Streptococcus pyogenes is a globally prominent bacterial pathogen that exhibits strict tropism for the human host， yet bacterial factors responsible for the ability of S. pyogenes to compete within this limited biological niche are not well understood. Using an engineered recombinase-based in vivo expression technology (RIVET) system， we identified an in vivo-induced promoter region upstream of a predicted Class IIb bacteriocin system in the M18 serotype S. pyogenes strain MGAS8232. This promoter element was not active under in vitro laboratory conditions， but was highly induced within the mouse nasopharynx. Recombinant expression of the predicted mature S. pyogenes bacteriocin peptides (designated SpbM and SpbN) revealed that both peptides were required for antimicrobial activity. Using a gain of function experiment in Lactococcus lactis， we further demonstrated S. pyogenes immunity function is encoded downstream of spbN. These data highlight the importance of bacterial gene regulation within appropriate environments to help understand mechanisms of niche adaptation by bacterial pathogens.