Rose Bengal as a Selective Aid in the Isolation of Fungi and Actinomycetes from Natural Sources

SUMMARY

The use of the bacteriostatic dye rose bengal as a selective additive for the isolation of both fungi and actinomycetes was examined. A medium has been developed, combining the useful bacteriostatic properties of rose bengal with the favorable effect of starch-casein-nitrate agar on development of fungi and actinomycetes. Rose bengal-starch-casein-nitrate agar allows easy isolation or enumeration of fungi and actinomycetes at the expense of bacteria. Rose bengal at a concentration of 670 mg/liter enhanced the number of fungi but suppressed a part of the actinomycete flora. Among 23 selected Streptomyces species, only Strcptomyces flavopersicus Oliver et al., hirsutus Ettlinger et al., S. hachijoensis Yama- guchi, S. netropsis Finlay & Sobin and an unidentified white-spored Streptomyces sp. FAL K 309 failed to grow on this medium at 670 mg/liter. No significant interference by the dye was observed at 350 mg/liter. At the latter concentration, rose bengal had no statistically significant effect on the number of actinomycetes enumerated on starch-casein-nitrate agar with or without the dye. The advantages of the additive rose bengal are threefold: (a) suppression of most bacteria, (b) coloring even pinpoint actinomycete colonies intense pink and (c) reducing the spreading growth of fungi. Media containing rose bengal should be used with care, as exposure to daylight may cause inhibition of actinomycete developments.