Genetics of the Metabolism of Lactose and Other Sugars

Abstract

The practice of fermentation is a fairly simple, cheap and safe way to preserve raw materials of plant and animal origin that is performed all over the world. The predominant microorganisms used in these fermentations are the lactic acid bacteria (LAB) that metabolize a variety of sugars present in the raw material as an energy source. These LAB are added as starter cultures or are selected during the fermentation process and added to the quality and functionality of the fermented raw materials. Moreover, many LAB have an important function in the animal gastro-intestinal tract where they metabolize a variety of sugars (Hudson and Marsh, 1995; Vaughan et al., 1999). They belong predominantly to the genera Lactococcus, Lactobacillus, Leuconostoc, and Streptococcus. All have a long history of safe use in the manufacture of foodstuffs and their primary function is the production of lactic acid that preserves the product and inhibits the growth of both spoilage and pathogenic bacteria. The metabolic routes by which sugars are converted into lactic acid have been well established and include the classical Embden-Meyerhof pathway for the homofermentative and the oxidative pentose phosphate pathway for the heterofermentative LAB (Kandler, 1983). Especially the conversion of lactose in milk into lactic acid is one of the most large-scale industrial applications of the LAB. In view of the economic importance of this dairy produce, lactose metabolism has been the subject of considerable research aimed at understanding, and, more recently, exploiting the process involved. The history of the genetics of lactose utilization stretches back to the 1930s when researchers observed the loss of lactose metabolism in Lactococcus lactis, but it was the early 1970s before this was explained by the plasmid-located nature of the lactose genes (McKay, 1982).