The effect of methylene blue dose when irradiated with 1.93 J/cm2 laser light on the activity of S. aureus sphingomyelinase. An equal volume of either 1, 5, 10 and 20 μM methylene blue (S+) or PBS (S-) was added to sphingomyelinaseand samples were either exposed to laser light with an energy density of 1.93 J/cm2 (L+) (black bars) or kept in the dark (L-) (white bars). Following irradiation, the activity of the enzyme was assessed spectrophotometrically using the substrate TNPAL-Sphingomyelin. Error bars represent the standard deviation from the mean. *** P < 0.001 (ANOVA). Experiments were performed three times in duplicate and the combined data are shown.