Q. Years ago in Key West, Fla., I bought a natural sponge as a souvenir. It has never developed mildew. Why?

A. Among the wealth of biologically active compounds in sea sponges are some enzymes that fight the growth not just of the fungus that causes mildew, but also molds and destructive bacteria.

Sponges also contain other substances that inhibit enzymes, leading researchers to hope that someday they will prove useful in treating diseases, like some cancers, linked to poor regulation of enzymes in humans.

Scientists are still debating whether some of these substances are produced by the sponge itself or by other beneficial bacteria that it harbors. At least two such potentially protective bacteria have been isolated from Theonella swinhoei, a sponge found in coral reefs in the Pacific and Indian Oceans. The researchers have proposed that they belong to a new phylum, which they called Tectomicrobia, meaning hidden microbes.

Bathers who use natural sponges can’t count on permanent resistance to funguses and are advised to let them dry thoroughly between uses.