Characterization of "Pyomelanin"-Producing Strains of Pseudomonas aeruginosa

Forty strains of Pseudomonas aeruginosa (Schroeter) Migula isolated from clinical specimens and producing a water-soluble brown pigment on peptone media were compared with the neotype and nine other non-brown strains of P. aeruginosa. The brown pigment is formed from either tyrosine or phenylalanine through the accumulation of homogentisic acid and has no identity with DOPA melanin. A new term, “pyomelanin,” is proposed for this brown pigment synthesized by certain strains of P. aeruginosa. Although the pyomelanin strains share the attributes of the species, some of them have a more limited capacity with regard to certain biochemical reactions than the neotype strain of P. aeruginosa. A careful examination is required to identify these aberrant strains. Actively growing cells of pyomelanin strains converted tyrosine to pyomelanin, but tyrosine itself does not appear to serve as a sole source of carbon and nitrogen for growth. A simple method of differentiating pyomelanin-producing strains of P. aeruginosa from pyorubin-producing strains is presented. To recognize properly the species, the attributes of pyomelanin strains should be included in descriptions of P. aeruginosa.