with low levels of piliation and TFP activity, which both progressively increase as the cells sense the presence of a surface. At present, it is not clear how twitching motility emerges from these initial minimal conditions. Here, we build a simple model for TFP-driven surface motility without complications from viscous and solid friction on surfaces. We discover the unanticipated structural requirement that TFP motors need to have a minimal amount of effective angular rigidity in order for cells to perform the various classes of experimentally-observed motions. Moreover, a surprisingly small number of TFP are needed to recapitulate movement signatures associated with twitching: Two TFP can already produce movements reminiscent of recently observed slingshot type motion. Interestingly, jerky slingshot motions characteristic of twitching motility comprise the transition region between different types of observed crawling behavior in the dynamical phase diagram, such as self-trapped localized motion, 2-D diffusive exploration, and super-diffusive persistent motion.
Highlights
Type IV Pili (TFP) are nanomotor/motility appendages capable of generating forces in the 100 pN range
We have designed a coarse-grained model to simulate early stage TFP motility of P. aeruginosa on surfaces and quantitatively compared the model predictions to the experiments performed on the cells tracked by high-speed video microscopy
Our single-pilus-resolved model allowed us to see how complex motility behavior reminiscent of twitching can result from a remarkably small number of pili simultaneously bound to the surface. We argue that this condition is commonly met in the early stages of bacterial surface adaptation when the cells are underpiliated
Summary
Type IV Pili (TFP) are nanomotor/motility appendages capable of generating forces in the 100 pN range. As schematically displayed in the dynamic phase diagram, a polarized TFP distribution results in persistent motion, while a wide-spread distribution can lead to persistent, diffusive or trapped behavior These qualitative observations are robust, the exact location of the boundaries between the different modes is likely to depend on the properties of the surface, especially in case of nonlinear friction effects such as shear thinning. The range of parameters where they are observed (marked green in Fig. 1), is close to the intersection of the three observed crawling modes This suggests that the slingshots - in addition to facilitating efficient motion on EPS covered surfaces - play an unanticipated important role in mediating changes in motility strategies in response to environmental conditions
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