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

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).