Sea creatures have long been screened for medically useful chemicals. Now US researchers say that the microbes that live on these animals might be a more fruitful place to look for new drugs.

Haiyin He of the pharmaceutical company Wyeth-Ayerst in Pearl River, New York, and colleagues have found a bacteria that produces an anti-cancer agent on the body of the Fijian sea squirt Polysyncraton lithostrotum.

The researchers were looking for the source of the chemical namenamicin, thought to come from the sea squirt. Namenamicin damages DNA, so can wreak havoc amongst cells that multiply rapidly, such as cancer cells. The chemical has shown some promise in treating tumours.

Many living organisms, especially sea creatures, make a wide range of poisons, and these are constantly being screened for possible use as anti-cancer agents. Sea squirts - a group of marine invertebrates - produce a range of nasty chemicals, some of which have pharmaceutical potential.

Namenamicin resembles the calicheamicins, chemicals made by actinomycetes bacteria that are also being studied as anti-cancer drugs. So He's group suspected that namenamicin might not be made by the sea squirts at all, but by bacteria living in or on their bodies.

When the researchers extracted actinomycetes from sea squirts, and grew them in fermentation vats, they discovered a previously unidentified strain that generates a substance highly damaging to tumour cells. They name the bacteria Micromonospora lomaivitiensis and its two related active agents lomaiviticin A and B.

These compounds also act as antibiotics, and may be produced by the actinomycete cells to ward off other bacteria. Lomaiviticin kills Staphylococcus aureus, one of the virulent, antibiotic-resistant bugs that causes infection and death in surgical treatments.

So although the researchers failed to identify the source of namenamicin, they stumbled across another class of potentially powerful drugs.

The lomaiviticins will need to pass through many hoops before they can be proclaimed clinically useful. Nonetheless, their discovery suggests that researchers might profitably search for new pharmaceuticals, not in squishy sea creatures, but in the microscopic bacteria that live on them.