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Using self-trapped Escherichia coli bacteria that have intact flagellar bundles on glass surfaces, we study statistical fluctuations of cell-body rotation in a steady (unstimulated) state. These fluctuations underline direction randomization of bacterial swimming trajectories and plays a fundamental role in bacterial chemotaxis. A parallel study is also conducted using a classical rotation assay in which cell-body rotation is driven by a single flagellar motor. These investigations allow us to draw the important conclusion that during periods of counterclockwise motor rotation, which contributes to a run, all flagellar motors are strongly correlated, but during the clockwise period, which contributes to a tumble, individual motors are uncorrelated in long times. Our observation is consistent with the physical picture that formation and maintenance of a coherent flagellar bundle is provided by a single dominant flagellum in the bundle.

Bacterial motion is strongly affected by the presence of a surface. One of the hallmarks of swimming near a surface is a defined curvature of bacterial trajectories, underlining the importance of counter rotations of the cell body and flagellum for locomotion of the microorganism. We find that there is another mode of bacterial motion on solid surfaces, i.e., self trapping due to fluid flows created by a rotating flagellum perpendicular to the surface. For a rod-like bacterium, such as Escherichia coli, this creates a peculiar situation in that the bacterium appears to swim along a minor axis of the cell body and is pressed against the surface. Although a full hydrodynamic theory is still lacking to explain the self-trapping phenomenon, the effect is intriguing and can be exploited to study a variety of biophysical phenomena of swimming bacteria. In particular, we showed that self-trapped E. coli cells display a chemotaxis response that is identical to the classical rotation assay in which antibodies are used to physically “glue” a flagellum to the surface.

We investigate bacterial chemotactic strategies using run-tumble and run-reverse-flick motility patterns. The former is typically observed in enteric bacteria such as Escherichia coli and Salmonella and the latter was recently observed in the marine bacteria Vibrio alginolyticus and is possibly exhibited by other polar flagellated species. It is shown that although the three-step motility pattern helps the bacterium to localize near hot spots, an exploitative behavior, its exploratory potential in short times can be significantly enhanced by employing a non-Poissonian regulation scheme for its flagellar motor switches.

We found recently that polar flagellated marine bacterium Vibrio alginolyticus is capable of exhibiting taxis toward a chemical source in both forward and backward swimming directions. How the microorganism coordinates these two swimming intervals, however, is not known. The work presented herein is aimed at determining the response functions of the bacterium by applying a stepwise chemoattractant stimulus while it is swimming forward or backward. The important finding of our experiment is that the bacterium responds to an identical chemical signal similarly during the two swimming intervals. For weak stimuli, the difference is mainly in the amplitudes of the response functions while the reaction and adaptation times remain unchanged. In this linear-response regime, the amplitude in the forward swimming interval is approximately a factor of two greater than in the backward direction. Our observation suggests that the cell processes chemical signals identically in both swimming intervals, but the responses of the flagellar motor to the output of the chemotaxis network, the regulator CheY-P concentration, are different. The biological significance of this asymmetrical response in polar flagellated marine bacteria is discussed.

Bacteria use different motility patterns to navigate and explore natural habitats. However, how these motility patterns are selected, and what their benefits may be, are not understood. In this article, we analyze the effect of motility patterns on a cell’s ability to migrate in a chemical gradient and to localize at the top of the gradient, the two most important characteristics of bacterial chemotaxis. We will focus on two motility patterns, run-tumble and run-reverse-flick, that are observed and characterized in enteric bacterium Escherichia coli and marine bacterium Vibrio alginolyticus, respectively. To make an objective comparison, master equations are developed on the basis of microscopic motions of the bacteria. An unexpected yet significant result is that by adopting the run-reverse-flick motility pattern, a bacterium can reduce its diffusivity without compromising its drift in the chemical gradient. This finding is biologically important as it suggests that the motility pattern can improve a microorganism’s ability to sequester nutrients in a competitive environment.

We recently found that marine bacteria Vibrio alginolyticus execute a cyclic three-step (run-reverse-flick) motility pattern that is distinctively different from the two-step (run-tumble) pattern of Escherichia coli. How this novel, to our knowledge, swimming pattern is regulated by cells of V. alginolyticus is not currently known, but its significance for bacterial chemotaxis is self-evident and will be delineated herein. Using a statistical approach, we calculated the migration speed of a cell executing the three-step pattern in a linear chemical gradient, and found that a biphasic chemotactic response arises naturally. The implication of such a response for the cells to adapt to ocean environments and its possible connection to E. coli's response are also discussed.

