| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
rglis *
bers * 
* University of Latvia, Zellu, Latvia; Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania; Latvian Biomedical Research and Study Centre, Riga, Latvia; and Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania
Correspondence: Address reprint requests to Dr. A. Cbers, E-mail: aceb{at}tesla.sal.lv.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONTHEORETICAL MODELSEXPERIMENTAL METHODSEXPERIMENTAL RESULTSCONCLUDING REMARKSACKNOWLEDGEMENTSREFERENCES |
|---|
40 ferromagnetic nanoparticles. The movements of the bacteria in suspension are analyzed by consideration of the orientation of their magnetic dipoles in the field, the hydrodynamic resistance of the bacteria, and the propulsive force of the flagella. Several novel features found in experiments include a velocity reversal during motion in the rotating field and an interesting diffusive wandering of the trajectory curvature centers. A new method to measure the magnetic moment of an individual bacterium is proposed based on the theory developed. |
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONTHEORETICAL MODELSEXPERIMENTAL METHODSEXPERIMENTAL RESULTSCONCLUDING REMARKSACKNOWLEDGEMENTSREFERENCES |
|---|
). It is a challenge to describe an integral behavior of bacterial population through individual active motions of the single cells and to find the relationship in the behavior of individuals with the characteristics of the population. Study of the individual movements of single cells is an obligatory prerequisite for such an approach, and magnetotactic bacteria may serve as a convenient model taking into consideration the possibility to control the magnetic field as one of the major factors influencing their motion.
Since their discovery in the early 1970s (2), magnetotactic bacteria have been intensively studied for more than 30 years. These bacteria assemble linear arrays of nanometer-scaled ferromagnetic particles called magnetosomes that are synthesized in the periplasmic space and internalized in the cytoplasm where chains of magnetosomes are stabilized by interaction with filaments analogous to eukaryotic cytoskeletal filaments (3–5). Orientation along the earth's magnetic field lines is thought to enable these bacteria to navigate to environments with suitably low oxygen concentrations. Recently the genes that encode the biochemical machinery responsible for synthesis of magnetosomes and their alignment into chains along the filamentary cytoskeleton-like structures of the bacteria have been partially identified and sequenced (6,7).
One of the well-characterized magnetotactic microorganisms is microaerophilic alphaproteobacterium Magnetospirillum gryphiswaldense, which contains crystals of magnetite Fe3O4. For this bacterium, a cultivation protocol in an oxystat fermentor has been elaborated, allowing one to study the physiology of magnetosome biomineralization (8) or to produce magnetite nanoparticles for possible nanotechnology applications (for review, see (9)).
Magnetotactic bacteria provide their motility through flagella, the nanoengine that is the key factor for the magnetotaxis (10). They respond to a number of environmental signals, where the responses to the oxygen concentration or oxygen gradient were described in the literature (11–13). There is the difference in the morphology of flagella between species, i.e., magnetotactic spirilla usually have two distal flagella, whereas magnetic cocci, the most abundant morphotypes in nature, have bundles of flagella (2).
Important information about the motion of magnetotactic bacteria can be obtained from the study of their behavior in time-dependent magnetic fields. The U-shaped trajectories of bacteria resulting from switching the direction of the magnetic field have been used to measure their magnetic properties (14). Rotating magnetic fields can be used to determine the critical frequency below which a rotation of single magnetic bacterium is synchronized with the field (15,16). This allows measurement of the ratio of the magnetic moment of the bacterium to its rotational drag coefficient.
In the previous study (17), the dynamics of magnetotactic bacteria under the action of a rotating magnetic field was theoretically investigated, and different trajectories with rather peculiar shapes were predicted depending on the frequency of the rotating field.
