Motility, manipulation and controlling of unicellular organisms

Introduction: Motility is a measure for vitality of unicellular organisms By using a microfluidic setup it is possible to analyse single-cell organisms and their motility. Thus it is possible to achieve several goals, from characterising the way of movement and the forces thereby generated to analysing drug effects and controlling pathogen displacement and spatial concentration to facilitate diagnosis. In order to do so, the microfluidic device as well as the manipulation and analysis tools have to be calibrated and adapted to the varying parameters as determined by the matter under study.
Methods: Using microfluidics in combination with optical trapping of unicellular organisms and high-speed microscopy, displacement trajectories were recorded and subsequently analysed using computer aided image analysis to characterise the flagellar propulsion of Trypanosoma brucei brucei and Caulobacter crescentus. Additionally, changes in the motility of T. b. brucei under the influence of drugs and different environments were determined and holdfast formation in C.crescentis was induced. The calibration parameters of the optical trap and the microfluidic devices were determined for different experimental setups in order to minimise phototoxic effects and maximise retention time of the organisms in the device.
Results: Swimming, Caulobacter crescentus generate an average force of 0.3 pN while being capable of a maximal force of 2.6 pN. C.crescentis and Trypanosoma brucei brucei rotate when they are inside an optical trap but for the trypanosomes this depends on the type of movement they were exhibiting directly before being trapped. The movement of T. b. brucei around the trap has a frequency of 15 Hz for the flagellar beat and a frequency of 1.5 Hz for the rotation itself.
The hydrodynamic interaction between swimming trypanosomes and the environment shows characteristic flow patters around the trypanosome that reveal it to be a pusher and not a puller. Their random-walk like migration can be directed by the geometry of the microfluidic device in order to contain them inside the device.
In our experimental setup, Caulobacter crescentus exhibits a phototoxic reaction when trapped with a laser of the wavelength of 808 nm.
The combination of optical traps and microfluidic devices can be furthermore used as a versatile methodology to study the impact of drugs and chemicals on motile unicellular organisms. Due to diffusion driven drug control, dosage-dependent effects can be determined through a motility factor.
Conclusion: Microfluidics in combination with optical trapping of cells and high speed microscopy can be used to analyse, manipulate, and control the motility of unicellular organisms, thus providing us with an interdisciplinary toolset to study living soft matter in a complex fluidic environment.