Fluoridation causes an obvious reduction of dental caries by interference with cariogenic streptococci. However, the effect of fluoride on group A streptococci that causes rheumatic fever and acute poststreptococcal glomerulonephritis is not known. We have used proteomic analysis to create a reference proteome map for Streptococcus pyogenes and to determine fluoride-induced protein changes in the streptococci. Cellular and extracellular proteins were resolved by two-dimensional polyacrylamide gel electrophoresis and identified by matrix-assisted laser desorption ionization mass spectrometry. 183 protein spots were visualized, and 74 spots representing 60 unique proteins were identified. A 16-h exposure to sodium fluoride caused decreased expression of proteins required to respond to cellular stress, including anti-oxidants, glycolytic enzymes, transcriptional and translational regulators, and protein folding. Fluoride caused decreased cellular expression of two well-characterized S. pyogenes virulence factors. Fluoride decreased expression of glyceraldehyde-3-phosphate dehydrogenase, which acts to bind fibronectin and promote bacterial adherence. We also performed proteomic analysis of protein released by S. pyogenes into the culture supernatant and observed decreased expression of M proteins following fluoride exposure. These data provide evidence that fluoride causes decreased expression by S. pyogenes proteins used to respond to stress, virulence factors, and implicated in nonsuppurative complications of S. pyogenes, including glomerulonephritis and rheumatic fever.The incidence of some sequelae of group A streptococcal infection such as rheumatic fever and acute post-streptococcal glomerulonephritis (APSGN) 1 has decreased over the last five decades in the United States and western countries (1-3). However, these disorders continue unabated and are important public health problems in developing countries (1,3,4). The effects of fluoride on cariogenic Streptococcus mutans and other bacteria have been extensively studied. Fluoride inhibits enolase, a glycolytic enzyme (5, 6), inhibits F-ATPase activity resulting in less acidurance (7,8), reduces glucan-binding lectin activity (9), and decreases glucose incorporation (10). However, the effects of fluoride on group A Streptococci have not been examined. To examine simultaneous changes in multiple virulence factors, we performed a proteomic analysis of S. pyogenes exposed to fluoride. Western blotting and other immunological methods have been successfully used to study protein expression of various microorganisms, cells, and tissues. However, these techniques are constrained by the limited number of proteins that can be studied in each experiment and the availability of specific antibodies. Proteomic techniques date to 1975, when two-dimensional PAGE was simultaneously described by O'Farrell and Klose (11, 12) and applied to the study of a large number of proteins simultaneously. In twodimensional PAGE proteins are separated by differential isoelectric point (pI)...