Here we report the results of experimental observations of the motion of the magnetotactic bacterium M. gryphiswaldense in rotating magnetic fields with different strengths and frequencies and interpret these motions in the framework of existing models (17) for an active magnetic particle. Although in general the behavior of the bacteria corresponds to what is expected from the theoretical model, several unanticipated phenomena were found in a rotating field, namely 1), a reversal of bacterium motion in a rotating field; 2), the possible escape into the third dimension out of the plane of a rotating field; and 3), a rotational Brownian motion around the steady state in a rotating field, as well as other features. For their interpretation, corresponding models are proposed.
|
THEORETICAL MODELS |
|---|
|
TOPABSTRACTINTRODUCTIONTHEORETICAL MODELSEXPERIMENTAL METHODSEXPERIMENTAL RESULTSCONCLUDING REMARKSACKNOWLEDGEMENTSREFERENCES |
|---|
coupled to the direction of its long axis 
along which the self-propelling motion of a bacterium occurs. The angular velocity of the bacterium 
is found from the viscous and magnetic torque balance, and the equation of the motion for its long axis reads
 | (1) |
is the rotational drag coefficient of the bacterium. Applying Eq. 1 to a field 
rotating in the x,y plane with angular frequency 
(
(
is the unit vector along the field direction), gives
 | (2) |
and it is assumed that motion of the long axis takes place in the plane of a rotating field. If a lag ß = t – 
of the bacterium axis (with the respect to the field direction) is introduced, then, from Eq. 2, follows
 | (3) |
c = mH/ is the critical frequency below which synchronous motion of the bacterium and the field takes place. If < c, Eq. 3 has a steady solution with ß determined by sin(ß) = /c. If > c, Eq. 3 has only a periodic solution:
 | (4) |
Angular velocity of the bacterium is c sin(ß). Since ß changes with time in the asynchronous regime, a characteristic back-and-forth motion of the magnetic dipole takes place, as shown schematically in Fig. 1 a. Corresponding to its change of orientation, angle is illustrated in Fig. 1 b.
|
ß 
) and backward ( ß 2) motion as a function of c/ (calculated according to the relation Tforth/Tback = (1 + a)/(1 – a), where 
), is shown in Fig. 1 c. This dependence is used in the next section to determine the critical frequency of a bacterium with one end attached to the microscope cover glass. It should be remarked that Eq. 4 is well supported by experimental observations of the motion of ferromagnetic particles under the action of a rotating field, which has interesting biological applications (18).
In the simplest model, the velocity of a bacterium is assumed to be in the direction of its long axis. Since the bacterium is not axisymmetric, there should be a velocity component perpendicular to its long axis but since a bacterium rotates during its motion this does not contribute to the overall shape of the trajectory. As the result, the trajectory of the bacterium is found from the equations
 | (5) |
 | (6) |
In a steady case of a synchronous motion dß/dt = 0, and for the curvature we have
 | (7) |
Although reversals of bacterium velocity, as experimental observations show, is not a frequent event nevertheless it can happen. At each such reversal, the center of the circle along which a bacterium moves makes a finite displacement. Since these reversals can take place at random times, the reversals lead to a curious diffusion of the particle trajectory curvature center. Assuming that the reversal of the bacterium is a random Poisson process with a probability of reversal per unit time interval given by 
exp(–t), the derivation of the corresponding diffusion coefficient is straightforward. The result reads as
 | (8) |
= 1/ is a characteristic time of the reversal event. In the case = 0, Eq. 8 coincides with the expression given in Berg (19) for the diffusion coefficient of a bacterium such as Escherichia coli in two dimensions. Experimental observation of the described diffusion process is discussed in the next section. Confirmation of the frequency dependence of the diffusion coefficient given by Eq. 8 remains a challenge for future experimental work.
Another issue that should be discussed here is the influence of rotational Brownian fluctuations. The fluctuations of the bacterium orientations around the steady state in a rotating field allow us to determine the magnetic moment of the bacterium as shown in the next section. The Boltzmann principle for the distribution of the bacterium orientation angle around the steady state P reads
 |
is the energy of a bacterium in a rigid dipole approximation when dipole interactions fix the magnetic moment along the long axis of a bacterium. As a result, the mean quadratic angular fluctuation is
 | (9) |
 | (10) |
is the period of the orientational motion and Tf = 2/. We note that, during part of the trajectory, the magnetic moment and field move toward each other. At a given condition, the periods of orientational motion and a field are commensurate, and the trajectory of the bacterium is a closed curve. In the general case when the ratio of periods is an irrational number, a bacterium covers a definite region of space by an unclosed curve but contains parts of a trajectory with a negative curvature. Experimental observation of paths with negative curvature is described in the next section.
