Optical Tweezers

The optical tweezers project is in collaboration with Rafael Jimenez at the Cal Poly Dairy Products Technology Center (DPTC) and Claudia Iniguez-Palomares at Centro de Investigación en Alimentación y Desarrollo, Mexico and is funded through the California Agricultural Research Initiative. Did you know that California is the sixth largest dairy producer in the world?

Optical tweezers can be used to manipulate and measure the forces on micron-sized particles. In our lab we are developing optical tweezers to measure the strength of adhesion of milk bacteria to mucin and milk fat globule membranes. The long term aim of this work is to understand the fundamentals of milk nutrition.

The tweezers system is built around a Zeiss Axiovert 200 microscope and uses a 1064 nm Nd:YVO laser. The system has two optical traps and one of the traps is steerable using a motorized gimbal mount.

So far we have been working on protocols for coating polystyrene spheres with the mucin proteins and membranes and how best to incubate the bacteria and the coated spheres so that we can make the force measurements. Unfortunately most of the milk bacteria (e.g. lactobacillus) are, well, bacilli, so they are long and it is very hard to know what forces the tweezers are exerting on them. Ideally, we would like to attach the opposite ends of a bacterium to coated spheres (it’s much easier to measure the forces on spheres) and then pull the spheres apart. The bacteria are not cooperating, however.

The figure below shows some images where we have pushed a Lactobacillus gallinarum against a 10 mm diameter polystyrene sphere coated with milk fat globule membrane (MFGM). The sphere is fixed to the cover slip. We then move the trap away from the sphere and adjust the power until we can just pull the bacterium off again. With some (rather crude) Stokes drag measurements we have been able to estimate the force to be about 20 pN, which is comparable to the forces measured by other groups using staphylococcus and fibronectin.

bacterium in optical trap

polysphere coated with MFGM

b

a

d

c

Frames from a binding experiment. a) shows the MFGM coated sphere and the bacterium in the trap. The bacteria, which are long, tend to point straight up in the traps when unattached and appear circular. b) shows the bacterium just as we push it against the sphere. c) shows the bacterium being pulled with one end of it attached to the sphere. In d) the bacterium has just detached.

microscope

Steerable mirror

IR laser

Quadrant photodetector

Here is the tweezers setup. There are two optical traps and one of them is precisely controlled in position using the steerable mirror. The quadrant photodetector system was put together by Adam Mednick as his senior project.