The human immune system is an astonishingly sophisticated defense system, but the bacteria and viruses that plague us have found some very clever ways to hide from it. The most recently discovered example is seriously metal; streptococcal bacteria use pieces of red blood cells they have torn apart as camouflage to hide from avenging immune cells.

Group A Streptococcus (GAS) bacteria is most familiar as the cause of “Strep throat”; which while unpleasant is seldom serious. However, the same group of bacteria sometimes invade the blood, lungs or muscles. In people with weakened immune systems or who are already fighting off other conditions, this can be lethal. An estimated half a million people die each year as a result, mostly in Africa.

Dr David Gonzalez of the University of California, San Diego has been exploring the methods GAS uses to evade the immune system. GAS cells release molecules that attach to red blood cells of the animal it has infected. Some go further, slicing these cells up like horror film serial killers.

Gonzalez noticed one of these molecules, named the S protein, appears to have many functions, one of which is binding the bacteria to the tattered remains of destroyed blood cells. After initial puzzlement, Gonzalez and his team wondered if they faced the Arya Stark of microbiology, hiding behind the faces of its victims. After all, the immune system wouldn't want to attack something that looked like blood cells.

The researchers modified the bacteria, removing the gene that codes for the S protein and discovered the immune cells have no difficulty finding, and destroying, the exposed invader in vitro. Likewise mice are killed by injections of the wild-type GAS, but show no symptoms when infected with the modified strain. Their findings are published in Cell Reports. 

The disguise is not just at a microscopic level. When placed in human blood, ordinary GAS bacteria turn so red they become hard to spot for scientists peering down the microscope. The modified version, however, just goes slightly pink.

GAS is currently treated with penicillin. Although it hasn't evolved resistance at anything like the speed of other pathogens, the rate of treatment failure is rising. Gonzalez hopes S protein-blocking medication could be the answer.

Better still, the mice injected with the modified version of GAS produced a strong immune response to the bacterium and vaccinated the mice against subsequent infections with the ordinary variety.

The usual disclaimers about mouse research not always translating to humans apply, and the experiments were done with only one of the many species of Streptococcus A, so a lot more work is required before we can vaccinate ourselves against GAS by removing its ill-gotten clothes.

[H/T: Smithsonian Mag]