8.3 Molecular Responses to Solvent Stress: Strategies for Living in Unpalatable Substrates

Abstract

This review article covers some general conclusions reached in research studies with respect to a number of solvent-tolerant Pseudomonas strains. The seminal observation was performed by Inoue and Korikoshi 1989 when they described in Nature a bacterium, belonging to the genus Pseudomonas, which was able to thrive in the presence of high concentrations of toluene. This property of extreme tolerance to solvents makes this Pseudomonas strain the first extremophile identified as able to survive in the presence of highly toxic solvents. Following this seminal observation, other solvent-tolerant strains, such as Pseudomonas putida DOT-T1E, S12, GM1, and MTB6, were described as able to grow in the presence of highly toxic solvents such as p-xylene (log Pow 3.15), styrene (log Pow 3.0), octanol (log Pow 2.92), and toluene (log Pow 2.69) (Aono et al. 1992; Cruden et al. 1992; Huertas et al. 2000; Isken and de Bont 1996; Kim et al. 1998; Ramos et al. 1995; Weber et al. 1994). These microbes can reach high cell densities in culture medium in the presence of 0.3% (v/v) of the mentioned solvents, a concentration that kills most of the microbes we work with in our laboratories. This solvent-tolerant trait allows us to consider this set of Pseudomonas strains as extremophiles. Normally solvent toxicity is due to these chemicals dissolving in the cell membranes, disorganizing them and altering the electron flow. They also prevent ATP synthesis and irrevocably provoke cell death. Currently, there is an inherent interest in deciphering the basis for survival and growth of extremophile microbes in such harsh conditions, particularly for the exploitation of these microbes in the bioremediation of heavily-polluted sites and in the biotransformation of water-insoluble compounds into added-value products. The growing interest in biodegradation derives from the fact that many pollutants are toxic above a certain threshold, killing many living organisms. The use of solvent-tolerant microorganisms, provided with an arsenal of enzymes to deal with pollutants, represents a powerful tool for in situ pollutant removal. In the case of biotransformation, many substrates of interest are poorly soluble in water and the use of a double-phase system, made of water and an organic solvent for the production of high added-value chemicals is of great interest in green chemistry. Among some examples we can cite the use of solvent-tolerant microorganisms to produce catechols or to carry out biotransformation of aromatic hydrocarbons including nitro substituted ones (Ju and Parales 2006; Neumann et al. 2005, 2006; Ramos-Gonza´lez et al. 2003; Ruhl et al. 2009; Rojas et al. 2004; Verhoef et al. 2009; Wierckx et al. 2005).