Group A streptococcus (GAS) is a human pathogenic bacteria. Many people carry GAS asymptomatically in their upper respiratory tract and other anatomic sites. However, these bacteria can cause a variety of human diseases ranging from superficial skin and throat infections to highly invasive life-threatening diseases such as toxic shock and necrotizing fasciitis, commonly known as the flesh-eating bacteria. Along with the consequences of autoimmune complications of rheumatic fever and rheumatic heart diseases, a conservative estimate of 500,000 deaths per year globally due to GAS infections has been calculated, placing this bacterium as one of the top 10 infectious causes of mortality.
Little is known about what controls the conversion of the bacteria from a non-harming form to the pathogenic state in GAS infections. Since these bacteria generally exist as communities and not as solitary microorganisms, bacterial communication systems are key elements in determining host-bacterial interactions. Most communication between bacterial cells is done by signaling molecules secreted and sensed by the bacteria. When the level of the signaling molecules is high enough, they can activate the expression of genes that coordinate their behavior. This activation only takes place in the presence of a sufficient number (a quorum) of bacteria, giving this mechanism the name quorum-sensing.