INACTIVATION OF BACTERIA AND YEASTS ON AGAR SURFACES WITH HIGH-POWER ND-YAG LASER-LIGHT

Near infrared light from a high-powered, 1064 nm, Neodymium:Yttrium Al uminium Garnet (Nd:YAG) laser killed a variety of Gram-positive and Gr am-negative bacteria and two yeasts, lawned on nutrient agar plates. A beam (cross-sectional area, 1.65 cm(2)) of laser light was delivered in 10 J, 8 ms pulses at 10 Hz, in a series of exposure times. For each microbial species, a dose/response curve was obtained of area of inac tivation vs energy density (J cm(-2)). The energy density that gave an inactivation area (IA) equal to 50% of the beam area was designated t he IA(50)-value and was plotted together with its 95% confidence limit s. Average IA(50)-values were all within a threefold range and varied from 1768 J cm(-2) for Serrati marcescens to 4489 J cm(-2) for vegetat ive cells of Bacillus stearothermophilus, There were no systematic dif ferences in sensitivity attributable to cell shape, size, pigmentation or Gram reaction. At the lowest energy densities where inactivation w as achieved for the majority of organisms (around 2000 J cm(-2)), no e ffect was observed on the nutrient agar surface, but as the energy den sity was increased, a depression in the agar surface was formed, follo wed by localized melting of the agar.