Distribution of microbes producing antimicrobial ε-poly-L-lysine polymers in soil microflora determined by a novel method

We developed a simple and sensitive screening method to investigate the distribution of microbes producing an antimicrobial poly(amino acid), epsilon-poly-L-lysine (epsilon-PL), in microflora. An acidic dye, Poly R-478, incorporated in an agar plate detected epsilon-PL producers by electrostatic interaction with the secreted basic polymers. All epsilon-PL producers, isolated after careful and sufficient screening of soil microflora, belonged exclusively to two groups of bacteria of the family Streptomycetaceae and ergot fungi. They were characterized based on the density and diameter of the concentric zone formed by the secreted polymers. The density depended on each isolate. The increase in the diameter of the concentric zone per unit of time varied among isolates and was negatively correlated with the molecular weight. Although the distribution of epsilon-PL producers was extremely limited, their products were structurally varied. The molecular masses of the secreted polymers among the isolates ranged from 0.8 to 2.0 kDa. There were also isolates producing unknown polymers inconsistent with the correlation or producing a mixture of polymers with original and modified structures. A chemically modified polymer was an epsilon-PL derivative, as determined by mass spectrometry. Since the structural variations had no relation to the phylogenetic position of the isolates, it is possible that enzymes involved in the synthesis diversified after putative horizontal transfers of relevant genes.