Electric Polarization Properties of Single Bacteria Measured with Electrostatic Force Microscopy

Abstract

We quantified the electrical polarization properties of single bacterial cells using electrostatic force microscopy. We found that the effective dielectric constant, ε(r,eff), for the four bacterial types investigated (Salmonella typhimurium, Escherchia coli, Lactobacilus sakei, and Listeria innocua) is around 3-5 under dry air conditions. Under ambient humidity, it increases to ε(r,eff) ∼ 6-7 for the Gram-negative bacterial types (S. typhimurium and E. coli) and to ε(r,eff) ∼ 15-20 for the Gram-positive ones (L. sakei and L. innocua). We show that the measured effective dielectric constants can be consistently interpreted in terms of the electric polarization properties of the biochemical components of the bacterial cell compartments and of their hydration state. These results demonstrate the potential of electrical studies of single bacterial cells.

Nanoscale dielectric microscopy of non-planar samples by lift-mode electrostatic force microscopy

“…We note that this geometry is very close to the hemiellipsoid geometry used in Ref. [30], but adapts slightly better to the geometry of the bacterial cell investigated here. The tip dimensions have been obtained from the calibration curve measured on the substrate shown in figure 4b, giving R = 115±1 nm, θ = 30.0±0.3º and kstray = 0.108±0.002 aF/nm.…”

“…In our previous work, we used equivalent 2D axisymmetric models preserving the bacterial cell volume and height [30] (i.e. representing hemiellipsoids by equivalent hemispheroids).…”

“…Among them, we can cite nanoscale capacitance microscopy [1][2][3], electrostatic force microscopy (EFM) [4][5][6][7][8][9][10], nanoscale impedance microscopy [11,12], scanning polarization force microscopy [13][14][15][16], scanning microwave microscopy (SMM) [17,18] and nanoscale non-linear dielectric microscopy [19]. These techniques have allowed measuring the electric permittivity with nanoscale spatial resolution on planar samples, such as thin oxides, polymer films and supported biomembranes [2][3][4]8,10], and on non-planar ones, such as, single carbon nanotubes, nanowires, nanoparticles, viruses and bacterial cells [20][21][22][23][24][25][26][27][28][29][30].…”

Internal Hydration Properties of Single Bacterial Endospores Probed by Electrostatic Force Microscopy

“…EFM is a scanning probe microscopy technique sensitive to the local dielectric properties of the samples. 33,34 Examples showing this ability include numerous applications to samples of non-biological origin (thin and thick oxides, 35 polymer films, [36][37][38] nanowires, 39 nanotubes 40,41 or nanoparticles [42][43][44][45] ), and of biological origin (single bacterial cells, 46,47 single virus particles, 45 solid supported biomembranes, 48 protein complexes 49 or DNA molecules). 50 EFM has two important properties relevant for the present application, namely, (i)…”

“…it is sensitive to the internal dielectric properties of the samples, since it is based in the measurement of long range electric forces [51][52][53] and, (ii) it is also sensitive to the presence of moisture in the sample, 46 due to the large electric permittivity of water (ε r,water~8 0). The above mentioned features of show that this technique is optimal to probe in situ and in a nondestructive way the internal hydration properties of small scale biological samples, and bacterial endospores in particular, under varying environmental humidity conditions.…”