Neil H. Mendelson

Mechanical behaviour of bacterial cell walls.

Publisher Summary The chapter discusses the value of investigating the mechanical behavior of bacterial cell walls and also the ways in which it can be done both experimentally and theoretically. The chapter presents the structure of cell-wall material and physical state of the cell wall. The cell wall has functions for which its mechanical properties are important and which are of significance in cell physiology. The three functions of cell wall are— permeability through the wall, anchorage of structures in the cell wall or their protrusion through it, and the possible role cell walls might play in sensing and/or transferring information into cells. Mechanical modeling suggests experiments involving dynamic changes in the observable geometry of live cells, such as wall thickness, cell length and diameter, and twisting rate in some cases. The chapter explores the geometrical models, models involving surface tension-like stress, a model involving anisotropic cell-wall material, and a cell-wall growth model.

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

BackgroundWhen 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.ResultsTime-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.ConclusionsBacterial 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.

Bacterial templating of zeolite fibres with hierarchical structure

Ordered macroporous zeolite fibres are prepared from the infiltration of swollen bacterial supercellular threads with as-synthesized silicalite nanoparticles.

Ordering nanometer-scale magnets using bacterial thread templates

Nanometer-scale ferromagnetic particles (Fe2O3, Fe3O4) are dispersed within a mutant bacterial chain which is drawn into a macroscopic fiber “rope.” Cross-sectional scanning electron microscopy images reveal that the iron oxide particles are intercalated between the walls of the bacterial cells which are bundled into parallel threads. The field-dependent switching is seen to markedly sharpen when the synthesis is conducted within an applied magnetic field.

The helix clock: a potential biomechanical cell cycle timer

Abstract A model based upon helical geometry that provides cylindrically shaped cells with a means to measure their length during growth and to time cell cycle events is presented. The helix clock arises from the change in pitch angle that accompanies the parallel packing of strands on a cylinder surface. A single strand inserted into the cylinder surface nearly perpendicular to the long axis of the cylinder starts the clock running. As additional strands are inserted parallel to those in place, the pitch angle of all strands must reorient. A limit is reached when all strands lie parallel to the long axis of the cylinder. By sensing either the pitch angle or a physical ramification thereof, cells can measure their length during growth and time events of the cell cycle. The helix clock model is discussed in relationship to the bacterial cell cycle. The idea that bacterial cells use one helix hand for cylinder elongation, the other for septation is presented. The negative twist so generated apparently drives folding in the helical bacterial macrofiber system of Bacillus subtilis .

Self-Assembly of Bacterial Macrofibers: A System Based Upon Hierarchies of Helices

Cylindrical-shaped cells of Bacillus subtilis (0.7 by 4 μm) are the building blocks of macrofibers, highly organized, helically twisted, multifilament structures millimeters to centimeters in length. The forces responsible for self-assembly and the cylinder-helix deformation trace to the assembly of cell wall polymers and restraint of the motions generated by cell growth. An hierarchical relationship exists involving: (i) molecular level events associated with cell surface assembly, which in turn govern, (ii) cellular level events concerned with motions that accompany cell growth, and these in turn drive, (iii) multicellular level events such as the folding and plying of cell filaments to form a mature macrofiber. Cell growth generates new material and engenders twisting of the cell cylinder along a screw axis as it elongates. The helix hand and degree of twist at the cellular level eventually dictate the hand and twist of the mature multifilament macrofiber. Although several different routes can lead to the initiation of macrofiber production, once initiated a repetitive cycle of folding and plying becomes established. The self-assembly proceeds until mechanical and geometrical factors preclude further folding cycles.

Minicells of Bacillus subtilis a unique system for transport studies

Abstract Ultrasound-purified minicells produced by Bacillus subtilis mutant div IV-Bl have been studied for their ability to transport and incorporate into macromolecules a variety of amino acids, uracil and thymine. Minicells transport all 12 amino acids examined, but are unable to incorporate them into macromolecules. No significant differences were found in the initial uptake rates of glutamic acid, aspartic acid, and alanine by minicells and parental cells. The uptake of methionine and proline by minicells was shown to be inhibited by metabolic poisons, indicating an energy-metabolism requirement for transport in this system. The proline pool in minicells was found to be readily exchangeable with exogenous proline. In contrast metabolically poisoned minicells only slowly lose their pool proline, indicating an energy requirement for pool maintenance. Packed-cell experiments reveal that minicells accumulate proline against a concentration gradient. In addition to amino acids, minicells are able to transport uracil but cannot incorporate uracil into acid-precipitable material (RNA). Neither thymine transport nor its incorporation into macromolecules can be demonstrated in minicells. Minicells appear to be a new system, therefore, in which transport may be studied in the absence of macromolecular biosynthesis.

