Cell wall mechanical properties as measured with bacterial thread made from Bacillus subtilis

Abstract

Engineering approaches used in the study of textile fibers have been applied to the measurement of mechanical properties of bacterial cell walls by using the Baciflus subtilis bacterial thread system. Improved methods have been developed for the production of thread and for measuring its mechanical properties. The best specimens of thread produced from cultures of strain FJ7 grown in TB medium at 20TC varied in diameter by a factor of 1.09 over a 30-mm thread length. The stress-strain behavior of cell walls was determined over the range of relative humidities between 11 and 98%. Measurements of over 125 specimens indicated that cell wall behaved like other viscoelastic polymers, both natural and man-made, exhibiting relaxation under constant elongation and recovery upon load removal. This kinetic behavior and also the cell wall strength depended greatly on humidity. The recovery from extension observed after loading even up to a substantial fraction of the breaking load indicated that the properties measured were those of cell wall material rather than of behavior of the thread assemblage. Control experiments showed that neither drying of thread nor the length of time it remained dry before testing influenced the mechanical properties of the cell walls. Specimens drawn from TB medium and then washed in water and redrawn were found to be stiffer and stronger than controls not washed. However, tensile properties were not changed by exposure of cells to lysozyme before thread production. This suggests that glycan backbones are not arranged along the length of the cell cylinder. The strength of the cell wall in vivo was estimated by extrapolation to 100% relative humidity to be about 3 N/mm2. Walls of this strength would be able to bear a turgor pressure of 6 atm (ca. 607.8 kPa), but if the increase in strength of water-washed threads was appropriate, the figure could be 24 atm (ca. 2,431.2 kPa).Very little information is currently available about the mechanical properties of bacterial cell walls, although such information would be useful in a number of areas ranging from fundamental studies of bacterial growth and shape determination to applied systems in which bacteria are used for industrial processes. The dimensions of individual bacterial cells preclude direct measurements similar to those made in the study of other materials. Nevertheless, it has been shown that cell walls, in particular the load-bearing polymer peptidoglycan, stretch appreciably and recover (1,5,6,10,11,19) and that cell filaments contract when the osmotic pressure is eliminated, indicating that the wall is stretched in living cells (7). The flexibility of the cell wall can be seen by observing the swimming motions of either long cells or cell separation-suppressed filaments. The degree to which wall can stretch and compress is also evident in the shape of cells found at the hairpin loop formed by folding of cell filaments during macrofiber formation (14).We recently described a new system, bacterial thread, in which it is possible to measure...

Mechanical properties of Bacillus subtilis cell walls: effects of ions and lysozyme

“…The significance of these results and those of earlier experiments (18,25,26) is discussed in relation to the molecular organization of the bacterial cell wall. These results are not in conflict with the presently accepted picture.…”

“…Regular structures must surely be crystalline, but there is much evidence to show that peptidoglycan in bacteria is not (12,21). All of the evidence from thread measurements (18,25,26) shows the cell wall to be in no way different from other amorphous polymers. There is no reason to suppose that the molecular arrangement in peptidoglycan is other than an entanglement network.…”

“…Bacterial threads were produced by using the methods described in the accompanying paper and previously (18,24,26). Standard threads were obtained by washing off the residual culture medium in deionized water and then redrawing the threads (26).…”

“…Such changes in the molecular arrangement ought to be accompanied by changes in the mechanical properties of the cell walls. The work reported here concerns the quantitative effects of ions on cell wall axial properties, measured directly by using the bacterial thread system (18,24). The effects of increasing concentrations of NaCl and of (NH4)2SO4, as well as the effects of changing pH, were measured.…”

“…They are also affected by the presence of residual growth medium (18,26). This growth medium (TB medium) contains NaCl.…”