**Kathrin I. Mohr**

Microbial Drugs (MWIS), Helmholtz Centre for Infection Research (HZI), 38124 Braunschweig, Germany; kathrin.mohr@helmholtz-hzi.de; Tel.: +49-531-6181-4251

Received: 6 June 2018; Accepted: 8 August 2018; Published: 11 August 2018

**Abstract:** The discovery of new antibiotics is mandatory with regard to the increasing number of resistant pathogens. One approach is the search for new antibiotic producers in nature. Among actinomycetes, *Bacillus* species, and fungi, myxobacteria have been a rich source for bioactive secondary metabolites for decades. To date, about 600 substances could be described, many of them with antibacterial, antifungal, or cytostatic activity. But, recent cultivation-independent studies on marine, terrestrial, or uncommon habitats unequivocally demonstrate that the number of uncultured myxobacteria is much higher than would be expected from the number of cultivated strains. Although several highly promising myxobacterial taxa have been identified recently, this so-called Great Plate Count Anomaly must be overcome to get broader access to new secondary metabolite producers. In the last years it turned out that especially new species, genera, and families of myxobacteria are promising sources for new bioactive metabolites. Therefore, the cultivation of the hitherto uncultivable ones is our biggest challenge.

**Keywords:** myxobacteria; diversity; uncultured; secondary metabolites; new antibiotics
