Production of brownish pigment by bacteria of the Morganella-Proteus-Providencia group.

All bacteria of the Morganella-Proteus-Providencia group tested produced a brownish pigment on agar containing L-forms of aromatic amino acids, i.e. histidine, phenylalanine, tryptophan, and tyrosine in the presence of iron ions under aerobic conditions. No pigment was observed without iron or when using D-forms of the amino acids. It is a melanin-like pigment with variable molecular weight under about 12000 D and an absorbance maximum between 330 and 410 nm depending on the concentration and the amino acid metabolized. Moreover, there is evidence that aldehydes are intermediates. In addition, the bacteria develop a characteristic almond-like smell on phenylalanine agar.