|
 |
is the unit vector along the direction of the magnetic moment of the magnetosomes, which is assumed to be the same for all magnetosomes, V is the total volume of the magnetosomes, and M is their saturation magnetization. As a result, the energy of the chain of magnetosomes in an external field reads
 | (11) |
M2 is an effective anisotropy constant of the chain of magnetosomes. The equation of the rotational motion of a bacterium in this case reads
 | (12) |
 | (13) |
is the operator for infinitesimal rotations. This model for the description of single domain particles of magnetic colloids was introduced in the late 1960s in Caroli and Pincus (20). It was found then that at 
the set of equations possess a steady solution when the axis of the particle (here the long axis of the bacterium) processes around the normal to the plane of a rotating field with its inclination 
given by the relation
 | (14) |
M/H. Since the magnetosomes of M. gryphiswaldense contain Fe3O4 with M = 500 G, then this ratio for an external field H = 10–20 Oe is large. Nevertheless, the solution of Eqs. 12 and 13 shows that at > c, a bacterium eventually should escape in the third dimension perpendicular to the plane of the rotating field. The characteristic features of this escape may be found by numerical solution of Eq. 12 with constraint Eq. 13, which makes the problem rather difficult. An efficient approach for the numerical solution of Eqs. 12 and 13 is proposed in Cbers, A., T. Cirulis, and O. Lietuvietis, unpublished. According to this report, the constraint Eq. 13 is put in the form
 | (15) |
are in the same plane. Equation 15 allows us to obtain a fourth-order polynomial equation for 
which is solved together with ODE for
 | (16) |
Numerical solution of Eq. 16 confirms that at 
the characteristic features of the bacterium motion in the asynchronous regime, shown in Fig. 2, may be observed as transients. This is illustrated by three-dimensional trajectories of the bacterium shown in Fig. 3 a (sin() = 0.85; s = 10) and Fig. 3 b (sin() = 0.9; s = 10)). In real experiments, the observation of three-dimensional trajectories of the bacterium is a challenge for future work. Some indications of the possible transition to the three-dimensional regime of the bacterium motion at rather large frequency of the rotating field are given in the next section.
|
|
EXPERIMENTAL METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONTHEORETICAL MODELSEXPERIMENTAL METHODSEXPERIMENTAL RESULTSCONCLUDING REMARKSACKNOWLEDGEMENTSREFERENCES |
|---|
). All chemicals of analytical grade were purchased from Sigma (St. Louis, MO). Cultivation of M. gryphiswaldense was carried out under microaerobic conditions in 15-ml tubes sealed by butyl rubber stoppers and saturated with a 1:99 oxygen/nitrogen mix (AGA, Riga, Latvia) before autoclaving. The culture was inoculated using 2-ml sterile syringes by injection through the stopper. Initial cell density after inoculation was 106 cells/ml, and the culture was incubated for 48 h at 25°C without agitation.
Samples for phase-contrast microscopy were taken by sterile 2-ml syringe and placed between two glass slides separated by 0.028-mm-thick double-stick tape with an aperture cut in the center. An AC magnetic field surrounding the field of view was created by four water-cooled coils with a variable power supply giving a rotating field of 0–20 Oe in a frequency range 0–5 Hz. The trajectories at room temperature were captured by a Zeiss Universal microscope (Carl Zeiss, Oberkochen, Germany) and a JAI CV-S3200 video camera (JAI, Copenhagen, Denmark) scanning at a rate of 25 frames per second. Tracking of trajectories from video frames was carried out by adapting the MatLab code (22) for our system.