It has been theoretically suggested that when a bacterium swims in a fluid, the disturbance it creates is long-ranged and can influence its locomotion. The contribution of these long-range hydrodynamic interactions to swimming cells is examined herein for a number of bacterial strains with well-defined flagellar geometries. We show experimentally for the first time that long-range hydrodynamic interactions are important for an accurate description of the swimming of a single cell, and the effect is more pronounced for bacteria with a large cell body. The commonly used local resistive force theory assumes a stationary background fluid while ignoring flows induced due to other moving parts of the cell. Although pedagogically attractive, resistive force theory is not generally applicable to experiment.

It has long been suspected that population heterogeneity, either at a genetic level or at a protein level, can improve the fitness of an organism under a variety of environmental stresses. However, quantitative measurements to substantiate such a hypothesis turn out to be rather difficult and have rarely been performed. Herein, we examine the effect of expression heterogeneity of lambda-phage receptors on the response of an Escherichia coli population to attack by a high concentration of lambda-phage. The distribution of the phage receptors in the population was characterized by flow cytometry, and the same bacterial population was then subjected to different phage pressures. We show that a minority population of bacteria that produces the receptor slowly and at low levels determines the long-term survivability of the bacterial population and that phage-resistant mutants can be efficiently isolated only when the persistent phage pressure10(10) viruses/cm(3) is present. Below this phage pressure, persistors instead of mutants are dominant in the population.

We use fluorescence correlation spectroscopy and fluorescence recovery after photobleaching to study vesicle dynamics inside the synapses of cultured hippocampal neurons labeled with the fluorescent vesicle marker FM 1–43. These studies show that when the cell is electrically at rest, only a small population of vesicles is mobile, taking seconds to traverse the synapse. Applying the phosphatase inhibitor okadaic acid causes vesicles to diffuse freely, moving 30 times faster than vesicles in control synapses. These results suggest that vesicles move sluggishly due to binding to elements of the synaptic cytomatrix and that this binding is altered by phosphorylation. Motivated by these results, a model is constructed consisting of diffusing vesicles that bind reversibly to the cytomatrix. This stick-and-diffuse model accounts for the fluorescence correlation spectroscopy and fluorescence recovery after photobleaching data, and also predicts the well-known exponential refilling of the readily releasable pool. Our measurements suggest that the movement of vesicles to the active zone is the rate-limiting step in this process.

Motility of polar flagellated bacteria is typically forward and backward in rapid succession. We recently found that one of the marine species, Vibrio alginolyticus, incorporates a flick movement at the end of the backward swimming interval, breaking the time reversal symmetry of the swimming trajectory. A flick in this bacterium is functionally equivalent to a tumble of peritrichously flagellated bacteria, such as Escherichia coli, causing the cell body to deflect in a new direction before the next run starts. Since V. alginolyticus is capable of swimming in both forward and backward directions, it raises an interesting question about how the chemotaxis behavior of this bacterium is regulated. Herein, we provide experimental evidence showing that the marine bacterium differentiates chemical signals detected in the two swimming intervals and responds in the manner that is consistent with the chemotaxis strategy where the forward swimming interval is exploratory and the backward interval is exploitative.

Vibrio alginolyticus is a marine bacterium with a single polar flagellum when growing in low viscous liquid media. Recently, we found that they carry out a novel cyclic 3-step motility pattern. In one cycle they swim forward, backward and at the end of the backward swimming they are able to use their flagellum to flick and actively change their cell body orientation. To understand how this novel motility pattern is regulated by V. alginolyticus, we studied their chemotactic response to a sudden increase of the attractant serine concentration. We made use of caged serine, which can be released by UV light in a fraction of second to achieve a sudden change in the serine concentration. We compared their swimming behavior before and after the UV flash. We found that in short time, they treat forward and backward swimming equally, i.e. they extend their current swimming interval regardless of the swimming direction. However, in a long time but before adaptation, cells become gradually forwardly biased. This chemotactic strategy enables the bacteria to swim up the attractant gradient quickly as well as to localize around the top of the gradient to get a maximum nutrient exposure. This perhaps is important in an ocean environment where nutrient are distributed intermittently in space and time.

