G. Ward

Bactericidal action of high-power Nd:YAG laser light on Escherichia coli in saline suspension

Infra‐red light (1064 nm) from a high‐power Nd:YAG laser caused more than 90% loss of viability of Escherichia coli during exposures that raised the temperature of PBS suspensions of the bacteria to 50 °C in a thermocouple‐equipped cuvette. In contrast, there was minimal loss of viability after heating the same suspensions to 50 °C in a water‐bath, or in a PCR thermal cycler. The mechanism of laser killing at 50 °C was explored by differential scanning calorimetry, by laser treatment of transparent and turbid bacterial suspensions, and by optical absorbancy studies of E. coli suspensions at 1064 nm. Taken together, the data suggested that the bactericidal action of Nd:YAG laser light at 50 °C was due partly to thermal heating and partly to an additional, as yet undefined, mechanism. Scanning electron microscopy revealed localized areas of surface damage on laser‐exposed E. coli cells.

Inactivation of bacteria and yeasts on agar surfaces with high power Nd : YAG laser light

G.D. WARD, I.A. WATSON, D.E.S. STEWART‐TULL, A.C. WARDLAW AND C.R. CHATWIN. 1996. Near infrared light from a high‐powered, 1064 nm, Neodymium : Yttrium Aluminium Garnet (Nd : YAG) laser killed a variety of Gram‐positive and Gramnegative bacteria and two yeasts, lawned on nutrient agar plates. A beam (crosssectional area, 1.65 cm2) 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 inactivation vs energy density (J cm−2). The energy density that gave an inactivation area (IA) equal to 50% of the beam area was designated the IA50‐value and was plotted together with its 95% confidence limits. Average IA50‐values were all within a threefold range and varied from 1768 J cm−2 for Serratia marcescens to 4489 J cm−2 for vegetative cells of Bacillus stearothermophilus. There were no systematic differences in sensitivity attributable to cell shape, size, pigmentation or Gram reaction. At the lowest energy densities where inactivation was achieved for the majority of organisms (around 2000 J cm−2), no effect was observed on the nutrient agar surface, but as the energy density was increased, a depression in the agar surface was formed, followed by localized melting of the agar.

Comparative bactericidal activities of lasers operating at seven different wavelengths

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.

Effect of laser and environmental parameters on reducing microbial contamination of stainless steel surfaces with Nd:YAG laser irradiation

Aims:  The effect of laser (pulse repetition frequency, pulse energy and exposure time) and environmental parameters (pH, NaCl concentration and wet or dry samples) of Nd:YAG laser decontamination of stainless steel inoculated with Escherichia coli, Staphylococcus aureus and Listeria monocytogenes was investigated.

Temperature distribution in Escherichia coli liquid suspensions during irradiation by a high-power Nd:YAG laser for sterilization applications

A time-dependent, heat diffusion equation was used to predict the three-dimensional temperature distribution of Escherichia coli in liquid-suspension during irradiation by a high-power Nd:YAG laser. The model may be used to calculate the temperature rise and the transient 3-D temperature profile in the liquid suspension under arbitrary combinations of laser wavelength, pulse shape, pulse width, repetition rate, energy density, and for different concentrations of bacteria. The temperature profiles in the liquid, for a range of energy densities, were measured to validate the theoretical model. The experimental results were in good agreement with the theoretical ones. A temperature gradient was found in the sample in the radial and axial directions during laser irradiation. The model enables the parameters that affect the temperature distribution of the liquid suspension to be identified and optimized when designing laser sterilization systems.

Bactericidal action of high-power Nd:YAG laser light onEscherichia coliin saline suspension: e. colikilling by nd: yag laser

Infra‐red light (1064 nm) from a high‐power Nd:YAG laser caused more than 90% loss of viability of Escherichia coli during exposures that raised the temperature of PBS suspensions of the bacteria to 50 °C in a thermocouple‐equipped cuvette. In contrast, there was minimal loss of viability after heating the same suspensions to 50 °C in a water‐bath, or in a PCR thermal cycler. The mechanism of laser killing at 50 °C was explored by differential scanning calorimetry, by laser treatment of transparent and turbid bacterial suspensions, and by optical absorbancy studies of E. coli suspensions at 1064 nm. Taken together, the data suggested that the bactericidal action of Nd:YAG laser light at 50 °C was due partly to thermal heating and partly to an additional, as yet undefined, mechanism. Scanning electron microscopy revealed localized areas of surface damage on laser‐exposed E. coli cells.

Bactericidal action of high-power Nd

Infra‐red light (1064 nm) from a high‐power Nd:YAG laser caused more than 90% loss of viability of Escherichia coli during exposures that raised the temperature of PBS suspensions of the bacteria to 50 °C in a thermocouple‐equipped cuvette. In contrast, there was minimal loss of viability after heating the same suspensions to 50 °C in a water‐bath, or in a PCR thermal cycler. The mechanism of laser killing at 50 °C was explored by differential scanning calorimetry, by laser treatment of transparent and turbid bacterial suspensions, and by optical absorbancy studies of E. coli suspensions at 1064 nm. Taken together, the data suggested that the bactericidal action of Nd:YAG laser light at 50 °C was due partly to thermal heating and partly to an additional, as yet undefined, mechanism. Scanning electron microscopy revealed localized areas of surface damage on laser‐exposed E. coli cells.