Biocides inducing resistance in Lascaux cave’s microbes

Biocides used in recent years to treat the growth of a black fungus on the cave-art-festooned walls of France’s Lascaux cave have eradicated some populations of human-introduced bacteria and fungi. However, those that remain — including some related to known human pathogens — are becoming biocide-resistant, researchers report in an upcoming issue of Naturwissenschaften.

The new findings come from analyses of around a dozen samples taken from several areas in the cave between April 2006 and January 2007, says Claude Alabouvette, a microbiologist at the University of Bourgogne in Dijon, France. Some of the samples came from areas that were obviously infested with fungal colonies, and others were taken from cave walls that lacked such infestations, he notes.

Human presence has caused problems in Lascaux almost since its discovery in 1940. Lights added so tourists could see the cave art, images of bulls and other creatures believed to be painted more than 17,000 years ago, triggered the growth of algae. The heat from those lights — as well as tourists’ body heat and exhalations, the skin cells they shed and the microbes they unwittingly tracked in — changed the cave environment drastically, Alabouvette says.

In areas deep within the cave — ones that have been visited only rarely, says Alabouvette — samples showed that microbial diversity is low. But in oft-visited portions of the cave, especially those that were frequented by tourists before the cave was closed to the public in 1963, microbial diversity is high, he and his colleagues report. In some areas, bacteria from the genera Pseudomonas and Ralstonia — groups that include several microbes that cause disease in humans — make up more than half the organisms found in the samples.

In 2001, scientists discovered that a black fungus had infested parts of the cave and was threatening to cover the art. Recent biocide treatments have, for the most part, successfully halted the spread of the fungus in most areas of the cave. But because the dead fungus has been left in place, the melanin pigment that gives the fungus its color still remains on the wall.

The shift in bacterial diversity that the survey uncovered and the results of lab tests that hint that two of the Pseudomonas species isolated from the Lascaux samples have adapted to survive biocide treatment concern the researchers. Though the cave’s microbes aren’t a proven threat to human health, they could be in the future, the researchers speculate.

The fact that the biocide breaks down to form carbon- and nitrogen-containing compounds that bacteria and fungi could use as nutrients also troubles Alabouvette and his colleagues.

“Now we are wondering what is more dangerous, treating or not treating?” Alabouvette says.

Robert J. Koestler, director of the Smithsonian Institution’s Museum Conservation Institute in Suitland, Md., disagrees with the idea of not treating Lascaux’s fungal infestation. Doing nothing could allow the fungus to grow and produce more melanin, he says. “I think that’s the wrong thing to do,” he notes, because even if researchers come up with a way to kill the fungus, the melanin will still taint the cave walls.

Whenever people discover and begin to frequent a cave, their presence fundamentally changes its ecology, says John C. Moore, an ecologist at Colorado State University in Fort Collins. “It’s a big challenge to balance the public desire to enjoy caves with preserving the natural balance of the caves.”