Mechanisms of solvent tolerance in gram-negative bacteria.
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.
Ramos
Juan L
JL
Department of Plant Biochemistry and Molecular and Cellular Biology, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, E-18008 Granada, Spain. jlramos@eez.csic.es
Duque
Estrella
E
Gallegos
Maria-Trinidad
MT
Godoy
Patricia
P
Ramos-Gonzalez
Maria Isabel
MI
Rojas
Antonia
A
Teran
Wilson
W
Segura
Ana
A
D000222:Adaptation, Physiological; D001426:Bacterial Proteins / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D005227:Fatty Acids / Q000145:classification / Q000378:metabolism; D006090:Gram-Negative Bacteria / Q000187:drug effects* / Q000378:metabolism; D016623:Ion Pumps / Q000096:biosynthesis* / Q000737:chemistry / Q000235:genetics; D008565:Membrane Proteins / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000145:classification / Q000378:metabolism; D008957:Models, Genetic; D009930:Organic Chemicals / Q000378:metabolism / Q000633:toxicity; D010743:Phospholipids / Q000032:analysis / Q000737:chemistry; D010802:Phylogeny; D016958:Pseudomonas putida / Q000235:genetics / Q000378:metabolism; D012997:Solvents / Q000493:pharmacokinetics / Q000633:toxicity*
D001426:Bacterial Proteins; D002352:Carrier Proteins; D005227:Fatty Acids; D016623:Ion Pumps; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D009930:Organic Chemicals; D010743:Phospholipids; D012997:Solvents; C430335:TtgG protein, Pseudomonas putida; C430336:TtgH protein, Pseudomonas putida; C430337:TtgI protein, Pseudomonas putida