A new antibiotic that selectively acts against Gram-negative bacteria have been discovered in the nematode, Photorhabdus. They named the antibiotic Darobactin. Darobactin is coded by a silent operon and has an unusual structure with two fused rings that are formed post-translationally. It is produced in very low quantity in laboratory conditions and is synthesized ribosomally. The Compound is active against Gram-negative bacteria both in vitro and in animal models of infection.
The last time we discovered a class of antibiotics against gram-negative was in the 1960s.
References: A new antibiotic selectively kills Gram-negative pathogens. Nature 2019