Diversity of Culturable Thermophilic Actinobacteria in Hot Springs in Tengchong, China and Studies of their Biosynthetic Gene Profiles

The class Actinobacteria has been a goldmine for the discovery of antibiotics and has attracted interest from both academics and industries. However, an absence of novel approaches during the last few decades has limited the discovery of new microbial natural products useful for industries. Scientists are now focusing on the ecological aspects of diverse environments including unexplored or underexplored habitats and extreme environments in the search for new metabolites. This paper reports on the diversity of culturable actinobacteria associated with hot springs located in Tengchong County, Yunnan Province, southwestern China. A total of 58 thermophilic actinobacterial strains were isolated from the samples collected from ten hot springs distributed over three geothermal fields (e.g., Hehua, Rehai, and Ruidian). Phylogenetic positions and their biosynthetic profiles were analyzed by sequencing 16S rRNA gene and three biosynthetic gene clusters (KS domain of PKS-I, KS alpha domain of PKS-II and A domain of NRPS). On the basis of 16S rRNA gene phylogenetic analysis, the 58 strains were affiliated with 12 actinobacterial genera: Actinomadura Micromonospora, Microbispora, Micrococcus, Nocardiopsis, Nonomuraea, Promicromonospora, Pseudonocardia, Streptomyces, Thermoactinospora, Thermocatellispora, and Verrucosispora, of which the two novel genera Thermoactinospora and Thermocatellisopora were recently described from among these strains. Considering the biosynthetic potential of these actinobacterial strains, 22 were positive for PCR amplification of at least one of the three biosynthetic gene clusters (PKS-I, PKS-II, and NRPS). These actinobacteria were further subjected to antimicrobial assay against five opportunistic human pathogens (Acinetobacter baumannii, Escherichia coli, Micrococcus luteus, Staphylococcus aureus and Streptococcus faecalis). All of the 22 strains that were positive for PCR amplification of at least one of the biosynthetic gene domains exhibited antimicrobial activities against at least one of the five test organisms. Among the remaining 36 actinobacteria that are negative for PCR amplification of the domains for the biosynthetic genes, 33 strains showed antimicrobial activities against at least one of the five test pathogens. In summary, the findings presented in this study emphasized the importance of underexplored habitats such as Tengchong hot springs as potential sources for search of bioactive molecules.