Application of Laser Manipulation for Evaluating the Antibacterial Effect and for Controlling DNA Conformation

We used laser tweezers as a rapid method for detecting a chemical damage of bacteria. We examined the effect of protamine, a natural polycation, on a culture of Pseudomonas aeruginosa. Under the attractive force by a focused IR laser (1064 nm), an assembly of multiple bacteria was formed both in the presence and absence of protamine. When the laser power was turned off, the assembly spontaneously dissociated into individual bacteria in the absence of protamine. Whereas, in the presence of protamine, the assembly remained stable after the power off. It is noted that this methodology on the detection of cell damage was sensitive even on a rather early stage of protamine treatment (ca. 10 min.), where no apparent change was observed by usual optical microscopic observations. These results were discussed in relation to the cell surface damage as depicted by electron microscopy.