Abstract
Seven laser instruments, delivering radiation at a selection of wavelengths in the range of 0.355 to 118 mm, were investigated for their ability to kill Escherichia coli as a lawn of the bacteria on nutrient agar culture plates. Easily the most effective was a 600-W CO2 laser operating at 10.6 mm, which produced 1.2- cm2 circular zones of sterilization at energy densities of around 8 J cm22 in a 30-msec exposure. Circular zones with an area of 0.7 cm2 were achieved with 200 W from a Nd:YAG laser delivering 8-ms, 10-J pulses of 1.06 mm radiation at 20 Hz. The exposure time, however, was 16 s and the energy density (1940 J cm22) was more than 240 times higher than with the CO2 laser. This difference is believed to be partly due to the much higher absorption of radiation at 10.6 mm than at 1.06 mm, by water in the bacterial cells and the surrounding medium (nutrient agar). Sterilization was observed after exposure to frequency-tripled Nd:YAG laser radiation at 355 nm (3.5 J cm22). Lasers that were totally ineffective in killing Escherichia coli (with their wavelength and maximum energy densities tested) were the far infrared laser (118 mm; 7.96 J cm22), the laser diode array (0.81 mm; 13,750 J cm22), and the argon ion laser (0.488 mm; 2210 J cm22). The speed at which laser sterilization can be achieved is particularly attractive to the medical and food industries.
