The invention provides nrdF polynucleotides encoding nrdF polypeptides from Staphylococcus aureus and methods for producing such polypeptides by recombinant techniques. The invention also provides polynucleotides having high identity to the nrdF polynucleotides. Also provided are preferred methods for utilizing these polynucleotides and polypeptides as diagnostic reagents and in diagnostic assays to screen for microbial infections in organisms and materials.