Penicillin, the wonder drug discovered in 1928, works in ways that are still mysterious almost a century later. One of the oldest and most widely used antibiotics, it attacks enzymes that build the bacterial cell wall, a mesh that surrounds the bacterial membrane and gives the cells their integrity and shape. Once that wall is breached, bacteria die - allowing us to recover from infection. That would be the end of the story, if resistance to penicillin and other antibiotics hadn’t emerged over recent decades as a serious threat to human health. While scientists continue to search for new antibiotics, they still don’t understand very much about how the old ones work. Now Thomas Bernhardt, associate professor of microbiology and immunobiology at Harvard Medical School, and his colleagues have added another chapter to the story. Their findings, published Dec. 4 in Cell, reveal how penicillin deals bacteria a devastating blow - which may lead to new ways to thwart drug resistance.