Abstract: A thermophilic microorganism lacks lactate dehydrogenase activity and preferably contains an active pyruvate formate lyase pathway. The thermophilic microorganism contains a gene encoding an NAD-linked formate dehydrogenase. The gene encoding an NAD-linked formate dehydrogenase is preferably a codon optimised version of the gene encoding a thermostable NAD-linked formate dehydrogenase. DNA constructs allow stable expression of the gene encoding an NAD-linked formate dehydrogenase in the thermophilic microorganism. The DNA constructs are based upon use of an insertion sequence to achieve stable expression or recombination to insert the gene encoding an NAD-linked formate dehydrogenase into the lactate dehydrogenase gene, thus achieving gene knockout and new functionality in a single step. The microorganisms are useful in fermentation of sugars to produce ethanol.