Samples for electron microscopy were adsorbed on carbon-formvar copper grids and stained with 1 percent uranyl acetate aqueous solution (pH 4.5). The grids were examined by a JEM-100C electron microscope (JEOL, Tokyo, Japan) at an accelerating voltage of 80 kV.
|
EXPERIMENTAL RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONTHEORETICAL MODELSEXPERIMENTAL METHODSEXPERIMENTAL RESULTSCONCLUDING REMARKSACKNOWLEDGEMENTSREFERENCES |
|---|
|
In most samples, some of the bacteria are attached to the cover glass of microscope, and these provide the possibility to observe the back-and-forth orientational motion of a bacterium in a rotating field. An example of the experimentally determined dynamics of the long axis of a bacterium is given in Fig. 5 (the field H = 14 Oe with frequency 
= 3 Hz is rotating clockwise). The ratio of the forth-and-back motion times for data shown in Fig. 5 is 1.30. Fitting this ratio to the data in Fig. 1 c gives c/ = 0.205. A theoretical curve calculated for this value of c/ = 0.205 and frequency = 3 Hz is shown in Fig. 5 by dashed line. It shows deviation from the experimental curve. Since the mean quadratic deviation in time interval 3 s of orientation angle calculated according to the rotational diffusion coefficient of a bacterium 7.8 x 10–3 s–1 is 0.21, then, presumably, the thermal fluctuations are not responsible for disagreement of the experimental and theoretical data.
|
c/ can be used to calculate the drag coefficient of the bacterium and its rotational diffusion coefficient. For a bacterium with a magnetic moment of 1.3 x 10–12 emu, corresponding to 40 magnetosomes with magnetite cores of size 50 nm, the rotational drag coefficient of the bacterium with an attached end is 4.74 x 10–12 erg/s, a value larger than the rotational drag coefficient of 2.4 x 10–12 erg/s for an ellipsoid with long and short axes 4 µm and 0.5 µm, respectively, in water. This difference is reasonable, since the bacterium is attached to the cover glass and its rotational drag coefficient accounts for both the motion of its center of mass and its interaction with the glass surface. It should be remarked here that, during the event shown in Fig. 5, an untethered nonmotile cell was rotating with the frequency of a rotating field, as should be expected since the critical frequency of the untethered cell is larger due to the smaller rotational drag coefficient in comparison with a tethered cell. To determine how the motion of a bacterium depends on the parameters of the rotating field, single bacteria were followed as the frequency of the rotating field was slowly altered. Fig. 6 a shows the tracked trajectory of a bacterium for 40 s in a rotating 14 Oe field as its frequency changes from 0.2 to 0.367 Hz. The dependence of the trajectory curvature on the field rotation frequency is obtained after fitting the data by smoothing splines and is shown in Fig. 6 b. Despite their somewhat noisy character, the data clearly show an increase of curvature with an increase in the frequency of the rotating field. A linear fit of the data in Fig. 6 b for the velocity of the bacterium gives 15.6 µm s–1, a value that is close to the value 16.9 ± 5.3 µm s–1 obtained by directly tracking the displacement of the bacterium between frames. A histogram of the velocity distribution for the trajectory shown in Fig. 6 a is given in Fig. 6 c. Since its width corresponds to the error in determining the displacement of a bacterium between video frames, 4.5 µm s–1, there is no evidence in this case for a significant change in velocity of the bacterium during the time interval shown. Velocity reversal events are not observed for the trajectory shown in Fig. 6 a.
|
= 1.35t + const, which is close to the expected value of = 1.26t + const for a synchronous regime of motion. Extracting this regular variation of the orientation angle from the tracked orientations of the bacterium, we obtain the data shown in Fig. 7. The error of orientation angle determined by MatLab routine (22) is estimated as 
d/L, where d is error in bacterium width 1 pixel but L is the bacterium length of 20 pixels. An estimate of the mean quadratic deviation 
(
)2
for the data shown in Fig. 7 gives 0.14. This value is considerably larger than the error estimated above. The ()2 value agrees with the expectation according to Eq. 8 if m cos(ß) = 2.04 x 10–14 emu. For a reasonable value of the magnetic moment of a bacterium m = 10–13 emu, this result gives cos(ß) = 0.2. Since the direction of the magnetic field is not registered in the present experiments, we were not able to determine the value of the angle ß directly.
|
We should remark here that, at present, different methods for the measurement of the magnetic moments of the magnetotactic bacteria have been proposed. Due to the finite orientational time of a bacterium at a field polarity change, the bacterium makes a characteristic U-shape trajectory (14,23). Its parameters allow one to determine the ratio of the magnetic moment of a bacterium to its rotational drag coefficient. By this method the magnetic moment m = 6.1 x 10–13 emu of the M. magnetotacticum (23) and magnetotactic cocci 2.4–54 x 10–12 (14) have been determined. We should draw the reader's attention to the interesting and not frequently referenced article by van Kampen (24), in which the characteristic time of U-turn is calculated by a singular perturbation theory.
