Future Trends And Potentials

In the foreseeable future, ME will continue to play a central role in the use of bacteria to produce food ingredients. One potential area for the application of ME is the removal of undesirable sugars present in many foods. A typical example is lactose, which is present in liquid dairy products. Many people are lactose-intolerant and cannot consume products in which lactose is present. Engineering LAB to obtain starter cultures with elevated ^-galactosidase activities has been proposed as a way to eliminate lactose (50). Raffinose is another example. Due to the absence of a-galactosidase, it cannot be degraded intestinally by humans. Consuming it causes intestinal disturbances. Engineering LAB to produce high levels of a-galactosidase and using it as starter cultures for removing raffi-nose has been proposed as a good solution to this problem (50). As a first step, the gene melA encoding a-galactosidase from Lactobacillus plantarum has been cloned (69) and is being expressed in Lac. lactis (50).

Energy metabolism is beginning to attract attention as a new target for ME, especially for engineering central metabolic pathways and transport systems. It was recently shown that an increase in ATP hydrolysis results in an increase in the glycolytic flux of E. coli (~70%), indicating that glycolytic flux is mainly (>75%) controlled by reactions hydrolyzing ATP (37). Therefore, one could engineer energy metabolism in order to improve the synthesis of products, such as organic acids, for which the glycolytic flux is a determinant (i.e., the introduction of an independent ATP "sink" would increase the glycolytic flux and the synthesis of the organic acid). One could even think that a similar approach will also be valid for synthesizing secondary metabolites whose precursors are provided by the glycolytic pathway. A second approach points toward a completely different direction, the creation of an energy surplus (in contrast to the energy "sink" created in the previous approach). For example, let us consider that the excretion of a certain product P (e.g., an amino acid) is an energy-requiring process. If a condition of energy surplus is created, cells will use any energy consuming process to dissipate the excess of energy, and therefore the secretion of product P will be stimulated.

The existence of sequenced genomes for several bacteria involved in producing food ingredients (e.g., Lac. lactis, C. glutamicum, B. subtilis, E. coli) will dramatically impact ME opportunities. Once genome sequences are available, technologies such as cDNA microarrays can be used to simultaneously monitor and study the expression levels of individual genes at genomic scale (known as the field of transcriptomics). This technology has already played an important role in understanding bacterial metabolism under a great variety of conditions, and in the improvement of numerous bacterial strains (70-74). However, the main contributions from the area of functional genomics are still to come and will result from the combined use of technologies that permit large-scale study of cell functioning and its interaction with the environment (i.e., combined analysis using genom-ic (DNA), transcriptomic (RNA), proteomic (protein), metabolomic (metabolites), and fluxomic (fluxes/enzymes) information). The era of functional genomics will contribute to an improved understanding of the molecular mechanisms underlying the relationships between microorganism and environment and in that way will establish the link between genotype and phenotype. Thus, in the future, functional genomics is expected to play a large role in the development of bacterial strains for the food industry.

You Are What You Eat

You Are What You Eat

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