The integrity and morphology of bacteria is sustained by the cell wall, the target of the main microbial inactivation processes. One promising approach to inactivation is based on the use of pulsed electric fields (PEF). The current dogma is that irreversible cell membrane electro-permeabilisation causes the death of the bacteria. However, the actual effect on the cell-wall architecture has been poorly explored. Here we combine atomic force microscopy and electron microscopy to study the cell-wall organization of living Bacillus pumilus bacteria at the nanoscale. For vegetative bacteria, exposure to PEF led to structural disorganization correlated with morphological and mechanical alterations of the cell wall. For spores, PEF exposure led to the partial destruction of coat protein nanostructures, associated with internal alterations of cortex and core. Our findings reveal for the first time that the cell wall and coat architecture are directly involved in the electro-eradication of bacteria.
Structural and mechanical mapping at the nanoscale by novel high-speed multiparametric Quantitative Imaging (QI) and PeakForce Quantitative Nanomechanical Mapping (PF-QNM) AFM modes was compared to the classical Force Volume (FV) mapping for the case of living Pseudomonas aeruginosa bacterial cells. QI and PF-QNM modes give results consistent with FV for the whole cells in terms of morphology and elastic modulus, while providing higher resolution and shorter acquisition time. As an important complement, the influence of scanning parameters on elastic modulus values was explored for small 0.2(2)μm(2) central area on top of cells. The modulus decreases with the indentation depth due to the effect of the hard cell wall, while it increases vs. tip oscillation frequency, displaying viscoelastic behaviour of the living bacterial cells. The ability of different AFM modes to follow correctly the bacteria viscoelastic behaviour at high oscillation frequency was tested.
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