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The current need for novel antibiotics is especially acute for drug-resistant Gram-negative pathogens1,2. These microorganisms have a highly restrictive permeability barrier, which limits the penetration of most compounds3,4. As a result, the last class of antibiotics that acted against Gram-negative bacteria was developed in the 1960s2. We reason that useful compounds can be found in bacteria that share similar requirements for antibiotics with humans, and focus on Photorhabdus symbionts of entomopathogenic nematode microbiomes. Here we report a new antibiotic that we name darobactin, which was obtained using a screen of Photorhabdus isolates. Darobactin is coded by a silent operon with little production under laboratory conditions, and is ribosomally synthesized. Darobactin has an unusual structure with two fused rings that form post-translationally. The compound is active against important Gram-negative pathogens both in vitro and in animal models of infection. Mutants that are resistant to darobactin map to BamA, an essential chaperone and translocator that folds outer membrane proteins. Our study suggests that bacterial symbionts of animals contain antibiotics that are particularly suitable for development into therapeutics.

Background Bunaken National Park (BNP) is one of the most famous marine national parks in Indonesia with an extraordinary diversity in marine life forms. However, this diversity is threatened by an increasing population on the islands, ongoing destructive fishing techniques and lately by an increase in tourism. Protecting and managing the future use of BNP resources will require the assessment of both, local marine biodiversity through monitoring efforts and the identification and subsequent reduction of any threats or changes in the park. A high diversity in marine Heterobranchia indicates a high diversity of metazoan life forms and a diverse habitat structure. Surveying the complete biological diversity across taxonomic groups found in BNP would be an extensive undertaking, so focus on heterobranch diversity as an indicator of coral reef health was initiated and a model group on which future monitoring and conservation efforts can be based is provided. This study follows up the first investigation of marine Heterobranchia in BNP, conducted 12 years ago, while assessing molluscan diversity, and intends to present a base line for future monitoring programs. Results The diversity of marine heterobranchs around BNP was surveyed with an emphasis on Bunaken Island by diving and snorkeling at nearly 20 sites. Species are listed with photographic documentation (81 species) and results compared with the former study on molluscan species diversity in BNP. Taking these two studies into account 135 species are now recorded from BNP. The low overlap of described species (21) between the two BNP studies illustrates the gap of knowledge about overall species diversity in this particular area. A comparison with other studies from the region and Indo-Pacific also provides evidence for undersampling, but show similar taxa composition except of a somewhat higher cladobranch number in relation to Anthobranchia. Conclusions BNP is still under-sampled with regard to sea slug diversity. Thus conclusions as to whether or not a shift in species has occurred during the 12 years since the first study cannot be drawn. More and extensive studies are necessary to completely document the species richness in this area.