Considerable information currently exists on many of the important genes and metabolic pathways that influence how lactic acid bacteria grow in yogurt, cheese, and other dairy products. Recently, the genome sequence of Lc. lactis was reported (Bolotin et al., 1999), and the genome sequence of Lb. acidophilus is expected to be completed soon. Efforts to use this information to improve or modify properties of lactic acid bacteria have already begun and are certain to be accelerated (Hugenholtz and Kleerebezem, 1999). As described earlier in this chapter, metabolic engineering could be used in several ways to improve dairy fermentations. Diverting pyruvate from lactate to the flavor compound diacetyl can be accomplished by genetically disrupting genes coding for lactate dehydro-genase or a-acetolactate decarboxylase. Similarly, cheese ripening can be accelerated by either increasing expression of genes involved in proteolysis or by induced expression of genes coding for lytic enzymes (de Ruyter et al., 1998; McGarry et al., 1994). Increased synthesis of an exopolysaccharide by Lc. lactis subsp. cremoris was achieved by overexpressing the gene coding for fructose-bisphosphatase, an enzyme that makes more precursors available for polysaccha-ride synthesis (Looijesteijn et al., 1999). Finally, efforts are underway in several laboratories to engineer S. thermophilus so that galactose is fermented rather than released back into the curd or cheese.
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