Mechanical properties of Bacillus subtilis cell walls: effects of removing residual culture medium

Abstract

Experiments are described in which the tensile strength, the initial (Youngs') modulus, and other mechanical properties of the bacterial cell wall were obtained as functions of relative humidity (RH) in the range of 20 to 95%. These properties were deduced from tensile tests on bacterial thread, a fiber consisting of many highly aligned cells of Bacillus subtilis, from which residual culture medium had been removed by immersion in water. Reasons are given to support the idea that the mechanical properties of bacterial thread relate directly to those of the cylinder wall and that they are not influenced by septa, cytoplasm, or the thread assembly. The data show that the cell wall, like many other heteropolymers, is visco-elastic. When dry, it behaves like a glassy polymer with a tensile strength of about 300 MPa and a modulus of about 13 GPa. When wet, its behavior is more like a rubbery polymer with a tensile strength of about 13 MPa and a modulus of about 30 MPa. Thus, the cell wall is stronger than previously reported. Walls of this strength would be able to bear a turgor pressure of 2.6 MPa (about 26 atm). The dynamic behavior suggests a wide range of relaxation times. The way in which mechanical behavior depends strongly on humidity is discussed in terms of possible hydrogen bond density and the ordering of water molecules. Cell walls in threads containing residual culture medium TB are, except at low RH, 10 times more flexible and about 4 times less strong. All of their mechanical properties appear to vary with change in RH in a manner similar to those of walls from which the culture medium has been washed, but with a downshift of about 18% RH.The bacterial cell wall has a substantial mechanical role in addition to other functions. It must be strong enough to protect the cytoplasmic membrane from forces originating outside of the cell and to stabilize it against turgor. The wall must be stiff enough to maintain cell shape but ductile enough to allow growth. It must also be elastic enough to recover from environmentally induced changes. In vivo the wall is under stress and is also stretched (7,17). Evidence for this has been obtained in several ways, including subjecting cells to electrochemical changes and osmotic shocks (1, 3, 5, 9, 18), but until recently estimates of cell wall mechanical properties have been largely qualitative. To understand how bacteria maintain their characteristic shape during growth and even during division, for example, to understand why rod shape is so stable, an investigation of the states of stress and deformation in the cell wall is necessary. For this, quantitative estimates of mechanical properties are required, but measurement, even indirectly, is difficult and often impossible in normal cultures. Bacterial thread is a fibrillar fiber consisting of many cellular filaments which lie parallel to the fiber axis and which adhere together very strongly. A thread contains hundreds of millions of highly aligned cells but closely resembles a textile fiber (whereas a macrofiber resem...

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.…”

“…Experiments are described in which the effects on cell wall mechanical properties of partial digestion by lysozyme have been measured in bacterial thread. Strong reasons for inferring these properties directly from those of bacterial thread are given in an accompanying paper (26). These reasons are equally valid for threads in which the cell walls have been modified by ions or by lysozyme attack.…”

“…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).…”

“…The mechanical properties of bacterial cell wall are greatly influenced by the degree to which the walls are hydrated (25,26). They are also affected by the presence of residual growth medium (18,26).…”

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