C&EN: SCIENCE & TECHNOLOGY - PLUMBING THE OCEAN DEPTHS FOR DRUGS

ELIZABETH K. WILSON, C&EN WEST COAST NEWS BUREAU

The discovery of modern antibiotics, beginning with penicillin in 1929, was arguably the most significant medical advance of the 20th century and signaled what many believed would be the end of infectious disease. Since then, scientists have isolated dozens of important natural antibiotic compounds, such as streptomycin and vancomycin, mostly from threadlike soil microbes in the Actinomycete family.


	CULTURED The deep-sea organism Salinospora grows colorfully in the lab. PHOTO BY TRACY MINCER

As is all too well known, however, antibiotics are losing their effectiveness against increasingly resistant bugs. The rate of discovery of new soil microbes has slacked off, and researchers are turning to combinatorial techniques and molecular design to generate new disease-fighting compounds.

Until recently, scientists had largely ignored a vast source of potentially useful microorganisms: the ocean. But the discovery of a new genus of actinomycete bacteria, ubiquitous in deep tropical ocean sediments and a producer of anticancer and antibiotic compounds, may usher in a new era of natural pharmaceutical discovery.

Oceanography professor and Center for Marine Biotechnology & Biomedicine Director William H. Fenical and his colleagues at Scripps Institution of Oceanography in La Jolla, Calif., first found the microbe, which they've dubbed Salinospora, off the coasts of the Bahamas and in the Red Sea [Appl. Environ. Microbiol., 68, 5005 (2002)].

And the first compound Fenical's group has isolated from Salinospora, called salinosporamide A, not only has a never-before-seen chemical structure, but is also a highly selective and potent inhibitor of cancer-cell growth [Angew. Chem. Int. Ed., 42, 355 (2003)].

SEVERAL MARINE researchers, themselves hot on the trail of new ocean microbes, say they're excited by Fenical's work. "This is the first reported instance of a new genus of actinomycete from the marine environment that shows potent biological activity," notes Peter McCarthy, head of microbiology in the biomedical marine research division at Harbor Branch Oceanographic Institution in Fort Pierce, Fla.

"There's a tremendous amount to be discovered in terms of microorganisms that produce new natural product structures," adds David H. Sherman, microbiology professor at the University of Minnesota. "I think this is just the tip of the iceberg."

In addition to illustrating the ocean's potential as a new microorganism resource, Fenical's group has shown that Salinospora species requires a salt environment to live. This lays to rest a long-standing debate over whether true marine actinomycetes actually exist or whether some microbes have merely washed in from terrestrial sources.

Fenical, who has distinguished his long career by finding natural products in sea creatures, started looking for microbes in the ocean 10 years ago. Most marine natural products have been found in sponges and other marine invertebrates in relatively shallow waters. Fenical's breakthrough came with the development of technology that could sample the deep ocean sediments thousands of meters down.

"At one time, the pharmaceutical industry spent $9 billion a year going over the planet, getting sand and muds, but they've never looked at the ocean," Fenical says. Fenical's group first cultured the organism in seawater, then determined the sequence for its 16S ribosomal RNA gene. Carried by all bacteria, the 16S rRNA gene is often used as an identifier of genetic diversity. The results confirmed that Salinospora was indeed a unique genus.

THE SHEER NUMBERS of Salinospora the researchers are finding continue to startle them, Fenical says--especially considering that, unlike its terrestrial relatives, Salinospora thrives in hostile ocean-bottom conditions: no light, low temperature, and high pressure. The group has now identified Salinospora in five oceans, and--with 10,000 organisms per cm³ of sediment and several distinct strains in each sample--they've quickly been able to isolate 5,000 strains.

A great percentage of the cultures they've tested have shown anticancer and antibiotic activity. Salinosporamide A, the first Salinospora product studied, contains a two-ring structure that bears resemblance to that of clasto-lactcystin--lactone. Also known as ormuralide, the compound is a transformation product of lactacystin, which was isolated from a terrestrial Streptomyces bacterium. Salinosporamide A has unique features, though, including a chloroethyl group and a cyclohexene functionality.

"What Fenical has found is something the combinatorial chemists never would have dreamed of," says natural products chemist Thomas Hemscheidt, associate professor at the University of Hawaii, Honolulu.

Like ormuralide, salinosporamide A inhibits the proteosome, an intracellular enzyme complex that destroys proteins the cell no longer needs. Without the proteosome, proteins would build up and clog cellular machinery. Fast-growing cancer cells make especially heavy use of the proteosome, so thwarting its action is a compelling drug strategy.

A FEW OTHER reports of new marine microbes are starting to trickle in. Sherman's lab also has discovered a new marine bacteria that produces natural products in the oceans off of Papua, New Guinea, and the Solomon Sea. Thanks to Fenical's sequence of Salinospora's 16S rRNA gene, Sherman's lab has been able to show that its microbe is also unique. Sherman is preparing to publish his work on the organism, which hasn't yet been named.

McCarthy and his colleagues are also exploring the oceans for organisms that coexist with deep-water sponges. They've gathered 16,000 bacterial samples, which they use in their drug discovery program. A recent National Science Foundation grant will enable them to sort out the bacteria's taxonomy and hopefully discover new genera.

Fenical says he hopes the recent work will attract the attention of the pharmaceutical industry. He's already interested La Jolla, Calif.-based Nereus Pharmaceuticals, on whose scientific advisory board he sits. The company is licensing the Salinospora discoveries and patents.

A few barriers remain to unfettered natural marine product discovery. Less than one-tenth of a percent of organisms have been studied, researchers say, because they're difficult to culture. And the ocean can be a forbidding place. "There are vast areas of the marine environment that remain difficult or impossible to access," McCarthy says.

Still, hopeful marine microbe researchers say the future can only get more interesting.

"Once microbiologists begin to focus on the diversity of bacteria and fungi in the marine environment," Sherman says, "we're going to find all sorts of new and exciting things."