The effect of 20 μM methylene blue when irradiated with different laser light doses on the activity of S. aureus sphingomyelinase. Sphingomyelinase was either kept in the dark (L-) or irradiated with laser light doses of 1.93 J/cm2, 3.86 J/cm2 and 9.65 J/cm2 (L+) in the presence of an equal volume of either PBS (S-) (white bars) or 20 μM methylene blue (S+) (black bars). Following irradiation with laser light doses of 1.93 J/cm2, 3.86 J/cm2 and 9.65 J/cm2, 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 triplicate and the combined data are shown.