Streptococcus pyogenes

Streptococcus pyogenes, also called group A streptococcus, is a gram-positive bacterium, meaning it stains purple with the Gram stain. It is uniquely adapted to humans, and it most often causes infections of the upper respiratory tract, which includes the throat, and of the skin.

One of its weapons is its capsule, an outer coat made of hyaluronic acid, a substance our own bodies also make. Because it looks like ours, the immune system does not detect it. It is like someone who walks into a building wearing the same uniform as the staff and gets through the door without anyone stopping them. Another of its weapons is M protein, which blocks phagocytosis, the process by which white blood cells engulf microbes.

The diseases it causes run from one extreme to the other. There are noninvasive infections, such as pharyngitis, an infection of the throat, or impetigo, on the skin. It can also cause severe invasive infections, such as necrotizing fasciitis, where necrotizing means tissue-killing. And it can leave behind sequelae caused by the immune system itself after the infection, such as rheumatic fever.