New approaches to antimicrobial discovery.

The spread of resistant organisms is producing a human health crisis, as we are witnessing the emergence of pathogens resistant to all available antibiotics. An increase in chronic infections presents an additional challenge - these diseases are difficult to treat due to antibiotic-tolerant persister cells. Overmining of soil Actinomycetes ended the golden era of antibiotic discovery in the 60s, and efforts to replace this source by screening synthetic compound libraries was not successful. Bacteria have an efficient permeability barrier, preventing penetration of most synthetic compounds. Empirically establishing rules of penetration for antimicrobials will form the knowledge base to produce libraries tailored to antibiotic discovery, and will revive rational drug design. Two untapped sources of natural products hold the promise of reviving natural product discovery. Most bacterial species, over 99%, are uncultured, and methods to grow these organisms have been developed, and the first promising compounds are in development. Genome sequencing shows that known producers harbor many more operons coding for secondary metabolites than we can account for, providing an additional rich source of antibiotics. Revival of natural product discovery will require high-throughput identification of novel compounds within a large background of known substances. This could be achieved by rapid acquisition of transcription profiles from active extracts that will point to potentially novel compounds.