In Kalmijn (25), the magnetic field strength dependence of the migration velocity along the field lines caused by orientational thermal fluctuations of a bacterium is used for the measurement of its magnetic moment. The values of m found for freshwater magnetic bacteria are 6–7 x 10–13 emu.
Among the other methods used to determine the magnetic moment of a single bacterium, magnetic atomic force microscopy should be mentioned (26)—which, for vibroid cells of the strain MV-1 (27), gives m = 4 x 10–13 emu.
Several methods use the ensemble of magnetic bacteria to extract the properties of a single bacterium, light scattering (28), and superconducting quantum interference device microscopy (29). The latter method is based on measurements of magnetic field fluctuations generated by an ensemble of magnetic bacteria and the dependence of the orientation relaxation time of a bacterium on the magnetic field strength, an effect that is well known from studies of magnetic colloids (for general review see (30)). This method determines a value of magnetic moment of M. magnetotacticum of 3 x 10–13 emu (29). This value is close to that determined earlier for A. magnetotacticum 2.2–2.7 x 10–13 by light scattering (28).
We are not aware of magnetic moment measurements of M. gryphiswaldense, except indirect estimates based on the number of magnetosomes in the cell, which may be as large as 60 (31). A similar estimate for M. magnetotacticum m = 10–12 emu is given in Lee et al. (32). Application of the method described here for the measurement of the magnetic moments of an individual M. gryphiswaldense bacterium will be considered in future work.
As illustrated above, an essential feature of the asynchronous regime is the existence of paths with a negative curvature. Fig. 8 shows an observation of such an event during a 4.5-s-long trajectory in a rotating field H = 10 Oe with frequency = 0.8 Hz. This event allows us to estimate the upper bound of the magnetic moment of the bacterium, if for the rotational drag coefficient we take its value for an ellipsoid with long and short semiaxes 4 µm and 0.5 µm, respectively, to be 2.4 x 10–12 erg/s. We get m < 1.2 x 10–12 emu, which is in reasonable agreement with data referenced above.
|
= 0.65 Hz). Tracking the orientation of the long axis of the bacterium and its position allows us to demonstrate that the bacterium changes direction of its motion at particular times corresponding to abrupt switches of the trajectory curvature center. Indeed, although at this moment both the long axis and magnetic moment aligned along it continue to follow the direction of magnetic field, the trajectory forms cusps. This finding may be explained only by an abrupt change in the direction of the velocity of the bacterium with respect to its magnetic moment, i.e., a bacterium with two flagellar motors reverses. It is essential to remark that after reversal, the speed of the bacterium changes significantly as seen in Fig. 10 where the distribution of bacterium velocities is shown for several reversal events. At each reversal the velocity switches between lower and higher mean values that can also be detected by the change in the curvature of the trajectory at each reversal.
|
|
). Two different models, to explain the bacterial band formation in an oxygen gradient and its behavior at a change of polarity of the field, have been proposed. According to the first one, the cocci switch their rotary motors at two critical oxygen concentrations, whereas the second model supposes that spirilla use the oxygen gradient sensor to switch the engine with some delay. A number of motional effects were observed for other magnetotactic prokaryotes caused by oxygen gradients, and/or redox potential (13,33,34). Particularly, very specific reversals of multicell prokaryotic organism, MMP, with significant acceleration during backward movement were observed when it reached an extreme oxygen concentration (33). All these circumstances are characterized by the nonuniform environment, e.g., gradients of oxygen/redox potential etc. Since in our experimental conditions bacteria were placed in a uniform environment we believe that the reversals are not caused by any significant gradient of the putative signal. Therefore, one may propose that random back-and-forth orientational "investigational" motions may be helpful for these bacteria in certain physiological conditions to look for additional niches for successful colonization.
