Antimicrobial activity of argon fluoride (ArF) excimer laser on gram-negative bacteria

The objective of this study was to evaluate the antibacterial activity of a rgon fluoride (ArF) excimer laser radiation on clinically important strains of gram-negative bacteria. The antibacterial activity of ArF excimer laser radiation was evaluated on two Acinetobacter baumannii, one Enterobacter c loacae, three Escherichia coli, two Helicobacter pylori, one Klebsiella pne umoniae and two Pseudomonas aeruginosa strains. The strains were isolated f rom clinical specimens and typed by the usual biochemical procedures. Squar e agar plates of 12x12 cm were divided into rectangular (2x3 cm) regions an d spread with 0.5x10(4) colony forming units (CFU)/ml of bacterial suspensi on. The excess liquid was removed and the plates were allowed to dry for 30 min. A total of 96 rectangular (2x3 cm) regions were used for each strain, in order to test an equal number of laser parameters. Each rectangular reg ion was irradiated with different laser parameters, using a 193 nm ArF exci mer laser, linked with a simple Galilean afocal system and a rectangular di aphragm of the same dimensions as the original laser beam cross-section, at a distance of 10 cm from the irradiated surface. This system was used in o rder to keep the laser pulse energy under 80 mJ and to cut-out the non-tran sverse electromagnetic mode branches of the laser beam. We then studied the bacterial survival ratio versus the number of laser pulses, the repetition frequency and the total laser beam fluence. Our results showed that the to tal laser beam fluence was the most important parameter to consider in eval uating the bactericidal effect of ArF excimer laser radiation. A critical v alue of the total fluence was determined for each strain, such that, for la ser beam fluences greater than this critical value, no colonies appeared to survive while, for laser fluences less than this critical value, the survi val ratio did not exceed 2x10(-7) CFU (2x10(-5%)). These critical values we re found to vary between 8 J/cm(2) and 16 J/cm(2) for the bacterial species studied. Under these conditions, ArF laser irradiation is promising for th e sterilisation of hard surfaces and for in situ application.