Motions caused by the growth of Bacillus subtilis macrofibres in fluid medium result in new forms of movement of the multicellular structures over solid surfaces

A new form of bacterial movement, dragging of multicellular aggregate structures over solid surfaces, is powered by macrofiber supercoiling

Growing Bacillus subtilis macrofibers use twist and supercoiling to: power their own self-assembly, join fibers together into multiclonal aggregates, move themselves over solid surfaces, and to drag other structures (cargo) over solid surfaces. The dragging of multiclonal aggregates attached to the ends of growing macrofibers is analyzed here. The linkage between fibers and cargo arose naturally in macrofiber cultures. Dragging was triggered when growing macrofibers became linked to cargo at both of their ends. Such macrofibers supercoiled, reduced their length, and dragged the cargo toward one another. In parallel experiments immobile wire was used in place of cargo at one end of macrofibers that were linked to cargo at the other. The cargo was dragged toward the wire when these fibers supercoiled. To estimate the force required for dragging we determined the dimensions of the cargo, the buoyant density of macrofibers in the growth medium where dragging occurred, the rate and distance over which the aggregate structures were dragged, and the viscosity of the growth medium. Friction resulting from contact with the solid surface over which the structures were dragged was estimated using the measured parameters. The results indicate that the supercoiling tension required to overcome limiting friction must have been approximately 10 nN, while that needed to overcome fluid drag was of the order of 1 nN. These values suggest that only a small fraction of the total power available from macrofiber supercoiling was needed to drive this new form of multicellular bacterial movement.

The dynamic behavior of bacterial macrofibers growing with one end prevented from rotating: variation in shaft rotation along the fiber's length, and supercoil movement on a solid surface toward the constrained end

BACKGROUND

Bacterial macrofibers twist as they grow, writhe, supercoil and wind up into plectonemic structures (helical forms the individual filaments of which cannot be taken apart without unwinding) that eventually carry loops at both of their ends. Terminal loops rotate about the axis of a fiber's shaft in contrary directions at increasing rate as the shaft elongates. Theory suggests that rotation rates should vary linearly along the length of a fiber ranging from maxima at the loop ends to zero at an intermediate point. Blocking rotation at one end of a fiber should lead to a single gradient: zero at the blocked end to maximum at the free end. We tested this conclusion by measuring directly the rotation at various distances along fiber length from the blocked end. The movement of supercoils over a solid surface was also measured in tethered macrofibers.

RESULTS

Macrofibers that hung down from a floating wire inserted through a terminal loop grew vertically and produced small plectonemic structures by supercoiling along their length. Using these as markers for shaft rotation we observed a uniform gradient of initial rotation rates with slopes of 25.6 degrees /min. mm. and 36.2 degrees /min. mm. in two different fibers. Measurements of the distal tip rotation in a third fiber as a function of length showed increases proportional to increases in length with constant of proportionality 79.2 rad/mm. Another fiber tethered to the floor grew horizontally with a length-doubling time of 74 min, made contact periodically with the floor and supercoiled repeatedly. The supercoils moved over the floor toward the tether at approximately 0.06 mm/min, 4 times faster than the fiber growth rate. Over a period of 800 minutes the fiber grew to 23 mm in length and was entirely retracted back to the tether by a process involving 29 supercoils.

CONCLUSIONS

The rate at which growing bacterial macrofibers rotated about the axis of the fiber shaft measured at various locations along fibers in structures prevented from rotating at one end reveal that the rate varied linearly from zero at the blocked end to maximum at the distal end. The increasing number of twisting cells in growing fibers caused the distal end to continuously rotate faster. When the free end was intermittently prevented from rotating a torque developed which was relieved by supercoiling. On a solid surface the supercoils moved toward the end permanently blocked from rotating as a result of supercoil rolling over the surface and the formation of new supercoils that reduced fiber length between the initial supercoil and the wire tether. All of the motions are ramifications of cell growth with twist and the highly ordered multicellular state of macrofibers.

The mechanisms responsible for 2-dimensional pattern formation in bacterial macrofiber populations grown on solid surfaces: fiber joining and the creation of exclusion zones

BACKGROUND

When Bacillus subtilis is cultured in a complex fluid medium under conditions where cell separation is suppressed, populations of multicellular macrofibers arise that mature into ball-like structures. The final sedentary forms are found distributed in patterns on the floor of the growth chamber although individual cells have no flagellar-driven motility. The nature of the patterns and their mode of formation are described in this communication.

RESULTS

Time-lapse video films reveal that fiber-fiber contact in high density populations of macrofibers resulted in their joining either by entwining or supercoiling. Joining led to the production of aggregate structures that eventually contained all of the fibers located in an initial area. Fibers were brought into contact by convection currents and motions associated with macrofiber self-assembly such as walking, pivoting and supercoiling. Large sedentary aggregate structures cleared surrounding areas of other structures by dragging them into the aggregate using supercoiling of extended fibers to power dragging. The spatial distribution of aggregate structures in 6 mature patterns containing a total of 637 structures was compared to that expected in random theoretical populations of the same size distributed in the same surface area. Observed and expected patterns differ significantly. The distances separating all nearest neighbors from one another in observed populations were also measured. The average distance obtained from 1451 measurements involving 519 structures was 0.73 cm. These spacings were achieved without the use of flagella or other conventional bacterial motility mechanisms. A simple mathematical model based upon joining of all structures within an area defined by the minimum observed distance between structures in populations explains the observed distributions very well.

CONCLUSIONS

Bacterial macrofibers are capable of colonizing a solid surface by forming large multicellular aggregate structures that are distributed in unique two-dimensional patterns. Cell growth geometry governs in an hierarchical way the formation of these patterns using forces associated with twisting and supercoiling to drive motions and the joining of structures together. Joining by entwining, supercoiling or dragging all require cell growth in a multicellular form, and all result in tightly fused aggregate structures.