INVESTIGATION INTO THE WAVELENGTH-DEPENDENT EFFECT OF LOW INCIDENT LEVELS OF LASER RADIATION ON THE GROWTH OF MICROBIAL CELLS

Abstract

The purpose of the present in vitro study was to compare the effect of N2 (5.4 J/cm2), HeNe (35.1 J/cm2) and GaAlAs (163.8 J/cm2) lasers on the microbes at doses in current clinical use. The suspensions of Escherichia coli NCTC 10418 with densities equivalent to 108-109 cells/ml and 5 x 103 cells/ml were subjected to viable count study after irradiation with the above lasers. The N2 laser revealed a distinct inactivating effect on bacteria at low cell densities and it was confirmed to be dose dependent for both Escherichia coli and Staphylococcus aureus. The N2 laser did not show any obvious lethal effect over high density bacterial suspensions, possibly due to the poor penetration of the 337 nm beam owing to the high absorption in water associated with UV energy, but exhibited reduced growth in growth curve studies in comparison to the non-irradiated control. The various microorganisms including Escherchia coli, Staphylococcus aureus, Klebsiella aerogenes and Pseudomonas aeruginosa revealed similar reduction in growth on irradiation with N2 laser. On the other hand, neither the HeNe nor GaAlAs diode lasers altered the growth and viability of E. coli at the varied cell densities.