Abstract
BackgroundDespite the availability of conventional devices for making single-cell manipulations, determining the hardness of a single cell remains difficult. Here, we consider the cell to be a linear elastic body and apply Young’s modulus (modulus of elasticity), which is defined as the ratio of the repulsive force (stress) in response to the applied strain. In this new method, a scanning probe microscope (SPM) is operated with a cantilever in the “contact-and-push” mode, and the cantilever is applied to the cell surface over a set distance (applied strain).ResultsWe determined the hardness of the following bacterial cells: Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and five Bacillus spp. In log phase, these strains had a similar Young’s modulus, but Bacillus spp. spores were significantly harder than the corresponding vegetative cells. There was a positive, linear correlation between the hardness of bacterial spores and heat or ultraviolet (UV) resistance.ConclusionsUsing this technique, the hardness of a single vegetative bacterial cell or spore could be determined based on Young’s modulus. As an application of this technique, we demonstrated that the hardness of individual bacterial spores was directly proportional to heat and UV resistance, which are the conventional measures of physical durability. This technique allows the rapid and direct determination of spore durability and provides a valuable and innovative method for the evaluation of physical properties in the field of microbiology.
Highlights
Despite the availability of conventional devices for making single-cell manipulations, determining the hardness of a single cell remains difficult
We developed methods to directly measure the physical properties of a single cell using bacterial spores and a scanning probe microscope (SPM)
The hardness of bacterial cells could be measured for the first time
Summary
Despite the availability of conventional devices for making single-cell manipulations, determining the hardness of a single cell remains difficult. We consider the cell to be a linear elastic body and apply Young’s modulus (modulus of elasticity), which is defined as the ratio of the repulsive force (stress) in response to the applied strain. In this new method, a scanning probe microscope (SPM) is operated with a cantilever in the “contact-and-push” mode, and the cantilever is applied to the cell surface over a set distance (applied strain). We developed a method using the cantilever probe of a SPM to determine the hardness of a cell, an organelle, or other microscopic specimen. The relationship between hardness, given by Young’s modulus, Nakanishi et al Journal of Nanobiotechnology 2012, 10:22 /10/1/22
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