Global natural products social (GNPS) molecular networking is a useful tool to categorize chemical space within samples and streamline the discovery of new natural products. Here, we demonstrate its use in chemically profiling the extract of the marine tunicate Synoicum kuranui, comprised of many previously reported rubrolides, for new chemical entities. Within the rubrolide cluster, two masses that did not correspond to previously reported congeners were detected, and, following MS-guided fractionation, led to the isolation of new methylated rubrolides T (3) and (Z/E)–U (4). Both compounds showed strong growth inhibitory activity against the Gram-positive bacteria Bacillus subtilis, with minimum inhibitory concentration (MIC) values of 0.41 and 0.91 μM, respectively.

LCMS analysis of an extract of the New Zealand tunicate Synoicum kuranui showed evidence for numerous new rubrolides. Following a mass spectrometry-guided isolation procedure, new hydrated rubrolides V and W (5 and 6), along with previously reported rubrolide G (3), were isolated and characterized using MS and NMR. The anti-bacterial and cell cytotoxic activity of the compounds were compared to the potent anti-MRSA compound rubrolide A; hydration across the C-5/C-6 bond was shown to abrogate antibacterial activity.

Declining rates of novel natural product discovery and exponential rates of rediscovery heralded the end of the 1940s to 1960s “golden era” of antibiotic discovery. Fifty years later, the implementation of molecular screening methodologies revealed that standard culture-based screening approaches had failed to capture the vast majority of environmental bacteria and that even for the cultivable isolates only a small fraction of the biosynthetic potential had been tapped. A diversity of metagenomic screening and synthetic biology approaches have been developed to address these issues. The nonribosomal peptides have received particular focus, owing to their high levels of bioactivity and the predictability of the biosynthetic logic of the genetically encoded assembly lines that produce them. By uniting advances in next-generation sequencing and bioinformatic analysis with a diversity of traditional disciplines, several pioneering teams have proven that this previously inaccessible resource is no longer out of reach.