Journal of Magnetism and Magnetic Materials 67 (1987) 291-294

North-Holland. Amsterdam 291

ZONED MIGRATION OF MAGNETOTACTIC BACTERIA

M.J. CARLILE

Department of Pure and Applied Biologv, Imperial College at Silwood Park, Ascot, Berks SLS 7PY, England

A.W.L. DUDENEY, B.K. HEBENSTREIT * and R.H. HEEREMA **

Department of Mineral Resources Engineering, Imperial College of Science and Technology, London S W7 2BP, England

Received 23 February 1987

Dense populations of magnetotactic bacteria, migrating under the influence of a magnetic field 10 to 100 times that of the

earth, rapidly break into bands that move at about the swimming speed of individual bacteria. These dense bacterial

populations have some of the properties of ferromagnetic fluids, and the formation of bands, for which a tentative explanation

is suggested, may be analogous to instabilities in such fluids.

1. Introduction

Magnetotactic bacteria [1,2] in the Northern Hemisphere swim towards the North Magnetic

Pole. Because of the vertical component of the

geomagnetic field this takes them to their pre-

ferred habitat, the surface layers of sediments at the bottom of natural bodies of water. Research

on magnetotactic bacteria has been hampered by

difficulties in their culture, only one species hav-

ing been grown in pure culture [3], and that to

yields low compared with those usual for many bacteria. We have tried to circumvent this prob-

lem by the magnetic separation of bacteria from

sediments. A magnetic field is applied to force the bacteria to migrate from the sediments and to pass through tubes of decreasing bore to concentrate

them. Our observations on the migrations of dense suspensions of bacteria at field strengths consider-

ably greater than that of the local geomagnetic

field have revealed a periodic effect - a stream of

* Present address: Institut fir Geowissenschaft, Montan-

universitlt Leoben, A8600 Leoben, Austria.

* * Present address: Department of Mineral and Petroleum Engineering, Delft University of Technology, Delft, The

Netherlands.

bacteria will break into zones which move at about

the swimming speed of individual bacteria.

2. Results and discussion

Mud from the pond in the Japanese Garden at the Imperial College Field Station at Silwood Park

was screened to remove coarse debris and placed in plastic trays in the laboratory, 3 cm of mud

being covered with 3 cm of pond water. The trays were covered with sheets of glass and evaporation

losses replaced with distilled water. Magnetotactic

bacteria became established in the surface sedi-

ments within a few weeks and persisted for many

months. The dominant form through most of this period had, as demonstrated by electron mi-

croscopy, the size, shape and flagellum and mag-

netite particle arrangement of the magnetococcus

described by Moench and Konetzka [4]. Our ex-

periments, unless otherwise stated, were con- ducted with mud in which there were few magne- totactic bacteria other than this form. Surface mud and detritus were placed in the apparatus shown in fig. 1. An applied magnetic field consid- erably greater (0.5-5 mT) than the local geomag- netic field (0.05 mT) was then used to direct

0304-8853/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

292

b / a

C d

Fig. 1. Apparatus for the concentration of magnetotactic bacteria. The reservoir a (total length 1X cm) is made from a 25 ml

volumetric pipette and has a rounded end at c. It is filled with surface mud (stippled) with a Pasteur pipette, and pond water with a

large suction bulb attached at b. The latter is then replaced by a small suction bulb and adjustahle clamp. A long (23 cm) Pasteur

pipette is cut to 15 cm to shorten the wider portion. This pipette (d, only wide end shown) is filled with pond water and attached at c

with plastic tubing. The end of the reservoir is slightly smaller than that of the Pasteur pipette and projects slightly into the opening

of the Pasteur pipette to avoid any discontinuity in which migrating bacterta could be trapped. Air buhhles that could obstruct

migration are expelled. The Pasteur pipette and tubing can he replaced by a photometric cuvette (internal dimensions, 33 x 10 x 1.

