Optical trapping and orientation of Escherichia coli
cells using two tapered fiber probes
Jianbin Huang, Xiaoshuai Liu, Yao Zhang, and Baojun Li*
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering,
Sun Yat-Sen University, Guangzhou 510275, China
*Corresponding author: stslbj@mail.sysu.edu.cn
Received July 14, 2015; revised August 26, 2015; accepted August 26, 2015;
posted August 27, 2015 (Doc. ID 245904); published October 1, 2015
We report on the optical trapping and orientation of Escherichia coli (E. coli) cells using two tapered fiber probes.
With a laser beam at 980 nm wavelength launched into probe I, an E. coli chain consisting of three cells was formed
at the tip of probe I. After launching a beam at 980 nm into probe II, the E. coli at the end of the chain was trapped
and oriented via the optical torques yielded by two probes. The orientation of the E. coli was controlled by adjust-
ing the laser power of probe II. Experimental results were interpreted by theoretical analysis and numerical
simulations. © 2015 Chinese Laser Press
OCIS codes: (350.4855) Optical tweezers or optical manipulation; (130.0130) Integrated optics; (060.2310)
Fiber optics.
http://dx.doi.org/10.1364/PRJ.3.000308
1. INTRODUCTION
Manipulation of single cells is of great importance in biomedi-
cal research, microbiology, cell–cell interaction, and microbi-
ology. Trapping and orientation of individual nonspherical
bacteria can promote the study of the interaction between
specific regions of biological objects, which provides an effi-
cient method to observe and describe the behavior of single
cells and, further, to study the dynamics of bacteria popula-
tions [1,2]. Thus, it is of great importance in understanding
the mechanisms of cell function [3]. In addition, scanning
the angle between the long cell axis and the optical axis of
the optical tweezer may allow for reconstruction of the
three-dimensional structures using standard computerized
tomography methods [4]. Moreover, when the cell was illumi-
nated in different orientations, by studding the scattering light
of different orientation, more information on the morphology
of the cell can be obtained which can be used to discriminate
between different cell types or, more importantly, between
different cell states [5]. One of the most widely investigated
nonspherical bacteria is Escherichia coli (E. coli) cells due
to their critical role in biological engineering and industrial
microbiology [6,7]. Some challenges, however, exist in realiz-
ing stable trapping and controllable orientation of E. coli cells.
First, the E. coli cells in liquids swim in two patterns, “runs”
and “tumbles” [8,9], which are related to the rotation direction
of the flagella [10]. Second, the E. coli cells are strongly af-
fected by Brownian motion in liquids due to their nanometer
size [11]. To solve these challenges, optical methods based on
resonant optical antennas [11] and optical tweezers [4,12]
have been proposed to trap and orientate E. coli cells.
However, the operations were limited to a fixed substrate
or a specific depth of cell solutions, and the bulk structure
of the optical system and focusing objective make it difficult
to move and focus. Moreover, the difficulty in penetrating
thick samples by the focus generated by the objective make
it difficult to apply the system to thick samples. Fortunately,
optical fibers provide an alternative approach to manipulate
E. coli cells [13–15]. Among them, the fiber probe is a minia-
turized and highly flexible tool for particle manipulation be-
cause light beams can be simply focused by the tip of the
probe without the use of complicated optical components
such as high-numerical-aperture objective. However, with
only one laser beam applied for the cell manipulation, the ori-
entation will only be determined once the cells are trapped
[12,13], and therefore the accessible rotational axes or fea-
sible angles are restrained. In this work, we report on the op-
tical trapping and orientation of E. coli cells using two tapered
fiber probes. With the 980 nm laser beams injected into the
probes, an E. coli cell chain was formed and trapped at the
tip of one probe while the cell at the end of chain can be
controllably orientated by adjusting the power injected into
the fiber probes.
2. RESULTS
Figure 1 shows the experimental scheme. Two probes with
different divergence angles were placed in an E. coli solution
[Fig. 1(a)], with an angle of 150° between the probe axes. After
launching a laser beam into probe I, an E. coli chain consisting
of three cells was formed at the tip of probe I [Fig. 1(b)].
Another beam was then injected into probe II. Due to the
superposition of the two beams, the energy density at the in-
tersection point of two probes, i.e., point O in Fig. 1(a), was
higher than those around it, resulting in a much stronger gra-
dient force than the scattering force. Therefore, an E. coli cell
can be trapped by the gradient force at point O and the ori-
entation of the cell can be realized by adjusting the powers in
the two probes. At P
2
>P
1
, the E. coli at point O will be ori-
entated with its axis along probe II due to the restoring torque
[Fig. 1(c)], while at P
1
>P
2
, the cell will be orientated along
probe I [Fig. 1(d)]. The experimental setup is schematically
308 Photon. Res. / Vol. 3, No. 6 / December 2015 Huang et al.
2327-9125/15/060308-05 © 2015 Chinese Laser Press