The mechanisms of the reversal movement are unclear. Some ways to solve the problem one can find in Berg (35), where polymorphic transformations of flagella were studied for E. coli. It was shown that at the change of the direction of rotation the flagella underwent transformation from left-handed helices rotating counterclockwise (normal waveform) to right-handed helices rotating clockwise. Since the direction of the propelling force at this transformation remains the same, this mechanism cannot be responsible for the observed velocity variation at the reversal. However, the fact that flagella can rotate in opposite directions motivates the analogous study of magnetotactic bacteria.
Nevertheless, since the change of the direction and value of velocity at a reversal of the rotary motor is observed for monotrichous bacteria (36) we may assume that the variation of speed at reversal could be connected with some change of flagellar helix pitch at the change of the rotation direction. This possibility is suggested by the results of Kim and Powers (37) where the mechanical deformation of a helix subjected to rotational flow is considered. Since the sign of helix deformation in rotational flow depends on its handedness (37), this hypothesis at least qualitatively corresponds to observations. Nevertheless, we should mention unpublished data by H. Berg referenced in Kim and Powers (37), which show that this effect is not sufficient to explain the experimental data. Thus, we conclude that the observed change in speed at reversals of magnetotactic bacterium is open for further theoretical and experimental investigations. For this purpose, the known methods of labeling flagella by fluorescent dyes (35) may be useful.
Velocity reversals of the bacterium lead to a diffusion process of the curvature center around which a bacterium moves in a rotating field. This is illustrated in Fig. 11 by a 200-s-long run of a bacterium in a rotating field with a strength H = 10 Oe and frequency slowly increasing from 0.47 to 0.62 Hz. To diminish the amount of the data, only each fifth frame has been tracked in the case of the trajectory shown in Fig. 11. We explicitly see that a reversal of the bacterium motor leads to the diffusive wandering of the centers of the circles.
|
|
|
CONCLUDING REMARKS |
|---|
|
TOPABSTRACTINTRODUCTIONTHEORETICAL MODELSEXPERIMENTAL METHODSEXPERIMENTAL RESULTSCONCLUDING REMARKSACKNOWLEDGEMENTSREFERENCES |
|---|
It is interesting to remark that trajectories similar to those of magnetotactic bacteria have also been observed in the case of nonmagnetic Listeria monocytogenes (38), due to the torque acting on the bacterium from the propelling actin comet (39). Quantitative studies of these trajectories may reveal those mechanisms of torque generation that are not yet clear.
|
ACKNOWLEDGEMENTS |
|---|
|
TOPABSTRACTINTRODUCTIONTHEORETICAL MODELSEXPERIMENTAL METHODSEXPERIMENTAL RESULTSCONCLUDING REMARKSACKNOWLEDGEMENTSREFERENCES |
|---|
rs Grns and Prof. Pauls Pumpns from Latvian BMC for bringing people together and ASLA Biotech for assisting in microbiology. This work was supported by the University of Latvia grant No. 2006/1-229701, Fogarty grant No. R03 TW-006954-01, National Science Foundation grant No. DMR-0079909, and Fulbright grant No. 69429947.
|
FOOTNOTES |
|---|
Submitted on February 23, 2007; accepted for publication April 17, 2007.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONTHEORETICAL MODELSEXPERIMENTAL METHODSEXPERIMENTAL RESULTSCONCLUDING REMARKSACKNOWLEDGEMENTSREFERENCES |
|---|
2. Blakemore, R. P. 1982. Magnetotactic bacteria. Annu. Rev. Microbiol. 36:217–238.[CrossRef][Medline]
3. Komeili, A., Z. Li, D. K. Newman, and G. J. Jensen. 2006. Magnetosomes are cell membrane invaginations organized by the actin-like protein MAMK. Science. 311:242–245.
