The incidence of infections by drug-resistant bacteria continues to increase and remains a serious threat to human health. In 2007, the US CDC (Centers for Disease Control and Prevention) reported that the number of serious infections caused by methicillin-resistant Staphylococcus aureus (MRSA) alone was close to 100 000 a year, with almost 19 000 related fatalities, a number that is larger than the US death toll attributed to HIV/AIDS (human immunodeficiency virus/acquired immunodeficiency syndrome) in the same year.1 Marine natural products offer a rich source of poorly explored bioactive molecules that are of potential value in the development of new pharmaceutical agents.2 Marine bacteria in particular have proven to be a good source of small molecules, with the vast majority of compounds being discovered from chemically prolific Gram+ bacteria belonging to the order Actinomycetales (actinomycetes). Of the marine actinomycetes from which novel natural products have been reported, the obligate marine genus Salinispora has yielded an impressive array of structurally diverse compounds including salinosporamide A, which is currently being used in clinical trials for the treatment of cancer.3 These compounds are produced in species-specific patterns and include potent antibiotics in the rifamycin class, which are consistently observed in extracts of Salinispora arenicola.4 In an effort to discover new antibiotics from a large collection of S. arenicola strains, a rifampin-resistant strain of MRSA was used to screen extracts for antibiotic activities that are derived from compounds other than rifamycins.

The S. arenicola strain CNR-647 was isolated from a sample of the ascidian Ecteinascidia turbinata collected from a mangrove channel at Sweetings Cay, Grand Bahama Island (tropical Atlantic Ocean) in June 2002. The animal was washed with sterile seawater, ground with an alcohol-sterilized mortar and pestle. A volume of 1 ml of the resulting suspension was added to 4 ml of sterile seawater, heated at 55 °C for 6 min, and 50 μl of the resulting suspension was plated onto seawater agar containing 5 μg ml−1 rifamycin and 100 μg ml−1 cycloheximide. The strain was identified as S. arenicola on the basis of the 16S rRNA sequence analysis (GenBank accession number FJ887039).

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