Relaxation Motions Induced in Bacillus subtilis Macrofibres by Cleavage of Peptidoglycan

Bacillus subtilis macrofibres exposed to lysozyme underwent characteristic rotations, termed relaxation motions, in which their twist changed. Intact macrofibres and macrofibre fragments devoid of loop ends responded in the same way. Macrofibre strains for which the helix hand is temperature-dependent and also those of fixed-hand (both left and right) underwent initial relaxation motions towards the right-hand end of the twist spectrum, the only exception being those in which the initial twist state was at or near the right-hand maximum. Often when the initial relaxation motions were completed immediately before structure breakdown the macrofibres underwent one or a few rotations in the opposite direction (towards the left-hand end of the twist spectrum). Crude autolysin extract obtained from wild-type B. subtilis also caused macrofibre relaxation motions at pH 5.6 but at pH 8.0 macrofibre breakdown occurred as a result of septal cleavage. This resulted in the release of helically shaped individual cellular filaments. These findings suggest that strain in the cell wall associated with helical shape was dependent on the integrity of the glycan backbone rather than peptide cross-bridges. In contrast, cleavage of peptide cross-bridges apparently was instrumental in the cell separation process. Left- and right-hand macrofibres, when exposed to lysozyme, exhibited different rates of relaxation, breakdown of fibre structure and protoplast formation. Similarly, the rate of macrofibre breakdown during the lag between temperature shift and inversion reflected the replacement of septal wall material by that of a new conformation corresponding to the new helix hand.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Bacillus subtilis macrofibres, highly ordered multicellular structures, undergo twisting and writhing motions when they grow in fluid medium as a result of forces generated by the elongation of individual cells. Macrofibres are denser than the fluid medium in which they are cultured, consequently they settle to the bottom of the growth chamber and grow in contact with it. The ramifications of growth on plastic and glass surfaces were examined. Macrofibres were observed to rotate about a vertical axis near the centre of their length in a chiral-specific direction. Right-handed fibres rotated clockwise on plastic surfaces at approximately 4 degrees min(-1), left-handed structures of lower twist rotate anti-clockwise at about half that rate. Very large ball structures produced late in macrofibre formation perched on many small protruding fibres but rotated only when driven by large fibres attached to their periphery. Closer examination showed that fibres made contact with surfaces at only a few points along their length (between 1 and 6 on glass). The regions in contact with the surface changed periodically as a result of rotation of the fibre shaft caused by growth. Every time the weight of a fibre transferred from one contact point to another, each section of the fibre took a small step approximately proportional to its distance from the fibre mid-point. The net result was a rolling of each section over the surface so that the fibre rotation about a vertical axis was produced. Macrofibres also took large steps when part of the structure rose off the floor, swept through an arc in the fluid and then returned to the floor at a new location. The rate of movement during a large step, measured as the change of angle between the moving and stationary portions of the fibre, was 5 degrees s(-1). These observations reveal that the forces derived from helical growth that lead to macrofibre formation also cause characteristic macrofibre motion that differs from classical motility.

Cell morphology of Bacillus subtilis: the effect of genetic background on the expression of a rod - gene.

SummaryThe Bacillus subtilis tag-1 mutation has been transferred into five different strains to determine the effect of genetic background on the expression of a gene regulating cell morphology. Two patterns of expression at restrictive temperature were observed:1.Irregular, unstable spherical cells unable to form colonies.2.Regular spherical cells which grow and divide producing luxuriant colonies. When tag-1 is combined with a ts-DNA- mutation, ts-132, cells which are part sphere and part rod are produced at 45 C. Electron micrographs of sections of the double mutant reveal grossly altered cell wall ultrastructure in both regions, suggesting that cell shape is not solely dictated by cell wall structure.