Figure 4

Single-cell membrane fluctuation spectroscopy as performed in E. coli (MG1655 strain) growing exponentially in a nutrient media at physiological temperature (37 °C). To record movies of the membrane fluctuations, single cells at pre-divisional status were observed under the optical microscope in the phase-contrast mode (see “Methods” section): (A) after tracking membrane fluctuations (see Suppl. Note N4), the bacterial profile is converted in spatial maps of dynamic properties estimated from the stochastic trajectories of every membrane emplacement, these are \(h\left(x,t\right)\). The spatial map obtained represents the rms-value of the fluctuation amplitudes, \(\Delta h\left(x\right)\equiv {\langle {h}^{2}\left(x,t\right)-{\langle h\left(x,t\right)\rangle }^{2}\rangle }_{t}\), which differentiates soft and rigid regions in a mechanically heterogeneous membrane (see main text for details). (B) Typical time series of membrane fluctuations \(h\left(x,t\right)\); top panel) stiff membrane site undergoing low amplitude fluctuations, down panel) soft spot undergoing larger fluctuations. Depending on the mechanical status, fluctuation amplitudes of variable standard deviation (\({\upsigma }_{h}\)) distribute near-normally as characterized by skewness (\(S\)) and kurtosis (\(K\)) (right panels). Stiff spots normally behave as low-amplitude thermal noise (Gaussian-like; \(S\approx 0\) and \(K\approx 3\)), and soft spots as high-amplitude, long-tailed trajectories with an out-of-equilibrium activity (\(S\approx 0\) and \(K>3\)). (C) Fluctuation analysis in the modified cells upon treatment with membrane elastoactive agents (false colour images): left panels) membrane stiffening by DDA; central panels) untreated E. coli cell; right panels) membrane softening by pentanol (see main text for details). (D) Single-cell statistics for the variance of the fluctuation amplitudes (\(\Sigma\)). Data sampling box plots in relevant cases (\(N>10\)); statistical significance is defined as differences on the fluctuation amplitudes with respect to the null hypothesis (control: untreated cells; significance \(99\%\)). The box size indicates SD, and the whiskers the significance band including outliers. In order to better visualization, the normal distributions are included as straight lines. Different biological situations are indicated in the legends.