Chuanfang Chen

Construction and operation of a microrobot based on magnetotactic bacteria in a microfluidic chip

Magnetotactic bacteria (MTB) are capable of swimming along magnetic field lines. This unique feature renders them suitable in the development of magnetic-guided, auto-propelled microrobots to serve in target molecule separation and detection, drug delivery, or target cell screening in a microfluidic chip. The biotechnology to couple these bacteria with functional loads to form microrobots is the critical point in its application. Although an immunoreaction approach to attach functional loads to intact MTB was suggested, details on its realization were hardly mentioned. In the current paper, MTB-microrobots were constructed by attaching 2 μm diameter microbeads to marine magnetotactic ovoid MO-1 cells through immunoreactions. These microrobots were controlled using a special control and tracking system. Experimental results prove that the attachment efficiency can be improved to ∼30% via an immunoreaction. The motility of the bacteria attached with different number of loads was also assessed. The results show that MTB can transport one load at a velocity of ∼21 μm/s and still move and survive for over 30 min. The control and tracking system is fully capable of directing and monitoring the movement of the MTB-microrobots. The rotating magnetic fields can stop the microrobots by trapping them as they swim within a circular field with a controllable size. The system has potential use in chemical analyses and medical diagnoses using biochips as well as in nano/microscale transport.

Configuration of redox gradient determines magnetotactic polarity of the marine bacteria MO-1

Magnetotactic bacteria are capable of aligning and swimming along the geomagnetic field lines; such a behaviour is called magnetotaxis. Previous studies reported that bacteria in the northern hemisphere migrate preferentially towards the North Pole of the Earths magnetic field (north-seeking, NS), whereas those in the southern hemisphere swim towards the South Pole (south-seeking, SS). The orientated swimming is thought to guide bacteria migrating downward to the favourable microaerobic or anaerobic regions in stratified water column or sediments. Recent identification of SS populations in northern hemisphere challenged the model of the adaptive value of magnetotaxis. To seek explanation for the apparent discrepancy, we analysed magnetotaxis polarity of axenic cultures under simulated growth conditions in hypomagnetic, northern-hemisphere-like or southern-hemisphere-like magnetic fields. We found that NS and SS cells could obviously coexist in hypomagnetic field and even, when the oxidation-reduction gradient configuration is suitable, in the geomagnetic field. These results reveal the selectivity of the redox gradient configuration on magnetotactic polarity of the cells and reconcile the discrepancy of the early reports.

Removal of the local geomagnetic field affects reproductive growth in Arabidopsis

The influence of the geomagnetic field-removed environment on Arabidopsis growth was investigated by cultivation of the plants in a near-null magnetic field and local geomagnetic field (45 µT) for the whole growth period under laboratory conditions. The biomass accumulation of plants in the near-null magnetic field was significantly suppressed at the time when plants were switching from vegetative growth to reproductive growth compared with that of plants grown in the local geomagnetic field, which was caused by a delay in the flowering of plants in the near-null magnetic field. At the early or later growth stage, no significant difference was shown in the biomass accumulation between the plants in the near-null magnetic field and local geomagnetic field. The average number of siliques and the production of seeds per plant in the near-null magnetic field was significantly lower by about 22% and 19%, respectively, than those of control plants. These resulted in a significant reduction of about 20% in the harvest index of plants in the near-null magnetic field compared with that of the controls. These results suggest that the removal of the local geomagnetic field negatively affects the reproductive growth of Arabidopsis, which thus affects the yield and harvest index.

