## 7 Recent Approaches for Discovery/Characterization of Cryptic Natural Products
### Top-Down Approach Based on Metabolomics  
The top-down approach is based on prior analysis of metabolites and subsequent analysis of the genomic data. Metabolomics is an effective analytical technique that encompasses the profiling of low molecular weight metabolites between medium and high throughput environments. In this approach, different biological samples are statistically analyzed and correlated, with the activity of interest focusing on differentially produced compounds as potential biomarkers. Therefore, metabolomics has the potential to lead to the discovery of novel NPs [79; 80]. Recently, the development in analytical techniques, such as nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry analysis has advanced the metabolomics approach to a new horizon. Different NMR techniques, such as J-resolved, COSY, TOCSY, HSQC, and HMBC, have the potential to precisely elucidate the chemical structure of the natural product. However, it is ineffective on impure organic compounds, because there are chances of signal-overlapping issues. To address these types of complexities, chromatography-hyphenated NMR (HPLC-NMR) has been introduced into metabolic analysis [81]. The incorporation of solid-phase-extraction (SPE) for injection sample refinement or integration of mass spectrometric instrumentation can fine-tune the structural evaluation of chemical components in NMR-based metabolomics studies. By using this LC-SPE-NMR approach [82] and LC-DAD-SPE-MSNMR instrumentation [83] comprehensive structural information can be deduced. Hence, it is feasible to get insight into the chemical composition of crude extracts through in-silico analysis leading to the assessment of the valuable components present on the extract or fraction at the early stage.  
A workflow of NMR-based metabolomics and bioinformatics was utilized for the identification of novel pyranonaphthoquinones. The constitutive expression of the pathway-specific activator, _qin_ in _Streptomyces_ sp. MBT76 resulted in the activation of the biosynthesis of a cryptic type II PKS compound belonging to the family of pyranonaphthoquinones. The genomics analysis facilitated by structural evaluation identified the chemical structures belonging to the family of 8-C-glycosyl-pyranonaphthoquinones. The quinimycins A-C possessed structural features including a rare 5,14-epoxidation and unprecedented 13-hydroxylation in pyranonaphthoquinones. A deoxyaminosugar was unusually attached to the pyranonaphthoquinone backbone similar to BE-54238A and BE-54238B [84].  
The continued advances in the large-scale acquisition and analysis of metabolomic data has made it possible to thoroughly process complex metabolite samples leading to the discovery of novel small molecules. The metabolomics-based comparison of supernatants of _Streptomyces chartreusis_ cultivated in different media, using liquid chromatography-coupled with tandem MS, and further analysis by molecular networking, led to the characterization of diverse NPs, including the isolation of a new polyether ionophore, named deoxacalcimycin [85]. By employing genome mining the previously unreported capability of a marine sponge-derived isolate, namely _Streptomyces_ sp. SM17, to produce surugamide A was identified [86]. Similarly, the detailed metabolomic analysis of 4160 fractions obtained from 13 actinomycetes maintained under 32 different culture conditions was performed by \({}^{1}\)H NMR spectroscopy and multivariate analysis [87]. Metabolomics screening using liquid chromatography-high resolution mass spectrometry enabled the identification and purification of terrosamycin A and B, and two polycyclic polyethers that are active against breast cancer from the fermentation broth of _Streptomyces_ sp. RKND004 [88].