Abstract

The resistance of bacterial endospores to both heat¹ and UV irradiation is well known. Laser light has been shown previously to have a killing effect on spores in relation to medical instruments². In the present case, Nd:YAG laser light and UV irradiation was combined into a treatment process and the subsequent killing effect statistically analysed. After exposing spores, lawned on nutrient agar plates, to UV irradiation, from a 2 × 8W source (distance 24cm, output 190 μWcm²), inactivation caused by laser light was increased. Exposure to the laser light alone (energy density 3000 Jcm⁻²) gave a 10% zone of inhibition, but with a previous exposure to UV for 4 min, the zone of inhibition increased to 55-65%. This is shown in Figure 1. The bacterial lawns were showing significant signs of inhibition after 6 min of UV exposure. To investigate further effects of laser and UV treatment, spore suspensions on stainless steel discs were pre-treated with laser and exposed to UV irradiation, with a varying delay between the two treatments. The experiment was a multifactorial design. Figure 2 shows that as the individual factors, laser and UV exposure, were increased, a reduction in remaining cell viability was observed. There was increased killing when a longer delay (15 min as opposed to 10 min) was employed. The results were analysed with the linear general model. It was found that as single factors, all three agents were significant. The laser was more significant than the UV at the exposures given. It was also found that a high laser pre-treatment and high delay had a significant interaction, (95% confidence limits). This could suggest that after a high enough laser treatment the spores were beginning to lose some of their resistant properties. The results did not suggest that there was a significant interaction between either the laser and UV, or the laser. UV and delay.

© 1998 IEEE