Bacterial microbes rely on chemical signals when invading human tissues. When their density is low in the bloodstream or other tissues and organs, each bacterium works as a single cell. However, when their numbers result in a higher density, bacteria work together in a collaborative fashion, often overwhelming the human immune system and leading to massive infection. This 'quorum sensing' system allows bacteria to ward off antibiotics that would otherwise kill individual microbes. Microbiologists and biochemists at the University of Wisconsin at Madison are working on methods that will block bacterial communication. At the American Chemical Society's annual conference, which was held in September 2006, Dr. Helen Blackwell revealed how the researchers are using acylated homoserine lactones (AHL) to block bacterial communication receptors. AHL is a substance that is secreted by many bacteria. When its concentration reaches high enough levels, AHL triggers behavioral changes in individual microbes, causing them to band together. At this stage, a protective layer known as a biofilm is formed from complex sugar molecules. It is this biofilm that causes the antibiotic resistance seen in many hospital-acquired, nosocomial infections. Dr. Blackwell's research group is screening and testing several hundred different AHL compounds to discover which ones are able to block certain microbes. The group was successful in a recent experiment involving cabbage white butterfly caterpillars that were infected with Pseudomonas aeruginosa. The group of caterpillars that were treated with AHL experienced a 50-percent survival rate, while nearly all of the caterpillars in the untreated group died.