Japanese researchers have good news for chocoholics. Not only can a bar of the stuff damp down cholesterol levels and raise the spirits, it can also help prevent tooth decay.

The researchers report in the Archives of Oral Biology that parts of the cocoa bean - chocolate's main ingredient - can thwart mouth bacteria. Streptococcus mutans bacteria make a sticky molecule, glucan.

They use this to anchor themselves to teeth and form plaque. With help from other bacteria, they convert sugars to acids which eat away the tooth's surface.

Chocolate is not as bad for teeth as many other sweets, perhaps because antibacterial agents in cocoa beans offset the sugar in chocolate.

Takashi Ooshima and colleagues at Osaka University found the husk of the cocoa bean produces even more of these antibacterial agents.

New Scientist magazine today reports that she and colleagues added husk extract to bacteria and stopped glucan production. The group plans to test it on human teeth.