Isolation and Characterization of Strain MMB-1 (CECT 4803), a Novel Melanogenic Marine Bacterium.

A novel marine melanogenic bacterium, strain MMB-1, was isolated from the Mediterranean Sea. The taxonomic characterization of this strain indicated that it belongs to the genus Alteromonas. Under in vivo conditions, L-tyrosine was the specific monophenolic precursor for melanin synthesis. This bacterium contained all types of activities associated with polyphenol oxidases (PPOs), cresolase (EC 1.18.14.1), catecholase (EC 1.10.3.1), and laccase (EC 1.10.3.2). These activities were due to the presence of two different PPOs. The first one showed all the enzymatic activities, but it was not involved in melanogenesis in vivo, since amelanogenic mutant strains obtained by nitrosoguanidine treatment contained levels of this PPO similar to that of the wild-type MMB-1 strain. The second PPO showed cresolase and catecholase activities but no laccase, and it was involved in melanogenesis, since this enzyme was lost in amelanogenic mutant strains. This PPO was strongly activated by sodium dodecyl sulfate below the critical micelle concentration, and it is a tyrosinase-like enzyme showing a lag period in its tyrosine hydroxylase activity that could be avoided by small amounts of L-dopa. This is the first report of a bacterium that contains two PPOs and also the first report of a pluripotent PPO showing all types of oxidase activities. The bacterium and the pluripotent PPO may be useful models for exploring the roles of PPOs in cellular physiology, aside from melanin formation. On the other hand, the high oxidizing capacity of the PPO for a wide range of substrates could make possible its application in phenolic biotransformations, food processing, or the cosmetic industry, where fungal and plant PPOs are being used.