Adsorption of lambda-phage on sensitive bacteria Escherichia coli is a classical problem but not all issues have been resolved. One of the outstanding problems is the rate of adsorption, which in some cases appears to exceed the theoretical limit imposed by the law of random diffusion. We revisit this problem by conducting experiments along with new theoretical analyses. Our measurements show that upon incubating lambda-phage with bacteria Ymel, the population of unbound phage in a salt buffer decreases with time and in general obeys a double-exponential function characterized by a fast (tau(1)) and a slow (tau(2)) decay time. We found that both the fast and the slow processes are specific to interactions between lambda-phage and its receptor LamB. Such specificity motivates a kinetic model that describes the interaction between the phage and the receptor as an on-and-off process followed by an irreversible binding. The latter may be a signature of the initiation of DNA translocation. The kinetic model successfully predicts the double exponential behavior seen in the experiment and allows the corresponding rate constants to be extracted from single measurements. The weak temperature dependence of the reversible and the irreversible binding rate suggests that phage retention by the receptor is entropic in nature and that a molecular key-lock interaction may be an appropriate description of the interaction between the phage tail and the receptor.

We identified a TTC1975-like protein from the thermophile Meiothermus taiwanensis WR-220 as a homolog of the protease domain of Lon protease. The lack of the ATPase domain excludes this protein, Mtw-TTC, from the AAA+ superfamily, even though Mtw-TTC possesses the conserved Ser-Lys dyad and TTC1975 proteins are classified as a member of the Lon protease family (S16) in the MEROPS database. We cloned, over-expressed and characterized Mtw-TTC. Far-UV CD measurements showed that Mtw-TTC consists of α-helices as the major secondary structure. Sedimentation velocity analysis indicated that the protein forms a 42S complex in solution. Gel-filtration chromatography revealed a major fraction near 2,000 kDa and a minor fraction of 481 kDa, which was confirmed by cryo-fixation transmission electron microscopy and native PAGE. In transmission electron micrographs, Mt-TTC was composed of hexamers and heptamers, owing to the dissociation of the 42S assembly in low pH buffer. Mtw-TTC was stable below 74 °C and displayed chaperone activity with chemically stressed or UV-irradiated proteins. Our results support a role of Mtw-TTC in the chaperone network of M. taiwanensis.

We have used the technique of resonance energy transfer in conjunction with distance geometry analysis to localize Cys 133 of troponin-I (TnI) with respect to troponin-C (TnC) in the ternary troponin complex and the binary TnC·TnI complex in the presence and absence of Ca 2+ . Cys 133 of TnI was chosen because our previous work has shown that the region of TnI containing this residue undergoes Ca 2+ -dependent movements between actin and TnC, and may play an important role in the regulatory function of troponin. For this purpose, a TnI mutant with a single Cys at position 133, and TnC mutants, each with a single Cys at positions 5, 12, 21, 41, 49, 89, 98, 133, and 158, were constructed by site-directed mutagenesis. The distances between TnI Cys 133 and each of the nine residues in TnC were then measured. Using a least-squares minimization procedure, we determined the position of TnI Cys 133 in the coordinate system of the crystal structure of TnC. Our results show that in the presence of Ca 2+ , TnI Cys 133 is located near residue 12 beneath the N-terminal lobe of TnC, and moves away by 12.6A upon the removal of Ca 2+ . TnI Cys 133 and the region of TnC that undergoes major change in conformation in response to Ca 2+ are located roughly on opposite sides of TnC's central helix. This suggests that the region in TnI that undergoes Ca 2+ -dependent interaction with TnC is distinct from that interacting with actin.

We investigate the protein expression pattern of the lamB gene in Escherichia coli bacterium. The gene product LamB is an important membrane protein for maltose transport into cells but it is also exploited by bacteriophage lambda for infection. Using a dual-colored phage labeling technique, we find that the LamB receptor distribution p(n) has a majority population with average receptor number n∼500 and a minority population at small n. This small-n distribution is scale invariant with p(n)∼ nˆα. A power law is also observed when LamB expression is chemically repressed by growing the bacteria in a glucose medium. We propose a heuristic model which can account qualitatively for our observations.