Two new strains of Escherichia coli B were engineered for the production of lactate with no detectable chiral impurity. All chiral impurities were eliminated by deleting the synthase gene (msgA) that converts dihydroxyacetonephosphate to methylglyoxal, a precursor for both L(+)- and D(-)- lactate. Strain TG113 contains only native genes and produced optically pure D(-)- lactate. Strain TG108 contains the ldhL gene from Pediococcus acidilactici and produced only L(+)-lactate. In mineral salts medium containing 1 mM betaine, both strains produced over 115 g (1.3 mol) lactate from 12% (w/v) glucose, > 95% theoretical yield.