The cytotoxic and photodynamic inactivation of micro‐organisms by Rose Bengal

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Abstract
Rose Bengal was cytotoxic to the following bacteria at the concentrations given in parentheses (highest concentrations of dye in mol/l at which growth occurred on nutrient medium): Brochothrix thermosphacta and Deinococcus radiodurans (1 .times. 10-6 or less); Streptococcus, Micrococcus, Staphylococcus, Bacillus, Arthrobacter and Kurthia spp. (1 .times. 10-5-10-4) and Pseudomonas spp. and Enterobacteriaceae (5 .times. 10-3-1 .times. 10-2 or greater). These organisms were killed rapidly when suspended in illuminated (170 .mu.E m2/s) solutions of Rose Bengal (1 .times. 10-4 mol/l), providing O2 was present. Singlet O was identified as the lethal agent because the rate of killing was increased by dissolving the dye in deuterium oxide, while the organisms were protected against photoinactivation by L-histidine or crocetin. Yeasts from chilled foods were killed in illuminated solutions of Rose Bengal but a light intensity of 315 .mu.E m2/s was needed for a death rate comparable with that of bacteria. The yeasts present in a range of chilled meat and dairy products failed to form colonies on Rose Bengal (5 .times. 10-5 mol/l) media exposed continuously to modest illumination (55-80 .mu.E m2/s).