Phototoxicity of rose bengal in mycological media--implications for laboratory practice

The effects of Rose Bengal (RB) on plate counts of the bacteria Escherichia coli and Pseudomonas aeruginosa, and the yeast Saccharomyces cerevisiae, were studied under natural sunlight and artificial fluorescent lighting. While RB was not inherently toxic in darkness at concentrations found in mycological media, the illumination of media containing RB caused a decrease in colony counts in all cases, and especially for surface spread plates. A negative synergy was observed between chloramphenicol, RB and illumination using a spring water sample containing substantial numbers of Gram-negative bacteria. Exposure of media containing RB to moderate amounts of light during standard laboratory procedures may inhibit microbial growth, with positive benefits in relation to the suppression of contaminant bacteria, or negative implications where fungi are inhibited.