Microbial responses to solvent and alcohol stress

Abstract

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 produced 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, conventional fermentation strains have been shown to have limited catabolic versatility. However, catabolically versatile engineered strains and novel isolates engineered with ethanologenic pathways 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 review a number of the more common cell responses to solvents with particular focus on alcohol tolerance.

Analysis of the Pseudomonas putida CA-3 proteome during growth on styrene under nitrogen-limiting and non-limiting conditions

“…On the one hand, styrene represents a carbon source that can be metabolized to yield carbon and energy for growth. On the other hand, with an octanol/water partition coefficient [log(P OW )] of 3, styrene falls into a category of organic solvents extremely toxic to microorganisms, due to their ability to bind to the cells, disturbing membranes by removing lipids and proteins and causing cell lysis (Ramos et al, 2001;Taylor et al, 2008). It is well documented that solvent-tolerant pseudomonads possess three membrane-associated solvent tolerance mechanisms: (A) active efflux of organic solvents, (B) outer membrane changes and (C) cytoplasmic membrane changes (Volkers et al, 2006).…”

Optimization of Growth Medium and Enzyme Assay Conditions for Crude Cellulases Produced by a Novel Thermophilic and Cellulolytic Bacterium, Anoxybacillus sp. 527

“…However, catabolically versatile engineered strains and novel isolates engineered with ethanologenic pathways have subsequently been shown to exhibit limitations in ethanol tolerance, hindering their full potential as economically viable production strains [15]. Also, the overall metabolic energy burden of both ethanol fermentation and cellulase production in an energy-frugal anaerobe is just beginning to be evaluated [16,17].…”

Dissecting and engineering metabolic and regulatory networks of thermophilic bacteria for biofuel production

“…Industrial strains produce solvent products through fermentation, however they can be sensitive to their own solvent products (Taylor et al, 2008), which reduce cell vitality, impair membrane integrity, inhibit enzymes and/or perturb intracellular pH balance (Taylor et al, 2008;Timmons et al, 2009). Interestingly, the sensitivity to solvent stress is likely linked to temperature, as thermophiles can be less tolerant to a high-level of solvents such as ethanol than the mesophilic ethanologen such as Z. mobilis and S. cerevisiae (Timmons et al, 2009).…”

“…In mesophiles, the general solvent responses involve increased membrane fluidity, solvent exclusion systems, energy-dependent efflux pumps of the resistancenodulation-cell division (RND) family, stress-response genes (soxS, marA and robA encoding DNA-binding proteins/transcriptional activators), mannose transporter of the phosphotransferase system (manXYZ), ATPase, heat shock proteins (GroESL) and redox balance maintenance (Ma and Liu, 2010;Okochi et al, 2007;Ramos et al, 2002;Rutherford et al, 2010;Taylor et al, 2008;Tomas et al, 2004). In thermophilic bacteria, however, the modes of solvent-induced membrane alterations are different from those of the mesophiles.…”