Streptococci are among the predominant bacterial species living in the human body. They are normally harmless bacteria, but have the ability to cause diverse infections, ranging from mild (e.g., tooth decay and sore throat) to life-threatening (e.g., endocarditis and meningitis). Streptococci have evolved various means of coping with the deleterious effects of environmental stressors and avoiding the host immune system. Recently, several studies have shown that streptococci colonizing the mouth and upper respiratory tract are able to mount complex stress responses in order to persist and successfully survive competition in their ecological niche. Using a small quorum-sensing peptide pheromone acting as a stress-inducible 'alarmone', oral streptococci synchronize the gene expression of a specific group of cells to coordinate important biological activities.