The taxonomic systems have led to the
The phylum Actinobacteria comprises of Gram+ve bacteria found in both terrestrial and marine habitats. The name Actinomyces indicates that this order of bacteria was first believed to be fungi and then reclassified into prokaryotes after the advent of techniques like electron microscopy (S.T. Williams, 1987). The genus Streptomyces was first introduced by Waksman & Henrici in the year 1943. Recent taxonomic systems have led to the reclassification of many species of the initially proposed genus into new genera like Actinopycnidium, Actinosporangium, Chainia, Elytrosporangium, Kitasatoa and, Microellobosporia (Anderson& Wellington, 2001). Streptomyces still remains one of the largest genera with over 500 discovered species. Actinomycetes have been extensively exploited in biotechnology for the assortment of secondary metabolites produced by them such as enzymes, antibiotics, food supplements and much more. Secondary metabolites serve as antibiotics, metal transporting agents, sexual hormones, signalling molecules, differentiation effectors, nutrition acquisition molecules and symbiosis agents for the microbe (Demain & Fang, 2001). Especially significant is the plethora of antibiotics produces by the genera Streptomyces.
Conventional drug development involves screening the bioactivity of extracts from sources like bacteria, fungi and plants (bioactive-guided screening), chemical screening and target?oriented screening. Modern approaches include the use of previously unknown microbial strains and genome mining (Wohlleben, Wolfgang, et al., 2016). The rapid development of genome sequencing methods over the past few decades has led to generation of voluminous amounts of data. Thousands of whole genome sequences have been created and relations have been mapped between the natural products produced or predicted and their role in the environment.
Genome mining involves the exploitation of available genome data to derive information about the organism and predict the possible natural products encoded in the genome. This approach is highly useful as all products which can be potentially produced by the organism are not necessarily produced in laboratory conditions. Genome mining reveals the gene clusters encoding for these products which can then be manipulated to produce the compound. Genome mining may be of various types including classical, comparative, phylogeny-based, resistance/target based, regulator mining and culture independent mining (Ziemert et al., 2016).
A BGC can be defined as a physically clustered group of two or more genes in a particular genome that together encode a biosynthetic pathway for the production of a specialized metabolite (including its chemical variants) (Medema et al, 2015).
A major problem encountered today is the emergence of antibiotic resistance which has rendered many of the antibiotics currently in use ineffective. These multi-drug resistant infections including Staphylococcus aureus, Klebsiella pneumoniae and Pseudomonas aeruginosa need novel compounds for their treatment. The various drug classes isolated from Actinomycetes includes ?-lactams, tetracyclines, aminoglycosides, rifamycins, macrolides, and glycopeptides (Genilloud, 2017). Even after over 70 years since its discovery, Streptomyces and other related genera continue to provide novel lead compounds and scaffolds to modify and create new compounds to overcome drug resistance.