Sugar-Coated Killer

The notorious "flesh-eating" bacterium appears to sweet-talk its way deep into vulnerable tissues. A report in the 7 November issue of the Journal of Clinical Investigation suggests that a coating of complex sugars may help vicious strains of streptococcal bacteria cause life-threatening infections. The finding may lead to a test someday that would help doctors identify strains likely to cause the severe infections.

Streptococcus can cause mild infections--witness strep throat--or horrifyingly invasive infections that turn tissue into soup. Researchers have long suspected that a coating made of a complex carbohydrate called hyaluronic acid influences the severity of strep infections. Michael Wessels and his colleagues at Harvard University and Brigham and Women's Hospital in Boston found that it does--but in an unexpected way.

They found that, as predicted, a strep strain genetically engineered not to manufacture the hyaluronic acid capsule failed to cause invasive infections in mice, while the encapsulated bacterium did. But when they exposed cultured human skin cells to the bacteria, they found that only the naked bacterium penetrated the cells. And that's what defeated the sugar-free bugs: The skin cells quickly killed the invaders. The sugar-coated bacterium, meanwhile, did not enter the skin cells and therefore escaped, which could explain how invasive strep slips into deeper tissues.

The finding is a step toward a better understanding of the molecular interactions between Streptococcus and its hosts, says James Musser, a molecular pathobiologist at Baylor College of Medicine in Houston. Indeed, it seems to dovetail with Musser's own work; he studies a strep toxin that appears to do its damage in tissues outside individual cells. The toxin sets off a chemical reaction that melts the protein bonds between cells and ``essentially causes the host tissue to eat itself,'' Musser says.

Both scientists say potential treatments that target the sugar coating are a long way off. But ``this does give us a better insight into how these organisms produce disease,'' says Wessels. ``And that's always the first step in figuring out something to do about it.''

Posted in Biology, Health