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The expression of genes in a metagenomics library by a single host expression system is a widely used strategy in recent times. The potentiality of an expression host is determined by its ability to replicate the vectors containing inserted DNA fragments, impeding recombination, and conferring resistance to background gene products and lytic phages. Escherichia coli is the most favored host system for the expression of foreign genes. However, only 40% of the genes with functional activity have been reported to express in an E. coli host system [15]. The metabolic potential of most functional genes of remotely related microbes may not be sufficiently expressed [56]. This may also be attributed to the lack of an appropriate biosynthetic pathway substrate in a single host or to the fact that the host transcription machines may not recognize the sequence of promoters or favor the expression of foreign genes by limiting the essential factors [19]. However, some of the shortcomings of the E. coli system are rectified by augmenting it with plasmids equipped with an additional tRNA gene, simultaneous expression of chaperonin genes, etc. [76, 77].

In order to mitigate the limitations of a single host, multiple hosts are used to increase the likelihood of expression of targeted genes. Bacillus, Burkholderia, Sphingomonas, Streptomyces, and Pseudomonas [78] are the alternate host systems used for the functional screening of metagenomic libraries. Parallel screening using multiple hosts allows the successful expression of gene products from the Metagenomic Clone Library by providing a diverse host cell environment. Furthermore, a multiple host system permits the screening of metagenomic libraries for biosynthetic pathways that may remain undiscovered through a single host expression system. Despite all these limitations and prospects, functional screening using multiple hosts is one of the viable options in discovery of novel biosurfactants from oil contaminated environments.