Isolation and antimicrobial activities of actinobacteria closely associated with liquorice plants Glycyrrhiza glabra L. and Glycyrrhiza inflate BAT. in Xinjiang, China

A total of 218 actinobacteria strains were isolated from wild perennial liquorice plants Glycyrrhiza glabra L. and Glycyrrhiza. inflate BAT. Based on morphological characteristics, 45 and 32 strains from G. inflate and G. glabra, respectively, were selected for further analyses. According to 16S rRNA sequence analysis, most of the strains belonged to genus Streptomyces and a few strains represented the rare actinobacteria Micromonospora, Rhodococcus and Tsukamurella. A total of 39 strains from G. inflate and 27 strains from G. glabra showed antimicrobial activity against at least one indicator organism. The range of the antimicrobial activity of the strains isolated from G. glabra and G. inflate was similar. A total of 34 strains from G. inflate and 29 strains from G. glabra carried at least one of the genes encoding polyketide synthases, non-ribosomal peptide synthetase and FADH2-dependent halogenase. In the type Il polyketide synthase KS alpha gene phylogenetic analysis, the strains were divided into two major clades: one included known spore pigment production-linked KS alpha sequences and other sequences were linked to the production of different types of aromatic polyketide antibiotics. Based on the antimicrobial range, the isolates that carried different KS alpha types were not separated from each other or from the isolates that did not carry KS alpha. The incongruent phylogenies of 16S rRNA and KS alpha genes indicated that the KS alpha genes were possibly horizontally transferred. In all, the liquorice plants were a rich source of biocontrol agents that may produce novel bioactive compounds.