mm”) with neck, filled with pond water and in contact with the reservoir in such a way as to maintain liquid continurty. I’he

apparatus is held by clamps between two 20 cm diameter Helmholr coils. along the axis through their centres. The length of the

apparatus means that the distance between the coils is several times greater than half their radtus. resulting in a field of non-uniform

intensity. A Bell 615 gaussmeter was used to determine the field strength along the axis between the coils for a range of cotI

separations and current strengths. Alignment of the coil axis parallel to the horizontal component of the earth’s field ensured that in

the horizontal plane the field direction close to the axis, as determined with a compass. was parallel to it. Cuvettes and Pasteur

pipettes were cleaned frequently with chromic acid and rendered hydrophobic with dimethyl dichlorosilane. This is essential to avoid

adherence of bacteria to glass and loss of motility. The direction of the applied field is such as to cause bacteria to mtgrate from the

mud towards the Pasteur pipette or cuvette.

magnetotactic bacteria from the mud into a Pas-

teur pipette or cuvette. If magnetotactic bacteria are abundant, they can be observed after a few minutes with the naked eye, moving as a continu-

ous stream along the bottom of the exit tube of

the reservoir. This stream soon breaks into numer-

ous fine bands that move at about the swimming

speed of individual bacteria and are oriented per-

pendicular to the direction of movement. If the

bacteria are not very abundant in the mud, they

may only be observable when they reach the nar-

row part of the Pasteur pipette and cell density

increases. Microscopy shows that migrating bacteria occur between bands but at a lower den- sity than within them. Bands can merge with each other (figs. 2a, b), split into two or more bands,

disperse, or appear in the space between bands. The tendency is for the number of bands to di- minish, resulting in a smaller number of promi- nent bands (figs. 2aad). Band form can vary (figs. 2a-g). Bands disperse in 2-3 s when the magnetic field is switched off, but its reapplication leads to renewed banding in about 10 s (fig. 2f). Reversal of the applied field leads to the migration of bands in the opposite direction.

Individual bands persist long enough for their migration speed to be determined. The time taken for bands in a Pasteur pipette to cross a 45 mm

microscope field was determined for magnetic field strengths of 0.9. 2.2, 2.9 and 3.5 mT. The average

times obtained indicated mean migration speeds

of 130, 125, 127 and 123 pm s ‘. These and other observations establish that migration speeds are similar for a wide range of magnetic field strengths

from a few times that of the geomagnetic field upwards. The rate of band migration in pipettes

and cuvettes was also determined from series of

photographs taken with an automatic timer and

by visual observation of bands passing points on

an adjacent scale. Videotapes were made of band

migration in pipettes and cuvettes- with adjacent

scale and speeds obtained by replaying the video- tape and noting the time taken for bands to pass

successive points on the scale. Twenty four de- terminations of band migration speed were made

with the above methods. Twenty two of these were

in the range 108 to 143 pm SC’ and two were much lower. As the latter may represent popula- tions of reduced vigour, they were omitted in the calculation of a mean band migration speed of 124 pm SC’. The speeds of individual bacteria were measured by placing a drop of detritus and pond water on a slide, observing with a microscope with eyepiece graticule, and timing the cells over a known distance on application of a magnetic field. The speeds obtained were similar to the mean

M.J. Carlile et al. / Zoned mrgrafion of magnefotactic bacteria 293

migration speeds of bands and considerably higher than those reported by Moench and Konetzka [4].

Zoning was initiated in a cuvette to determine a

cell density that permits zone formation. Bacteria were directed by an applied magnetic field from

the reservoir into a cuvette which was then re-

moved, stoppered and shaken. The cuvette was replaced in the field, zoning observed, and the cuvette shaken again to give a uniform suspension.

This was mixed with an equal volume of iodine solution to kill and stain the bacteria. A haemo-

cytometer count showed that zoning had occurred

in a suspension with an initial cell density of

about lo8 cells ml-‘. The limits of the range of

cell densities at which zoning can be observed remain to be determined.

