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ABSTRACT: Lactic acid bacteria (LAB) are Gram positive bacteria, widely distributed in nature, and industrially important as they are used in a variety of industrial food fermentations. The use of genetic engineering techniques is an effective means of enhancing the industrial applicability of LAB. However, when using genetic engineering technology, safety becomes an essential factor for the application of improved LAB to the food industry. Cloning and expression systems should be derived preferably from LAB cryptic plasmids that generally encode genes for which functions can be proposed, but no phenotypes can be observed. However, some plasmid-encoded functions have been discovered in cryptic plasmids originating from Lactobacillus, Streptococcus thermophilus, and Pediococcus spp. and can be used as selective marker systems in vector construction. This article presents information concerning LAB cryptic plasmids, and their structures, functions, and applications. A total of 134 cryptic plasmids collated are discussed.
KEY WORDS: lactic acid bacteria, genetic engineering, cryptic plasmid, cloning vector, food-grade vector construction, hcterologous gene expression.
1. INTRODUCTION
Lactic acid bacteria (LAB) constitute a group of Gram-positive bacteria, including Lactococcus, Lactobacillus, Leuconostoc, Pediococcus, Streptococcus, and Bifidobacterium species that share the ability to ferment sugars primarily into lactic acid. Several species of LAB have a long history of use in the traditional production of fermented foods, beverages, and animal feed and are therefore generally recognized as safe (GRAS) (Davidson et al., 1996; Stiles and Holzapfel, 1997; Axelsson, 1998). LAB are also used as production organisms for various food ingredients such as preservatives, flavor compounds, gums and thickeners, and some cheese flavoring enzymes. Metabolites are secreted directly into the fermentation medium, which is quite advantageous from a technological standpoint. To maximize production, commercial strains can be genetically modified to improve inherent properties, introduce desirable characteristics and novel phenotypes, or remove unwanted traits.
While genetic engineering of LAB could have a great positive impact on the food and pharmaceutical industries, its progress could be impeded by legal issues related to the controversy surrounding this technology. The safe use of genetically modified LAB requires the development of food-grade cloning systems composed solely of DNA from the homologous host or GRAS organisms and that do not rely on antibiotic markers.
The rationale for the development of cloning vectors derived from LAB cryptic...