Optimal optical trap for bacterial viability.

Optical trapping is a powerful tool for the micromanipulation of living cells--especially bacteria--but photodamage induced by the laser beam can adversely affect viability. We have explored optical trapping conditions in the near infrared (840-930 nm) that preserve the viability of E. coli, as measured by gene expression of green fluorescent protein. We have found that time-sharing the optical traps, i.e., dwelling only 10 micros-1 ms on the cell, improves viability relative to continuous wave (CW) exposure for the same exposure time. We have also observed that similar to CW traps the photodamage in a time-shared trap depends weakly on wavelength, but linearly on peak power, implying an effect induced by single photon absorption. Taken altogether, integrating the exposure time and peak power, the data indicate that there is a lethal energy dose of about 5 J for E. coli. Thus a single parameter--the energy--can be used to describe the limitation on viability.