This study assessed the effectiveness of methylene blue (MB) and malachite green (MG) on photodynamic inactivation (PDI) of Escherichia coli. The photosensitizers methylene blue (1000 mol L -1 ) and malachite green (250 mol L -1 ) were activated with a red light-emitting diode (LED) lamp ( max = 636 nm). Bacterial suspensions containing 10 6 CFU mL -1 were irradiated for 5, 10 and 15 minutes (energy density = 119.9 J cm -2 , 223.9 J cm -2 and 335.8 J cm -2 , respectively). The following

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... imental conditions were performed for each photosensitizer: no light irradiation or photosensitizer, irradiation only, photosensitizer only or irradiation in the presence of a photosensitizer. Next, serial dilutions were prepared and seeded onto PCA medium for the determination of the number of colony-forming units per milliliter (CFU mL -1 ). The results were subjected to analysis of variance (ANOVA) and Tukey test (P<0.05). Photodynamic inactivation using MB and MG was effective in reducing the number of E. coli. Malachite green (250 µmol L -1 ) photosensitization was able to achieve reductions of over 89% in the viable counts after 15 min of irradiation and methylene blue (1000 µmol L -1 ), at the same conditions of irradiation, showed a rate growth inhibition of 94.6%. The red LED light used has proven to be effective in the photosensitizing dyes and proved a good alternative to conventional light sources such as laser. Inativação fotodinâmica de Escherichia coli por azul de metileno e verde malaquita sob luz vermelha LED RESUMO Este estudo avaliou a eficiência do azul de metileno (AM) e do verde malaquita (VM) na inativação fotodinâmica (IFD) de Escherichia coli. Os fotossensibilizadores (FS) azul de metileno (1000 mol L -1 ) e verde malaquita (250 mol L -1 ) foram ativados com lâmpada de luz LED vermelha ( max = 636 nm). Suspensões bacterianas contendo 10 6 UFC mL -1 foram irradiadas por 5, 10 e 15 minutos (densidade de energia = 111,9 J cm -2 , 223,9 J cm -2 e Photodynamic inactivation of Escherichia coli by methylene blue ... Rev. Ambient. Água vol. 10 n. 2 Taubaté -Apr. / Jun. 2015