Melanin Production By Azospirillum Lipoferum Strains

Abstract

Melanin production is a common property among fungi and streptomycetes, but it is less known among unicellular bacteria. Melanin formation from tyrosine is due to the catalytic activity of tyrosinase. Generally, this pigment is not essential for growth, but rather it enhances the survival and competitive abilities of species in certain environments. Among nitrogen-fixing bacteria, Rhizobium leguminosarum, R. meliloti and Sinorhizobium fredii produce a dark-brown melanin pigment (4). One hypothesis concerning functions of melanin production in Rhizobium is that the tyrosinase functions in detoxifying polyphenolic compounds in senescing nodules (4). According to Rhizobium species, genetic studies have shown that mel genes are located either in the symbiotic plasmid or in cryptic plasmids (4). Among Azospirillum spp., melanin formation has only been detected in A.brasilense Sp7 (8). However dark-brown pigment is only expressed in aging culture under influence of the cultural conditions conducive for encystement (8). In most A.lipoferum strains tested, spontaneous non-motile forms appeared. A. lipoferum 4BpHT2 is a non-motile individual derived from A.lipoferum 4B, the parental strain isolated from the rhizosphere of rice (5).