Nile Blue, Aniline Blue and Neutral Red as Indicators of Lipolysis

SUMMARY: Dyes which have been used to detect lipolysis (Nile Blue, Aniline Blue and Neutral Red) and others (Methylene Blue, Toluidine Blue and Thionin) were critically examined for their inhibitory effects in both liquid and solid media. It was confirmed that Nile Blue would inhibit the Gram‐positive bacteria tested, even at 2 × 10 −5 . Gram‐negative bacteria were not inhibited at 1 × 10 −4 and the colour change in fat media was good. Aniline Blue and Neutral Red did not inhibit Gram‐positive organisms at 1 × 10 −4 and the colour change was moderately good. The other three dyes were not toxic at 2·5 × 10 −5 , but there was no colour change. When butter fat saturated with the precipitated bases of Nile Blue, Basic Fuchsin, Crystal Violet or Malachite Green was incorporated in peptone‐Yeastrel agar, the resulting media were toxic to Gram‐positive bacteria. Fat saturated with the bases of Aniline Blue or Neutral Red permitted growth, but the colour change with lipolytic species was not striking, being one of intensity only. Satisfactory growth and colour changes were obtained using butter fat stained with the inert dye Waxolene Green in medium containing Neutral Red in the aqueous phase, and with butter fat stained with the oxazine base of Nile Blue in agar containing Aniline Blue.