Optical Tweezers Group
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UCL Optical Tweezers Group


CoMPLEx MRes Summer Project

Quantifying bacterial adhesion using optical tweezers

Abstract:  Bacterial infection is absolutely dependent on the ability of bacteria to adhere to the correct site in the host and is controlled by bacterial cell surface proteins called adhesins. Each bacterium expresses a multiplicity of adhesins producing an ‘adhesion system’ whose properties (particularly binding strength) we need to model and understand. In this project we will be applying optical tweezer methodology to study a novel and unusual bacterial adhesin, which binds both to fibronectin and to DNA with different affinities.An optical tweezers, that is a trap for microscopic objects formed by a single focussed laser beam have several applications in physical, chemical and biological science (see: Neuman & Block 2004). In particular a carefully calibrated device can be used for the measurement of piconewton forces with sub-piconewton precision, and are thus ideal for quantifying forces in biological systems.

Plan: This project will be a systematic investigation of the adhesion strength of the novel recombinant adhesin described with fibronectin and DNA, using coated polystyrene beads as a model system. This simple system will be probed, using optical tweezer methodology, to define the relationships between surface concentrations of adhesin and the two adhesin ligands and the strengths of the binding complexes. It is suitable for a student with a background either in physics or in the life sciences. 

Functionalised polystyrene beads are readily available and can be coated with adherin molecules or fibronectin at a controlled surface density. A suspension of coated beads can then be transferred to the optical tweezers where a number of beads may be trapped an individually manipulated (see figure 1 below).


Figure 1: A line of three polystyrene beads held and individually manipulated in an optical tweezers

If the trapping strength (i.e. trap restoring force per unit displacement from trap centre) of the optical tweezers is calibrated then controlled pairwise contacts between coated beads can be made and the force required to rupture the bond between then quantified (see Movie 1 below). 

Movie 1: The small bead is coated with a bacterial adhesin, and the large bead with fibronectin. The trapping laser power is decreased until the tweezers can no longer pull the two apart

This can then be repeated in a systematic manner for all the different adherin molecules found on the surface of the bacterium.


Neuman KC, Block SM (2004) Optical trapping, Rev Sci Instrum 75: 2787-2809

Jones PH, Stride E, Saffari, N (2006) Trapping and manipulation of microscopic bubbles with a scanning optical tweezer, Appl Phys Lett 89: 081113

Mullen L, Nair SP, Rycroft A, Ward JM, Henderson B (2006) Phage display in the study of infectious diseases. Trends Microbiol 14:141-147.

Williams RJ, Henderson B, Nair SP (2002) The identification of a fibronectin-binding protein from Staphylococcus epidermidis. Infect Immun 70: 6805-6810.

Contact Phil Jones or Brian Henderson for more about this project

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