Optical regulation of cell chain

Abstract

Formation of cell chains is a straightforward and efficient method to study the cell interaction. By regulating the contact sequence and interaction distance, the influence of different extracellular cues on the cell interaction can be investigated. However, it faces great challenges in stable retaining and precise regulation of cell chain, especially in cell culture with relatively low cell concentration. Here we demonstrated an optical method to realize the precise regulation of cell chain, including removing or adding a single cell, adjusting interaction distance, and changing cell contact sequence. After injecting a 980-nm wavelength laser beam into a tapered optical fiber probe (FP), a cell chain of Escherichia colis (E. colis) is formed under the optical gradient force. By manipulating another FP close to the cell chain, a targeted E. coli cell can be trapped by the FP and removed from the chain. Further, the targeted cell can be added back to the chain at different positions to change the cell contact sequence. The experiments were interpreted by numerical simulations and the impact of cell sizes and shapes on this method was analyzed.

Figure 1. Schematic of the regulation process and experimental setup.

(a) A cell chain is formed after a laser beam is injected into FP 1. (b) With a laser beam injected into FP 2, E. coli 5 is rotated and then removed from the cell chain. (c) E. coli 5 is added back at a new position into chain. (d) After turning off the laser in FP 2, E. coli 5 is orientated along the axial direction of FP 1. (e) Schematic of the experiment setup.

Figure 2. Simulated energy density distribution and calculated optical torque.

(a,b) Simulated energy density distributions of FPs 1 and 2 with normalized energy densities along the fiber axial direction. The yellow dashed lines show the positions of fiber tips. The insets are the microscopic images of the FPs. Scale bar: 5 μm. (c) Calculated torques exerted on the 6 E. colis as a function of the power in FP 2 (power in FP 1: 30 mW). The inset shows the torques on the E. coils 1, 2 and 3 with the power ranged from 60 to 80 mW to distinguish the curves for the three cells.

Figure 3. Optical microscopic images for removing E. coli 6 from the cell chain.

(a–c) After the laser was injected into FP 2, E. coli 6 was rotated and gradually orientated along the axial direction of FP 2. (d–f) E. coli 6 was removed from the cell chain and then shifted with FP 2. The insets schematically show the removal process.

Figure 4. Calculated optical torque and force during the removal and shifting progress.

(a) Calculated optical torque exerted on the six E. colis of the chain as a function of azimuthal angle. The inset shows the simulated energy density distribution at θ = 180°. The arrows indicate the rotation direction. (b) Calculated optical force F_x and torque on E. coli 6 with the shift of FP 2 along the −x direction.

Figure 5. Optical microscopic images and calculated optical torque for adding E. coli 6 back into the cell chain.

(a–c) After the laser in FP 2 was turned off, E. coli 6 was rotated and gradually orientated along the axial direction of FP 1. The insets schematically show the adding process. (d) As FP 2 shifted, the E. coli kept trapped by FP 1. (e) Calculated optical torque exerted on the six E. colis at different azimuthal angles (θ) of E. coli 6 during the rotation process. The inset shows the simulated energy density distribution at θ = 130°.

Figure 6. Optical microscopic images for adjusting the cell contact sequence.

(a) FP 2 was adjusted with the microstage 1 to approach E. coli 3. (b–d) With the shift of FP 2 along the −x direction, E. coli 3 was rotated and then trapped by FP 2. (e,f) E. coli 3 kept trapped and shifted with FP 2 along the +y and +x direction. (g–i) After turning off laser in FP 2, E. coli 3 was added back into the cell chain between E. coils 4 and 5 and the contact sequence was changed to 1–2–4–3–5–6–7–8–9–10. The two tips of E. coil 3 were indicated by yellow and red dots.

Figure 7. Calculated optical torque and force during the sequence regulation progress.

(a) Calculated optical torque exerted on the 10 E. colis as a function of azimuthal angle θ (FP 1: 30 mW, FP 2: 50 mW). The inset shows the energy density distribution at θ = 0°. (b) Calculated optical torque exerted on the 10 E. colis as a function of azimuthal angle θ (FP 1: 30 mW, FP 2: 0 mW). The inset shows the energy density distribution at θ = 130°. (c) Calculated optical force F_x and torque on the E. coli 3 as a function of shifting distance.

Figure 8. Impact of cell sizes and shapes on regulation.

(a) Calculated optical torques on E. colis with lengths varied from 1.2 to 1.7 μm at different azimuthal angles. (b) Calculated optical torques on E. colis with diameters varied from 0.35 to 0.60 μm at different azimuthal angles. (c) Calculated trapping forces F_x on the targeted yeast cells with diameters varied from 3.0 μm to 4.5 μm as functions of the shift distance of FP2.