By complementing a non-fermentative Escherichia coli (ldhA(-) pflB(-)) strain with the recombinant Zymomonas mobilis ethanol pathway (pdc, adhB), we evaluated the effect of different levels of enzymatic activity on growth rate demonstrating that there is a direct relationship between anaerobic growth rate and the total specific activity of pyruvate decarboxylase, which is the limiting enzyme of this specific fermentative NAD(+) regenerating pathway. This relationship was proved to be useful to establish a selection strategy based on growth rate for the analysis of lctE libraries, which encode lactate dehydrogenase from Bacillus subtilis.