BackgroundIn Escherichia coli, MinD-GFP fusion proteins show rapid pole to pole oscillations. The objective was to investigate the effects of extracellular cations on the subcellular oscillation of cytoplasmic MinD within Escherichia coli.Methodology/Principal FindingsWe exposed bacteria to the extracellular cations Ca++, Mg++, the cationic antimicrobial peptide (CAP) protamine, and the cationic aminoglycoside gentamicin. We found rapid and substantial increases in the average MinD oscillation periods in the presence of any of these polyvalent cations. For Ca++ and Mg++ the increases in period were transient, even with a constant extracellular concentration, while increases in period for protamine or gentamicin were apparently irreversible. We also found striking interdependence in the action of the small cations with protamine or gentamicin, distorted oscillations under the action of intermediate levels of gentamicin and Ca++, and reversible freezing of the Min oscillation at high cationic concentrations.Conclusions/SignificanceIntracellular Min oscillations provide a fast single-cell reporter of bacterial response to extracellular polycations, which can be explained by the penetration of polycations into cells.

The well calibrated force‐extension behaviour of single double‐stranded DNA molecules was used as a standard to investigate the performance of phase‐only holographic optical tweezers at high forces. Specifically, the characteristic overstretch transition at 65 pN was found to appear where expected, demonstrating (1) that holographic optical trap calibration using thermal fluctuation methods is valid to high forces; (2) that the holographic optical traps are harmonic out to >250 nm of 2.1 μm particle displacement; and (3) that temporal modulations in traps induced by the spatial light modulator (SLM) do not affect the ability of optical traps to hold and steer particles against high forces. These studies demonstrate a new high‐force capability for holographic optical traps achievable by SLM technologies. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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Probing relatively short molecules, such as the protein elastin (~200 nm), with optical tweezers requires manipulating trapped polystyrene beads at separations small compared to their micron dimensions. We discuss experimental complications arising from this proximity, such as hydrodynamic and optical interactions, and our efforts to minimize them.