Uncovering the biodiversity and biosynthetic potentials of rare actinomycetes

Background

Antibiotic resistance is on the rise, and new antibiotic research has slowed in recent years, necessitating the discovery of possibly novel microbial resources capable of producing bioactive compounds. Microbial infections are gaining resistance to existing antibiotics, emphasizing the need for novel medicinal molecules to be discovered as soon as possible. Because the possibilities of isolating undiscovered actinomycetes strains have decreased, the quest for novel products has shifted to rare actinomycetes genera from regular environments or the identification of new species identified in unusual habitats.

Main body of the abstract

The non-streptomyces actinobacteria are known as rare actinomycetes that are extremely difficult to cultivate. Rare actinomycetes are known to produce a variety of secondary metabolites with varying medicinal value. In this review, we reported the diversity of rare actinomycetes in several habitat including soil, plants, aquatic environment, caves, insects and extreme environments. We also reported some isolation methods to easily recover rare Actinobacteria from various sources guided with some procedures to identify the rare Actinobacteria isolates. Finally, we reported the biosynthetic potential of rare actinomycetes and its role in the production of unique secondary metabolites that could be used in medicine, agriculture, and industry. These microbial resources will be of interest to humanity, as antibiotics, insecticides, anticancer, antioxidants, to mention but a few.

Short conclusion

Rare actinomycetes are increasingly being investigated for new medicinal compounds that could help to address existing human health challenges such as newly emerging infectious illnesses, antibiotic resistance, and metabolic disorders. The bioactive secondary metabolites from uncommon actinomycetes are the subject of this review, which focuses on their diversity in different habitats, isolation, identification and biosynthetic potentials.

Characterization of Antibacterial Strains against Kiwifruit Bacterial Canker Pathogen

Kiwifruit (Actinidia spp.) is an economically important crop and a bacterial canker disease, caused by Pseudomonas syringae pv. actinidiae (Psa), is the most destructive disease in kiwifruit production. Therefore, prevent and control of the disease is a critical issue in kiwifruit industry worldwide. Unfortunately, there is no reliable control methods have been developed. Recently, interest in disease control using microbial agents is growing. However, kiwifruit microbiota and their roles in the disease control is mainly remaining unknown. In this study, we secured bacterial libraries from kiwifruit ecospheres (rhizosphere, endospere, and phyllosphere) and screened reliable biocontrol strains against Psa. As the results, Streptomyces racemochromogenes W1SF4, Streptomyces sp. W3SF9 and S. parvulus KPB2 were selected as anti-Psa agents from the libraries. The strains showed forcible antibacterial activity as well as exceptional colonization ability on rhizosphere or phyllosphere of kiwifruit. Genome analyses of the strains suggested that the strains may produce several anti-Psa secondary metabolites. Our results will contribute to develop biocontrol strains against the kiwifruit canker pathogen and the disease management strategies.

Cave Actinobacteria as Producers of Bioactive Metabolites

Recently, there is an urgent need for new drugs due to the emergence of drug resistant pathogenic microorganisms and new infectious diseases. Members of phylum Actinobacteria are promising source of bioactive compounds notably antibiotics. The search for such new compounds has shifted to extreme or underexplored environments to increase the possibility of discovery. Cave ecosystems have attracted interest of the research community because of their unique characteristics and the microbiome residing inside including actinobacteria. At the time of writing, 47 species in 30 genera of actinobacteria were reported from cave and cave related habitats. Novel and promising bioactive compounds have been isolated and characterized. This mini-review focuses on the diversity of cultivable actinobacteria in cave and cave-related environments, and their bioactive metabolites from 1999 to 2018.

Oligotrophy is Helpful for the Isolation of Bioactive Actinomycetes

It is necessary to develop new methods for the isolation of unknown actinomycetes from soils. To evaluate the effects of oligotrophic medium on the isolation of soil actinomycetes and develop a new isolation method, the Gause's synthetic medium was diluted to one tenth the recommended concentration in the present study. Soil dilution plate technique was used to isolate actinomycetes from the soil samples. Oligotrophy decreased actinomycete and streptomycete counts, as well as the number of antagonistic actinomycete species. Oligotrophy also decreased the number of actinomycete species in five samples. Some actinomycete species were cultured only on the oligotrophic medium, whereas other species could not be cultured. Oligotrophy decreased actinomycete counts more significantly for soils with organic matter content >40 g/kg. We used 16S rRNA sequence analysis to identify 22 actinomycete species that were only cultured on the oligotrophic medium. Oligotrophic medium was helpful for the isolation of Streptomyces spp., Micromonospora spp. and Streptosporangium spp. Slightly more than 80 % of the identified actinomycete species were biologically active. Therefore, we could draw a conclusion that oligotrophic medium could be helpful for the discovery of new antibiotic producers and the exploitation and utilization of new, biologically active compounds.