© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Biotechnol. J. 2008, 3 DOI 10.1002/biot.200800158 www.biotechnology-journal.com 1 Introduction The pursuit of suitable renewable alternatives to oil-based fuels has become a socio-economic pri- ority, starkly pertinent in South Africa at both the level of government and consumer because of dra- matic increases in the price of petrol at the pump and increases in the price of basic foodstuffs. The negative impacts of fossil fuel combustion on hu- man health and the local and global environments are well established [1, 2] and include the emission of sulphur oxide, nitrous oxide, carbon monoxide and carbon dioxide, potentially contributing to glo- bal warming [3, 4]. In consequence, there is significant interest in blending alcohols, particularly anhydrous ethanol and butanol, with gasoline, having the twofold ef- fect of limiting CO 2 and particulate emissions and reducing national dependency on fossil fuels. The best example of successful national transition from fossil fuel dependency to renewable fuel depend- ency in the transport sector is in Brazil where the national alcohol programme was responsible, in the 1996/97 season, for 273 million tons of har- vested (wet weight) sugar cane, leading to 13.7 mil- lion m 3 ethanol destined for transportation [5–7]. Internationally, the biological production of al- cohol for fuel supplementation has always focused on conventional fermentation using established or- ganisms (1st generation processes) such as Saccha- romyces cerevisiae [8, 9] and Zymomonas mobilis [10–12]. Such organisms have distinct advantages in terms of ethanol yields, high solvent tolerance and very well understood fermentations. However, a major drawback to these processes is that they Review Microbial responses to solvent and alcohol stress Mark Taylor 1,2 , Marla Tuffin 1 , Stephanie Burton 3 , Kirstin Eley 2 and Don Cowan 1 1 Institute for Microbial Biotechnology and Metagenomics (IMBM), University of the Western Cape, Cape Town, South Africa 2 TMO Renewables Ltd, Guildford, Surrey, UK 3 Department of Chemical Engineering, University of Cape Town, Cape Town, South Africa Increasing fuel prices and doubts over the long-term availability of oil are currently major global concerns. Such concerns have led to national policies and objectives to develop microbially pro- duced alcohols as fuel additives or substitutes. However, in South Africa this solution poses the further dilemma of sourcing a suitable fermentative carbohydrate that will not impact negatively on the availability of staple foods. The solution lies in the use of lignocellulosic materials, currently a waste product of the food and agriculture industries, which could be used in conjunction with a catabolically suitable production strain. In the pursuit of lignocellulosic biofuel production, con- ventional fermentation strains have been shown to have limited catabolic versatility. However, catabolically versatile engineered strains and novel isolates engineered with ethanologenic path- ways have subsequently been shown to exhibit limitations in solvent tolerance, hindering their full potential as economically viable production strains. A considerable volume of research has been reported on the general cellular mechanisms and physiological responses to solvent shock as well as adaptive changes responsible for solvent tolerant phenotypes in mutant progeny. Here we re- view a number of the more common cell responses to solvents with particular focus on alcohol tolerance. Keywords: Ethanol · Solvent · Tolerance · Biofuels · Fermentation Correspondence: Professor Donald Cowan, Institute for Microbial Biotech- nology and Metagenomics, University of the Western Cape, Bellville 7535, Cape Town, South Africa E-mail: dcowan@uwc.ac.za Received 1 August 2008 Revised 2 September 2008 Accepted 3 September 2008