Congeners of etamycin produced by Streptomyces griseoviridus

Activity of the streptogramin antibiotic etamycin against methicillin-resistant Staphylococcus aureus

The alarming rise of hospital- and community-associated methicillin-resistant Staphylococcus aureus (HA- and CA-MRSA) infections has prompted a desperate search for novel antibiotics. We discovered the streptogramin antibiotic, etamycin, for the first time from a newly discovered marine actinomycete and characterized its activity against a panel of HA- and CA-MRSA strains. Etamycin was extracted and purified from a previously uncharacterized marine-derived actinomycete, designated strain CNS-575, as a three-rotamer species as determined by two-dimensional nuclear magnetic resonance (NMR) spectroscopy. Etamycin demonstrated potent activity against hospital- and community-associated strains of MRSA in microbroth dilution assays, with minimum inhibitory concentrations (MIC) as low as 1 – 2 mg/L against HA- and CA-MRSA strains. Furthermore, etamycin was also active against other Gram-positive and several Gram-negative pathogens and was found to be non-cytotoxic at concentrations more than 20-fold above the MIC. Etamycin displayed favorable time-kill kinetics compared to the first-line MRSA antibiotic, vancomycin, and also conferred significant protection from mortality in a murine model of systemic lethal MRSA infection. These data emphasize the utility of the marine environment as a relatively untapped source of antibiotics against major drug-resistant human pathogens. These studies will also guide future isolation and preclinical development of depsipeptide anti-MRSA compounds from marine-derived actinomycetes.