Bacteria may hide in hunks of gunk.

Processed sludge from waste treatment plants that is sold as fertilizer for home gardens may harbor disease-causing fecal organisms, contends microbiologist David L. Lewis of the Environmental Protection Agency in Athens, Ga.

To test his suspicion that fats and petroleum products in sludge interfere with assays for bacteria, Lewis mixed bacteria from sludge with a silicone lubricant. When he analyzed the resulting mixture for colonies of live bacteria, he found few. However, when he dissolved the silicone with acetone and tested again, he found 100,000 times as many colonies.

Lewis suggests that bacteria such as Salmonella get caught in clumps of gunk in the sludge. If the clumps get coated with water-repellent substances like chicken fat, petroleum, or industrial lubricants, standard tests may significantly underestimate the number of bacteria hidden inside them, he says. If people accidentally ingest clumps sticking to unwashed hands or vegetables, he says, the acids and churning action of the digestive tract would expose the bacteria.

Although EPA regulates the methods used to decontaminate and test sludge, Lewis remains "skeptical as to whether the regulations protect us or not." Alan B. Rubin of EPA's Washington, D.C., office says sludge used as fertilizer does not make people sick when applied according to the regulations.

The agency should use Lewis' methods to test sludge certified as free of pathogens to see whether the bacteria indeed escape detection, says Ellen Z. Harrison of the Cornell Waste Management Institute in Ithaca, N.Y.