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Thermophiles and their bioproducts
Scientific interest in thermophiles can be divided into three main endeavours: (i) the general isolation, characterization and exploration of thermophilic life and the boundaries defining its limits, (ii) the physiological and biochemical characterization of the various adaptive mechanisms required for microbial survival at high temperatures, and (iii) the characterization and development of thermostable biocatalysts/bioproducts. Advances in the first two areas have been well summarized elsewhere (Gerday and Glansdorff, 2007; Robb et al., 2008) but developments in thermophilic whole‐cell biocatalysts have, with a few exceptions, been modest.
Although many enzymes from thermophilic organisms have reached full commercialization (the best‐known examples being a number of polymerases such as Taq and Pfu) the general commercial approach to the bulk production of commercial thermostable enzymes has been to engineer thermostability rather than seek it from a thermophilic organism/source material (Haki and Rakshit, 2003). This approach is perfectly sound and perhaps has arisen because of the greater mesophilic diversity in genomic databases. It has resulted in a wide range of thermostable enzyme mutants that are applicable across a broad range of biotechnological targeted markets such as the food, feed and textile industries (Turner et al., 2007).
The industrial use of thermophilic whole‐cell biocatalysts has been widely anticipated, but has largely remained undeveloped. Some advantages in the use of thermophiles as whole‐cell biocatalysts are: (i) for anaerobic strains, high‐temperature fermentations retain anaerobic status more readily, (ii) thermophiles may have lower sensitivity to organic solvents, (iii) there may be a reduced risk of contamination and (iv) the ability to operate at elevated temperatures allows the chemistry of some processes to be ‘accelerated’ (Zeikus et al., 1981). Drawbacks include the technical challenges of high‐temperature culturing and differences in codon usage and folding processes which may lead to low levels of expression or recombinant enzymes with reduced or no activity (Haki and Rakshit, 2003; Turner et al., 2007). Process economics may also be exacerbated through associated heating costs.
A major barrier to development of thermophilic biocatalysts has been the general inability to genetically modify the parent strains. Until recently there has been a dearth of reliable methods for inducing competence, genetic material transfer, gene expression and genome...