Background: Thiol-disulfide oxidoreductases catalyze disulfide bond formation in extracellular proteins. Results: A new oxidoreductase, SdbA, affects multiple phenotypes in Streptococcus gordonii and is required for production of disulfide-bonded proteins. Conclusion: SdbA is a new type of oxidoreductase that has important biological functions. Significance: This is the first indication that thiol-disulfide oxidoreductases are important to the physiology of Firmicute bacteria.