Dynamic characteristics of the Typhimurium wild-type strain: the (bottom) and (upper) coordinates of the wild-type bacteria during a 7 s time interval.

<p>The the switch of the flagellar motor from the running to the tumbling state is shown. The trace of the coordinate is shifted in order to avoid overlap with that of the coordinate.</p

Dynamic characteristics of the motion of a dead bacterium: time-traces of the and positions during 1 s (a) and their corresponding PSD (b), ACF, and CCF (c).

<p>In (a), the trajectory is shifted in order to improve its visibility. In (c) the red and blue curves are the ACFs of the and positions, and the green curve is the CCF.</p

A simplified explanation of the two possible flagellar conformations.

<p>On the left is a cell with all flagella rotating CCW and consequently forming a bundle that gives a translation velocity to the bacteria, which runs. The motion of a living bacterial cell is characterized by a body roll with frequency , and by flagellar bundle rotation . On the right is a tumbling cell with the flagella rotating CW such that no bundle is formed. This process gives extra energy to the cell’s dynamics, but does not produce any rotation around the optical axis of the trapping system.</p

Dynamic characteristics of the mutant: the trajectories of the and positions during 1 s (a) and their corresponding PSD (b), ACF and CCF (c).

<p>The red and blue curves are the trajectories, PSD, and ACF of the and positions, and the green curve is the CCF.</p

The chemotaxis pathway.

<p>(a) When the methyl-accepting chemotaxis proteins (MCPs) are highly methylated or unbound (yellow), CheA is activated by phosphorylation. Once activated, it phosphorylates the CheY and CheB response regulators. CheB-P demethylates the MCPs and the high level of CheY-P interacts with the flagellar motor, increasing the frequency of clockwise (CW) rotation, which causes the cell to tumble. (b) MCPs associated with a ligand (attractant) or less methylated (orange), maintain the CheA in a non-phosphorylated, inactive state. Consequently, the CheB methylesterase is not active and does not demethylate the MCPs. CheR methylation of the MCPs, decreases the sensitivity of these receptors. In addition, the CheY-P levels are reduced, leading to an increase in counterclockwise (CCW) flagellar rotation frequency, causing the cell to run. PE, periplasm; IM, inner membrane; Z, CheZ; W, CheW. The grey discontinuous arrows indicate non-occurring reactions (based on <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061630#pone.0061630-Parkinson1" target="_blank">[11]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061630#pone.0061630-Jurica1" target="_blank">[12]</a>).</p

The slope, , of the CCF near obtained from the trajectories of single optically trapped dead (diamonds), running (circles and crosses, corresponding to and mutants, respectively), mutant tumbling (squares), and wild-type (triangles) bacteria.

<p>The histograms of the distributions can be classified into three main groups: (1) The histograms with a single maximum centered at zero that corresponds to tumbling bacteria ( mutant); (2) those with a single maximum not centered at zero correspond to a deterministic rotation associated with the rotation of a solid sphere, as is the case for running bacteria ( and mutants); and (3) a combination of both profiles, as occurs in the wild-type strain.</p

Dynamic characteristics of the mutant: the trajectories of the and positions during 1 s (a) and their corresponding PSD (b), ACF, and CCF (c).

<p>The red and blue curves are the trajectories, PSD, and ACFs of the and positions, and the green curve is the CCF.</p

The slope, , of the CCF near , obtained from the trajectories of the optically trapped mutants.

<p>The slope, , of the CCF near , obtained from the trajectories of the optically trapped mutants.</p

The chemotactic response of the <i>S</i>. Typhimurium Δ<i>recA</i> mutant is impaired.

<p>The chemotactic responses of <i>S</i>. Typhimurium wild-type (WT), Δ<i>recA</i>, Δ<i>recA</i> complemented (Δ<i>recA</i> pUA1109), and Δ<i>cheB</i> (tumbling) cells were assessed using Adler's capillary assay <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0105578#pone.0105578-Adler1" target="_blank">[30]</a> with the modifications described in Materials and Methods. Values are expressed as the number of viable cells (in cfu/ml) in a capillary tube containing either 10 mM aspartate (+) or tethering buffer alone (−). The results are the mean of five independent experiments of three capillaries each. Error bars indicate the standard deviation. ***p<0.001 and *p<0.05 as determined by two-way ANOVA with Bonferroni correction.</p

Bacterial strains and plasmids used in this work.

<p>Bacterial strains and plasmids used in this work.</p