Method of selecting a polypeptide of interest

A method of identifying a variant polypeptide of interest, or its encoding polynucleotide, the method comprising:

• (i) generating a plurality of polynucleotides encoding a plurality of variant polypeptides comprising the variant polypeptide of interest;
• (ii) expressing the plurality of variant polypeptides in an obligate or facultative anaerobe that is incapable of, or displays a reduction in, the oxidation of NADH and/or NADPH under anaerobic fermentation conditions;
• (iii) culturing, in growth media, the obligate or facultative anaerobe under anaerobic fermentation conditions, wherein the anaerobic fermentation conditions comprise the absence of an external electron acceptor, in the presence of a substrate, wherein the polypeptide of interest enables the obligate or facultative anaerobe to oxidise, or to increase oxidation of, NADH and/or NADPH in the presence of the substrate;
• (iv) selecting an obligate or facultative anaerobe that grows or displays a growth advantage in the growth media;
• and
• (v) identifying the variant polypeptide of interest expressed, or its encoding polynucleotide, in the obligate or facultative anaerobe of step (iv);
• wherein the substrate is exogenously added to the growth media.