transfer holds much promise for improved dairy strains. Genetic engineering techniques allow directed changes in existing traits, exchange of traits (protease, bac-teriocins, carbohydrate metabolism) among closely related strains or species, and introduction of foreign genes from unrelated strains (Sanders, 1991). Methods for introducing genes into a new strain include conjugation, transformation, and transduction.
Although consumption of lactic acid bacteria modified by deletion of genetic information or other self-cloning procedures might not influence potential hazards of consuming fermented milk products, safety concerns should still be taken into account. These include possible interactions with the foodborne pathogenic microorganisms as well as possible influences on process technology and on nutritional value and allergenic potential of products (Klein et al., 1995).
Among all lactic starter species, mesophilic lactococci are the most studied host system. Table 9 provides references on the significant genetic modifications applied to dairy starter species.
Bacteriophage infection is the single most important cause of slow acid production in dairy fermentations. Consequently, there has been a worldwide research effort focusing on transferring of different phage resistance traits to improve dairy starter culture performance in the presence of industrial phage. However, genetically engineered defense strategies (nonnatural) in lactic acid bacteria suffer the weakness of being highly specific. At present, conjugal transfer of naturally occurring plasmids is the only accepted approach for genetic improvement of starter cultures, although food-grade cloning systems may be considered in the future (Coakley et al., 1997). In lactococci, there is a genetic linkage between phage resistance and bacteriocin production. Phage resistance transconju-gants were identified by their ability to ferment lactose and their resistance to the produced bacteriocin (Coakley et al., 1997). Phage resistance is associated with the sucrose-nisin transposon in lactococci (Gireesh et al., 1992).
Although genetic information presently available allows construction of tailor-made genetically modified lactic acid bacteria, the use of genetically engineered starter cultures will depend on cost of strain development, regulations, and consumer acceptance. Although, recombinant DNA technology provides a potential improvement over the classic methodology of selection, mutation and strain screening will continue in the immediate future. See Chapter 8 for further discussion of genetics of lactic acid bacteria.
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