Architectural bacteria. Spain's limestone Puerta de las Granadas at the Alhambra and micrographs of stonework (insets) before and after treatment.

Can Microbes Save Historic Treasures?

Researchers have identified a strain of bacterium that may help preserve valuable, centuries-old stonework. The microbes are now being tested on Spain's 9th-century Alhambra palace to determine whether they can help conserve it.

Minerals such as limestone, dolostone, and marble are particularly susceptible to weathering and pollution because they are porous, exposing a large surface area to the elements. In recent years, scientists have tried using carbonate-producing bacteria to coat delicate stonework with a tough layer of calcium carbonate. But the newly deposited mineral often clogs stone pores rather than lining them, preventing the escape of moisture and accelerating decay. Now, a team led by mineralogist Carlos Rodríguez-Navarro at the University of Granada reports promising results from tests of an abundant soil bacterium, Myxococcus xanthus, on samples of limestone used extensively in historic buildings in Spain.

They found that the bacterium produces carbonate crystals that form a cement that binds tightly to existing calcite grains, lining the walls of the pores without plugging them. The newly deposited calcite matches the orientation of the existing crystals, and organic molecules that harden calcite make it even tougher than the original rock, the scientists report in the April issue of Applied and Environmental Microbiology.

"The beauty of the treatment is that the repair material is chemically identical to the original limestone," says Princeton University materials scientist George Scherer. Although a "natural" method for healing damaged stone would be a great advance, Scherer says, the calcite layer this bacterium deposits is shallow and thus still vulnerable to long-term damage.