Cancer tends to stick around because it’s practically invisible to the body’s own defenses: The immune system doesn’t recognize the rogue cells since they aren’t foreign invaders. To activate the immune system to attack cancer, scientists have tried all sorts of tricks, including infecting cancerous tissue with bacteria. Now, scientists have modified Salmonella bacteria to trigger a particularly powerful immune response against human cancer cells implanted in mice, shrinking the tumors and—for the first time—preventing them from metastasizing. If the technique can be replicated in humans, it would be a significant step forward for the field of bacterial cancer therapy.
“This team did very solid work, very rigorous,” says Roy Curtiss III, an infectious diseases researcher at the University of Florida in Gainesville who has pioneered similar bacterial techniques to combat cancer.
Because bacteria often home in on necrotic, oxygen-depleted tissue—present in most solid tumors—scientists can easily “target” cancerous tissue with the microbes. Only one such treatment has so far received FDA approval (a therapy to treat bladder cancer), though others are in the pipeline. But even with the most effective of these techniques, tumors tend to come back and the bacteria themselves can be toxic.
Enter Salmonella, a rod-shaped microbe notorious for causing most cases of food poisoning. In 2006, researchers at Chonnam National University in Gwangju, South Korea, were looking to create a new cancer-fighting agent. They were also searching for a vaccine for the bacterium Vibrio vulnificus, which infects shellfish off the South Korean coast. As they worked with Vibrio, they noticed that a protein in its flagellum—a whiplike tail used for swimming—triggered a particularly strong response from immune cells. So they took a harmless version of Salmonella typhimurium and “weaponized” it, genetically modifying it to secrete the protein, known as FlaB.