Required for cytochrome c synthesis and stage V of sporulation. Might transfer reducing equivalents across the cytoplasmic membrane, promoting efficient disulfide bond isomerization of proteins localized on the outer surface of the membrane or in the spore coat. Cytochrome c oxidase activity is partially restored by mutations in bdbC or bdbD that are defective in the synthesis of disulfide bond containing proteins. The same mutations abrogate the need for CcdA in sporulation. Belongs to the DsbD family.