Microbial strains and methods of making and using

A mutant heterodisulfide reductase-like protein subunit B (HdrB) polypeptide, wherein the mutant HdrB polypeptide comprises an amino acid sequence with at least 95% sequence identity to the amino acid sequence of SEQ ID NO:

• 3, wherein methionine at the amino acid corresponding to position 249 of SEQ ID NO:
• 3 is substituted with valine in the mutant HdrB polypeptide, and wherein when the mutant HdrB polypeptide is combined with a heterodisulfide reductase-like protein subunit A (HdrA) polypeptide and a heterodisulfide reductase-like protein subunit C (HdrC) polypeptide forms an HdrABC enzyme that has heterodisulfide reductase activity.