Construction of a microrobot system using magnetotactic bacteria for the separation of Staphylococcus aureus

Magnetotactic bacteria exhibit superiority over other bacteria in fabricating microrobots because of their high motility and convenient controllability. In this study, a microrobot system is constructed using magnetotactic bacteria MO-1 and applied in pathogenic separation. The feasibility of this approach is demonstrated using Staphylococcus aureus. The MO-1 magnetotactic bacterial microrobots are fabricated by binding magnetotactic bacteria MO-1 with their rabbit anti-MO-1 polyclonal antibodies. The efficient binding of MO-1 magnetotactic bacterial microrobots to Staphylococcus aureus is corroborated by phase contrast microscopic and transmission electron microscopic analyses. Further, a microfluidic chip is designed and produced, and the MO-1 microrobots are magnetically guided toward a sample pool in the chip. In the sample pool, Staphylococcus aureus samples are loaded on the microrobots and then carried away to a detection pool in the chip, suggesting the microrobots have successfully carried and separated pathogen. This study is the first to demonstrate bacterial microrobots carrying pathogens and more importantly, it reflects the great potential of using magnetotactic bacteria to develop magnetic-guided, auto-propelled microrobots for pathogen isolation.

Dynamic Model and Motion Mechanism of Magnetotactic Bacteria with Two Lateral Flagellar Bundles

Magnetotactic Bacteria (MTB) propel themselves by rotating their flagella and swim along the magnetic field lines. To analyze the motion of MTB, MTB magneto-ovoid strain MO-1 cells, each with two bundles of flagella, were taken as research object. The six-degrees-of-freedom (6-DoF) dynamic model of MO-1 was established based on the Newton-Euler dynamic equations. In particular, the interaction between the flagellum and fluid was considered by the resistive force theory. The simulated motion trajectory of MTB was found to consist of two kinds of helices: small helices resulting from the imbalance of force due to flagellar rotation, and large helices arising from the different directions of the rotation axis of the cell body and the propulsion axis of the flagellum. The motion behaviours of MTB in various magnetic fields were studied, and the simulation results agree well with the experiment results. In addition, the rotation frequency of the flagella was estimated at 1100 Hz, which is consistent with the average rotation rate for Na+-driven flagellar motors. The included angle of the magnetosome chain was predicted at 40° that is located within 20° to 60° range of the observed results. The results indicate the correctness of the dynamic model, which may aid research on the operation and control of MTB-propelled micro-actuators. Meanwhile, the motion behaviours of MTB may inspire the development of micro-robots with new driving mechanisms.

Killing of Staphylococcus aureus via Magnetic Hyperthermia Mediated by Magnetotactic Bacteria

ABSTRACT Staphylococcus aureus is a common hospital and household pathogen. Given the emergence of antibiotic-resistant derivatives of this pathogen resulting from the use of antibiotics as general treatment, development of alternative therapeutic strategies is urgently needed. Here, we assess the feasibility of killing S. aureus cells in vitro and in vivo through magnetic hyperthermia mediated by magnetotactic bacteria that possess magnetic nanocrystals and demonstrate magnetically steered swimming. The S. aureus suspension was added to magnetotactic MO-1 bacteria either directly or after coating with anti-MO-1 polyclonal antibodies. The suspensions were then subjected to an alternating magnetic field (AMF) for 1 h. S. aureus viability was subsequently assessed through conventional plate counting and flow cytometry. We found that approximately 30% of the S. aureus cells mixed with uncoated MO-1 cells were killed after AMF treatment. Moreover, attachment between the magnetotactic bacteria and S. aureus increased the killing efficiency of hyperthermia to more than 50%. Using mouse models, we demonstrated that magnetic hyperthermia mediated by antibody-coated magnetotactic MO-1 bacteria significantly improved wound healing. These results collectively demonstrated the effective eradication of S. aureus both in vitro and in vivo, indicating the potential of magnetotactic bacterium-mediated magnetic hyperthermia as a treatment for S. aureus-induced skin or wound infections.

Bacterial Magnetic Nanoparticles as Peroxidase Mimetics and Application in Immunoassay

Although progress in nanosynthesis has succeeded in making nanoscale particles from iron oxide, the research about natural magnetic nanoparticles, magnetosomes, is still a current interest because of their intrinsic magnetic features, nano‐features, membrane‐enclosed features and genetic control of size and morphology properties. In this study, we investigated magnetosomes’ intrinsic peroxidase‐like activity similar to that found in artificial magnetic nanoparticles. We characterized the catalytic activity by varying the method of extraction and storage, the pH value, the temperature and the H2O2 concentration. Based on these finding, we developed a simplified immunoassay approach to use magnetosomes as a peroxidase mimic catalyst and a magnetic separator as well.

