is associated with various gastrointestinal disorders. Lethal photosensitisation was investigated as a possible technique for killing which might offer a better alternative to antibiotics. The susceptibility of to lethal photosensitisation was determined by mixing suspensions of with various photosensitisers and plating out on blood agar before irradiation with low-power laser light. Five sensitisers were studied further by mixing them with in a tissue-culture plate and counting survivors after irradiation as a function of laser exposure time, dye concentration and pre-irradiation time. Crystal violet and thionin were ineffective as sensitisers, but zones of inhibition appeared with methylene blue (MB), protoporphyrin IX (PPIX), haematoporphyrin derivative (HPD), toluidine blue O (TBO) and disulphonated aluminium phthalocyanine (S2). Laser light or sensitiser alone did not affect bacterial viability. S2 (100 μg/ml) with a laser light energy density of 16 J/cm, HPD (100 μ/ml) with 160 J/cm, MB (100 μg/ml) with 21 J/cm, PPIX (150 μg/ml) with 320 J/cm and TBO (50 μg/ml) with 160 J/cm all reduced bacterial viability by > 99%. The killing of sensitised by laser light offers a new approach to the treatment of localised infections when all colonised areas are accessible to light.


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