4. Scheffel, A., M. Gruska, D. Faivre, A. Linaroudis, J. M. Plitzko, and D. Schuler. 2006. An acidic protein aligns magnetosomes along a filamentous structure in magnetotactic bacteria. Nature. 440:110–114.[CrossRef][Medline]
5. Pradel, N., C. L. Santini, A. Bernadac, Y. Fukumori, and L. F. Wu. 2006. Biogenesis of actin-like bacterial cytoskeletal filaments destined for positioning prokaryotic magnetic organelles. Proc. Natl. Acad. Sci. USA. 103:17485–17489.
6. Grunberg, K., C. Wawer, B. R. Tebo, and D. Schuler. 2001. A large gene cluster encoding several magnetosome proteins is conserved in different species of magnetotactic bacteria. Appl. Environ. Microbiol. 67:4573–4582.
7. Ullrich, S., M. Kube, S. Schubbe, R. Reinhardt, and D. Schuler. 2005. A hypervariable 130-kilobase genomic region of Magnetospirillum gryphiswaldense comprises a magnetosome island, which undergoes frequent rearrangements during stationary growth. J. Bacteriol. 187:7176–7184.
8. Heyen, U., and D. Schuler. 2003. Growth and magnetosome formation by microaerophilic Magnetospirillum strains in an oxygen-controlled fermentor. Appl. Microbiol. Biotechnol. 61:536–544.[CrossRef][Medline]
9. Lang, C., D. Schuler, and D. Faivre. 2007. Synthesis of magnetite nanoparticles for bio- and nanotechnology: genetic engineering and biomimetics of bacterial magnetosomes. Macromol. Biosci. 7:144–151.[CrossRef][Medline]
10. Schultheiss, D., M. Kube, and D. Schuler. 2004. Inactivation of the flagellin gene FLAA in Magnetospirillum gryphiswaldense results in nonmagnetotactic mutants lacking flagellar filaments. Appl. Environ. Microbiol. 70:3624–3631.
11. Spormann, A. M., and R. S. Wolfe. 1984. Chemotactic, magnetotactic and tactile behavior in a magnetic spirillum. FEMS Microbiol. Lett. 22:171–177.[CrossRef]
12. Frankel, R. B., D. A. Bazylinski, M. S. Johnson, and B. L. Taylor. 1997. Magneto-aerotaxis in marine coccoid bacteria. Biophys. J. 73:994–1000.
13. Smith, M. J., P. E. Sheehan, L. L. Perry, K. O'Connor, L. N. Csonka, B. M. Applegate, and L. J. Whitman. 2006. Quantifying the magnetic advantage in magnetotaxis. Biophys. J. 91:1098–1107.
14. Esquivel, D. M. S., and H. G. P. Lins de Barros. 1986. Motion of magnetotactic microorganisms. J. Exp. Biol. 121:153–163.
15. Steinberger, B., N. Petersen, H. Petermann, and D. G. Weiss. 1994. Movement of magnetic bacteria in time-varying magnetic fields. J. Fluid Mech. 273:189–211.[CrossRef]
16. Winklhofer, M., L. G. Abracado, A. F. Davila, C. N. Keim, and H. G. Lins de Barros. 2007. Magnetic optimization in a multicellular magnetotactic organism. Biophys. J. 92:661–670.
17. Cbers, A., and M. Ozols. 2006. Dynamics of an active magnetic particle in a rotating magnetic field. Phys. Rev. E. 73:021505.[CrossRef]
18. McNaughton, B. H., K. A. Kehbein, J. N. Anker, and R. Kopelman. 2006. Sudden breakdown in linear response of a rotationally driven magnetic microparticle and application to physical and chemical microsensing. J. Phys. Chem. B. 110:18958–18964.[Medline]
19. Berg, H. C. 1993. Random Walks in Biology. Princeton University Press.
20. Caroli, C., and P. Pincus. 1969. Response of an isolated magnetic grain suspended in a liquid to a rotating field. Zeitschrift für Physik B Cond. Matter. 9:311–319.