Sometimes the magnetotactic bacteria in a tray

consisted almost entirely of a smaller coccus (di- ameter ca. 1 pm) which lacked the prominent refractile granule present in the usual larger mag- netococcus. Zone formation was obtained with

this coccus also, with migration speeds of cells and bands being similar to or higher than those of the

larger coccus. The smaller coccus was used in

experiments to determine the lowest field strength at which zoning could occur. Unambiguous zone

Fig. 2. Magnetotactic zoning in Pasteur pipettes. The length of

pipette shown is 1 cm and migration is towards the right in the

direction of the pipette tip. Photography was from the side,

with the camera focussed on the mid-plane of the pipette. The

horizontal stripes are reflections from the pipette surface. (a),

(b) there are about a dozen prominent vertical bands of

bacteria with numerous fainter bands just perceptible. Move-

ment of bands to the right has occurred in the 2 s interval

between photographs, and at A fusion between two bands is

taking place; (c), (d) photographs taken several minutes later at

the same point, with a 10 s interval between them. Bacteria,

having travelled for a longer time, form fewer, more sharply

defined large bands although fainter bands still occur between

them. The distance moved in 10 s is clear from the position of

the pair of bands at B; (e) following a period in which the

magnetic field was reversed, it was reapplied to cause renewed

migration towards the pipette tip. Since the field is not strictly

uniform in direction throughout the pipette, the vertical com- ponent of the earht’s field has finally limited the bacteria to the

lower side of the pipette. As well as the four prominent bands

there are many smaller bands close to the bottom of the

pipette; (f) a continuous layer of bacteria breaking into bands

15 s after the application of a magnetic field; (g) a different but common zoning pattern.

formation was seen at 0.5 mT, about ten times the

geomagnetic field strength.

An adequate theory of magnetotactic zoning will require studies on band formation with uni-

form magnetic fields, known cell numbers and

accurate determination of the velocity distribution

of individual bacteria. A tentative explanation of

the way in which zones may be formed can how-

ever be made. In the earth’s magnetic field the

orientation of magnetotactic bacteria, although

adequate for taxis, is imprecise, but in a field with

a strength several times that of the earth will be

nearly perfect [1,2]. A population of magnetotactic bacteria in such a field will be a population of

precisely aligned dipoles. If the cell density is

high, then many individuals will be close enough to each other for magnetic interaction to occur. Chance groupings of bacteria could arise in which interaction results in the members of a group

swimming at the same speed. Groups could grow

by attracting near by cells and by merging with

other groups. Although the sites of group ini- tiation would be random, for any one experiment

their numbers per unit volume through a long

portion of a Pasteur pipette should be similar.

Continuing merging of groups could result in a

few seconds in bands stretching across the pipette

and approximately evenly spaced. Switching off the applied field will give poorer orientation of

bacteria and reduced attraction. The less precise orientation will result in intersecting paths and

near collisions between cells. Since bacteria, like

other cells, have a net negative charge at the cell surface, this will lead to electrostatic repulsion,

further disorientation and rapid dispersal of bands. A dense suspension of magnetotactic bacteria

can be regarded as a living ferromagnetic liquid.

The break up of a continuous stream of magneto-

tactic bacteria into zones has some resemblance to

the instabilities that occur in ferromagnetic liquids

[5,6]. The study of cell interactions in dense popu-

lations of magnetotactic bacteria could provide models for the behaviour of such magnetic materi-

als.

Acknowledgements

We thank Heerema Engineering Services (UK) Ltd. for financial support, Professor D.W. Rib- bons and Dr. David Leak for facilities at the

Imperial College Centre for Biotechnology, and Dr. H.M. Flower, Dept. of Metallurgy and

Materials Science, Imperial College, for electron

microscopy.

Note added in proof

Following submission of our manuscript an independent discovery of zoning by magnetotactic

bacteria has been reported: A.M. Spormann.

FEMS Microbial. Ecol. 45 (1987) 37.

References

[I] R.P. Blakemore. Ann. Rev. Microbial. 36 (19X2) 217.

[2] R.B. Frankel. Ann. Rev. Biophys. Bioeng. 13 (7984) 85.

[3] R.P. Blakemore, D. Maratea and R.S.J. Wolfe. J. Bact. 140

(1979) 720.

[4] T.T. Moench and W.A. Konetzka. Arch. Microhiol. 119

(197X) 203.

(51 R.E. Rosensweig, Scientific American 24, pt. 4 (19X2) 124.

[6] B. Berkovsky and V. Bashtovoi. IEEE Trans. Magn. MAG-

16 (1980) 288.