Mechanisms of Solvent Tolerance in Gram-Negative Bacteria

Abstract

Organic solvents can be toxic to microorganisms, depending on the inherent toxicity of the solvent and the intrinsic tolerance of the bacterial species and strains. The toxicity of a given solvent correlates with the logarithm of its partition coefficient in n-octanol and water (log Pow). Organic solvents with a log Pow between 1.5 and 4.0 are extremely toxic for microorganisms and other living cells because they partition preferentially in the cytoplasmic membrane, disorganizing its structure and impairing vital functions. Several possible mechanisms leading to solvent-tolerance in gram-negative bacteria have been proposed: (a) adaptive alterations of the membrane fatty acids and phospholipid headgroup composition, (b) formation of vesicles loaded with toxic compounds, and (c) energy-dependent active efflux pumps belonging to the resistance-nodulation-cell division (RND) family, which export toxic organic solvents to the external medium. In these mechanisms, changes in the phospholipid profile and extrusion of the solvents seem to be shared by different strains. The most significant changes in phospholipids are an increase in the melting temperature of the membranes by rapid cis-to-trans isomerization of unsaturated fatty acids and modifications in the phospholipid headgroups. Toluene efflux pumps are involved in solvent tolerance in several gram-negative strains, e.g., Escherichia coli, Pseudomonas putida, and Pseudomonas aeruginosa. The AcrAB-TolC and AcrEF-TolC efflux pumps are important for n-hexane tolerance in E. coli. A number of P. putida strains have been isolated that tolerate toxic hydrocarbons such as toluene, styrene, and p-xylene. At least three efflux pumps (TtgABC, TtgDEF, and TtgGHI) are present in the most extensively characterized solvent-tolerant strain, P. putida DOT-T1E, and the number of efflux pumps has been found to correlate with the degree of solvent tolerance in different P. putida strains. The operation of these efflux pumps seems to be coupled to the proton motive force via the TonB system, although the intimate mechanism of energy transfer remains elusive. Specific and global regulators control the expression of the efflux pump operons of E. coli and P. putida at the transcriptional level.

Two naphthalene degrading bacteria belonging to the genera Paenibacillus and Pseudomonas isolated from a highly polluted lagoon perform different sensitivities to the organic and heavy metal contaminants

“…Abilities to degrade aliphatic and aromatic hydrocarbons are not necessarily mutually exclusive in bacteria (Churchill et al 1999). Bacteria that metabolize aromatic hydrocarbons face the challenge of acquiring carbon and energy from compounds that are potentially toxic (Ramos et al 2002;Sikkema et al 1995). The inability of potential biodegrading populations to tolerate aromatic hydrocarbon toxicity may contribute to the persistence of pollutants in the environment.…”

“…ORNaP1. A higher tolerance of Pseudomonas strains towards monoaromatic organic solvents has already been described and is based on the high adaptability of these bacteria mainly on the level of membrane composition as well as by active efflux pumps for organic solvents Ramos et al 2002).…”

Naphthalene metabolism and growth inhibition by naphthalene in Polaromonas naphthalenivorans strain CJ2

“…Bacteria that metabolize aromatic hydrocarbons face the challenge of acquiring carbon and energy from compounds that are potentially toxic (Ramos et al, 2002;Sikkema et al, 1995). The inability of potential biodegrading populations to tolerate aromatic hydrocarbon toxicity may contribute to the persistence of pollutants in the environment.…”

Ionic liquid-tolerant microorganisms and microbial communities for lignocellulose conversion to bioproducts

“…Microorganisms have been modified to address other challenges in biofuel production, including biofuel toxicity, which has been summarized in several reviews (Boyarskiy and Tullman-Ercek 2015;Dunlop et al 2011;Jones et al 2015;Mukhopadhyay 2015;Nicolaou et al 2010;Ramos et al 2002). (Bokinsky et al 2011).…”