Enhancement of the hydrolysis activity of F0F1‐ATPases using 60 Hz magnetic fields

The effects of extremely low frequency (ELF) magnetic fields on membrane F(0)F(1)-ATPase activity have been studied. When the F(0)F(1)-ATPase was exposed to 60 Hz magnetic fields of different magnetic intensities, 0.3 and 0.5 mT magnetic fields enhanced the hydrolysis activity, whereas 0.1 mT exposure caused no significant changes. Even if the F(0)F(1)-ATPase was inhibited by N,N-dicyclohexylcarbodiimide, its hydrolysis activity was enhanced by a 0.5 mT 60 Hz magnetic field. Moreover, when the chromatophores which were labeled with F-DHPE were exposed to a 0.5 mT, 60 Hz magnetic field, it was found that the pH of the outer membrane of the chromatophore was unchanged, which suggested that the magnetic fields used in this work did not affect the activity of F0. Taken together, our results show that the effects of magnetic fields on the hydrolysis activity of the membrane F(0)F(1)-ATPases were dependent on magnetic intensity and the threshold intensity is between 0.1 and 0.3 mT, and suggested that the F1 part of F(0)F(1)-ATPase may be an end-point affected by magnetic fields.

Effects of pulsed magnetic field on the formation of magnetosomes in the Magnetospirillum sp. strain AMB-1.

Magnetotactic bacteria are a diverse group of microorganisms which possess one or more chains of magnetosomes and are endowed with the ability to use geomagnetic fields for direction sensing, thus providing a simple and excellent model for the study of magnetite-based magnetoreception. In this study, a 50 Hz, 2 mT pulsed magnetic field (PMF) was applied to study the effects on the formation of magnetosomes in Magnetospirillum sp. strain AMB-1. The results showed that the cellular magnetism (R(mag)) of AMB-1 culture significantly increased while the growth of cells remained unaffected after exposure. The number of magnetic particles per cell was enhanced by about 15% and slightly increased ratios of magnetic particles of superparamagnetic property (size <20 nm) and mature magnetosomes (size >50 nm) were observed after exposure to PMF. In addition, the intracellular iron accumulation slightly increased after PMF exposure. Therefore, it was concluded that 50 Hz, 2 mT PMF enhances the formation of magnetosomes in Magnetospirillum sp. strain AMB-1. Our results suggested that lower strength of PMF has no significant effects on the bacterial cell morphologies but could affect crystallization process of magnetosomes to some extent.

Preliminary estimation of rotary torque produced by proton-motive force in fully functional F0F1-ATPase.

F(0)F(1)-ATPase is a rotary molecular motor. It is well known that the rotary torque is generated by ATP hydrolysis in F(1) but little is known about how it produces the proton-motive force (PMF) in F(0). Here a cross-linking approach was used to estimate the rotary torque produced by PMF. Three mutant E. coli strains were used in this study: SWM92 (deltaW28L F(0)F(1), as control), MM10 (alphaP280CgammaA285C F(0)F(1)) and PP2 (alphaA334C/gammaL262C F(0)F(1)). The oxidized inner membranes from mutant MM10 having a disulfide bridge in the top of gamma subunit exhibited good ATP synthesis activity, while the oxidized PP2 inner membranes having a disulfide bridge in the middle of gamma subunit synthesized ATP very poorly. We conclude that the rotary torque generated by PMF is sufficient to uncoil the alpha-helix in the top of gamma subunit (MM10) and to overcome the Ramachandran activation barriers (25-30 kJ/mol, i.e. about 40-50pNnm), but cannot cleave the disulfide bond in the middle of the gamma subunit (200 kJ/mol, i.e. 330pNnm) (PP2). Consequently a preliminary estimation is that the rotary torque generated by PMF in the fully functional F(0)F(1) motor is greater than 40-50pNnm but less than 330pNnm.