Formation of a carcinogenic aromatic amine from an azo dye by human skin bacteria in vitro

Hum Exp Toxicol. 1999 Sep;18(9):552-9. doi: 10.1191/096032799678845061.


Azo dyes represent the major class of dyestuffs. They are metabolised to the corresponding amines by liver enzymes and the intestinal microflora following incorporation by both experimental animals and humans. For safety evaluation of the dermal exposure of consumers to azo dyes from wearing coloured textiles, a possible cleavage of azo dyes by the skin microflora should be considered since, in contrast to many dyes, aromatic amines are easily absorbed by the skin. A method for measuring the ability of human skin flora to reduce azo dyes was established. In a standard experiment, 3x10(11) cells of a culture of Staphylococcus aureus were incubated in synthetic sweat (pH 6.8, final volume 20 mL) at 28 degrees C for 24 h with Direct Blue 14 (C.I. 23850, DB 14). The reaction products were extracted and analysed using HPLC. The reduction product o-tolidine (3,3'-dimethylbenzidine, OT) could indeed be detected showing that the strain used was able to metabolise DB 14 to the corresponding aromatic amine. In addition to OT, two further metabolites of DB 14 were detected. Using mass spectrometry they were identified as 3,3'-dimethyl-4-amino-4'-hydroxybiphenyl and 3, 3'-dimethyl-4-aminobiphenyl. The ability to cleave azo dyes seems to be widely distributed among human skin bacteria, as, under these in vitro conditions, bacteria isolated from healthy human skin and human skin bacteria from strain collections also exhibited azo reductase activity. Further studies are in progress in order to include additional azo dyes and coloured textiles. At the moment, the meaning of the results with regard to consumer health cannot be finally assessed.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Benzidines / metabolism*
  • Carcinogens / metabolism*
  • Chromatography, High Pressure Liquid
  • Coloring Agents / metabolism*
  • Gas Chromatography-Mass Spectrometry
  • Humans
  • In Vitro Techniques
  • Skin / microbiology*
  • Species Specificity
  • Staphylococcus aureus / isolation & purification
  • Staphylococcus aureus / metabolism*
  • Sweat
  • Trypan Blue / metabolism*


  • Benzidines
  • Carcinogens
  • Coloring Agents
  • 2-tolidine
  • Trypan Blue