21. Reference deleted in proof.
22. Crocker, J. C., and D. G. Grier. 1996. Methods of digital video microscopy for colloidal studies. J. Coll. Int. Sci. 179:298–310.[CrossRef]
23. Bahaj, A. S., P. A. B. James, and F. D. Moeschler. 1996. An alternative method for the estimation of the magnetic moment of non-spherical magnetotactic bacteria. IEEE Trans. Magn. 32:5133–5135.[CrossRef]
24. Van Kampen, N. G. 1995. The turning of magnetotactic bacteria. J. Stat. Phys. 80:23–33.[CrossRef]
25. Kalmijn, A. J. 1981. Biophysics of geomagnetic field detection. IEEE Trans. Magn. 17:1113–1124.[CrossRef]
26. Proksch, R. B., T. E. Schaffer, B. M. Moskowitz, E. D. Dahlberg, D. A. Bazylinski, and R. B. Frankel. 1995. Magnetic force microscopy of the submicron magnetic assembly in a magnetotactic bacterium. Appl. Phys. Lett. 66:2582–2584.[CrossRef]
27. Bazylinski, D. A., R. B. Frankel, and H. W. Jannach. 1988. Anaerobic magnetite production by a marine, magnetotactic bacterium. Nature. 334:518–519.[CrossRef]
28. Rosenblatt, C., F. F. Torres de Araujo, and R. B. Frankel. 1982. Light scattering determination of magnetic moments of magnetotactic bacteria. J. Appl. Phys. 53:2727–2729.[CrossRef]
29. Chemla, Y. R., H. L. Grossman, T. S. Lee, J. Clarke, M. Adamkiewicz, and B. B. Buchnan. 1999. A new study of bacterial motion: superconducting quantum interference device microscopy of magnetotactic bacteria. Biophys. J. 76:3323–3330.
30. Blums, E., A. Cbers, and M. M. Maiorov. 1997. Magnetic Fluids. de Gruyter, New York.
31. Schuler, D. 2002. The biomineralization of magnetosomes in Magnetospirillum gryphiswaldense. Int. Microbiol. 5:209–214.[CrossRef][Medline]
32. Lee, H., A. M. Purdon, V. Chu, and R. M. Westervelt. 2004. Controlled assembly of magnetic nanoparticles from magnetotactic bacteria using microelectromagnets arrays. Nano Lett. 4:995–998.[CrossRef]
33. Greenberg, M., K. Canter, I. Mahler, and A. Tornheim. 2005. Observation of magnetoreceptive behavior in a multicellular magnetotactic prokaryote in higher than geomagnetic fields. Biophys. J. 88:1496–1499.
34. Simmons, S. L., D. A. Bazylinski, and K. J. Edwards. 2006. South-seeking magnetotactic bacteria in the northern hemisphere. Science. 311:371–374.
35. Berg, H. C. 2003. E. coli in Motion. Springer, New York.
36. Taylor, B. L., and D. E. Koshland. 1974. Reversal of flagellar rotation in monotrichous and peritrichous bacteria: generation of changes in direction. J. Bacteriol. 119:640–642.
37. Kim, M., and T. R. Powers. 2005. Deformation of a helical filament by flow and electric or magnetic fields. Phys. Rev. E. 71:021914.[CrossRef]
38. Robbins, J. R., and J. A. Theriot. 2003. Listeria monocytogenes rotates around its long axis during actin-based motility. Curr. Biol. 13:R754–R756.[CrossRef][Medline]
39. Zeile, W. L., F. Zhang, R. B. Dickinson, and D. L. Purich. 2005. Listeria's right-handed helical rocket-tail trajectories: mechanistic implications for force generation in actin-based motility. Cell Motil. Cytoskeleton. 60:121–128.[CrossRef][Medline]
This article has been cited by other articles:
 |
|
 |
|
  M. Tanaka, Y. Nakata, T. Mori, Y. Okamura, H. Miyasaka, H. Takeyama, and T. Matsunaga Development of a Cell Surface Display System in a Magnetotactic Bacterium, "Magnetospirillum magneticum" AMB-1 Appl. Envir. Microbiol., June 1, 2008; 74(11): 3342 - 3348. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||
is the angle between the long axis of the bacterium and the horizontal axis of the image. The magnetic field rotates clockwise. Theoretical dependence of 
