Germs cause rust.

A few scientists have known this for years but, until recently, knowledge of the role that bacteria and other microbes play in metal corrosion has not been appreciated by materials engineers and others who design critical metal objects such as bridges, oil and chemical pipelines and containers for toxic materials.

As these and other objects corrode, they cause an estimated $167 billion worth of damage each year in the United States.

At a conference last month at the National Bureau of Standards in Gaithersburg, some of the top experts in biocorrosion met and concluded that much more research is needed to understand how microorganisms induce and accelerate corrosion and how to prevent it.

Corrosion is a chemical process in which atoms in a metal surface break free and bind with nearby nonmetal atoms to form a combination that remains separate from the metal. Iron, for example, binds readily with oxygen, especially in the presence of water, to form iron oxide, or rust. When enough metal atoms have broken away, the metal is weakened.

When certain combinations of common bacteria colonize a steel surface, a British researcher told the conference, the rate of corrosion becomes up to four times faster.

The exact mechanism is not clear, but it appears to be the result of substances secreted by the microbial colony growing on the metal.