Handbook of Solvents - George Wypych - ChemTech - Ventech!

862 M Matsumoto, S Isken, JAMdeBont
14.4.1.3 Solvent-tolerant bacteria
As described in the previous section, the organic solvents with 1 < log PO/W < 4 are considered
to be toxic to microorganisms. In 1989, Inoue and Horikoshi 21 found a toluene-tolerant
Pseudomonas putida strain that grew in a two-phase toluene-water system (log PO/W = 2.5
for toluene). This finding was surprising and went against the dominant paradigm at that
time. Solvent tolerance was confirmed by other strains of P. putida 22-26 and by other representatives
of the genus Pseudomonas. 27-30 Furthermore, solvent tolerance has been found in
the strains of Gram-positive bacteria Bacillus 31,32 and Rhodococcus. 33 The key question now
is: How do solvent-tolerant bacteria overcome the toxic effects of organic solvents? Some
of the possible mechanisms involved in solvent tolerance according to various researchers
are shown in Fig.14.4.1.3. 32
Current research on changes in the structure of the cytoplasmic membrane shows the
involvement of: 1) the composition of the fatty acids of the phospholipids like the cis/trans
isomerization of unsaturated fatty acids; 2) composition of phospholipid headgroups and 3)
rate of turnover of membrane components.
Organic solvents cause a shift in the ratio of saturated to unsaturated fatty acids. 34,35 In
a solvent-tolerant strain, an increase in the saturation degree has been observed during adaptation
to the presence of toluene. Solvent-tolerant strains also have the ability to synthesize
trans-unsaturated fatty acids from the cis-form in response to the presence of organic solvents.
34,36-38 Increases in the saturation degree and the ratio of trans-form change the fluidity
of the membrane and the swelling effects caused by solvents are depressed.
Alterations in the headgroups of lipids during the adaptation to solvents have also been
observed in some solvent-tolerant strains. 37,39 The changes in the composition of the
headgroups cause changes in the affinity of the lipids with the organic solvents and in the
stability of membrane due to an alteration of bilayer surface charge density. These changes
compensate the effect caused by the solvents. In one strain, the rate of phospholipid synthesis
increases after exposure to a solvent. 40 This strain has a repairing system which is faster
than the rate of damage caused by the organic solvent.
Unlike Gram-positive bacteria, Gram-negative bacteria such as Pseudomonas have an
outer membrane. The outer membrane has been shown to play a role in the protection of the
cell from solvent toxicity. Ions such as Mg 2+ or Ca 2+ stabilize the organization of the outer
membrane and contribute to solvent tolerance. 38 Low cell surface hydrophobicity caused by
changes in the lipopolysaccharide (LPS) content has been reported to serve as a defensive
mechanism. 41,42 It has also been reported that the porins which are embedded in the outer
membrane are relevant to solvent tolerance. 37,42-44
The metabolism of organic solvents in solvent-tolerant strains contributes to solvent
tolerance by degradation of the toxic compounds. This contribution, however, is considered
to be limited 33,45 because many solvent-tolerant strains show non-specific tolerance against
various organic compounds.
Non-specific tolerance to toxic compounds is well known in the field of antibiotic resistance.
A wide range of structurally dissimilar antibiotics can be exported out of the cell
by multidrug-efflux pumps. Could the export of organic solvents contribute to solvent tolerance?
Isken and de Bont 46 conducted experiments to determine whether the solvent tolerant
Pseudomonas putida S12 was able to export toluene by monitoring the accumulation of 14 C
labeled toluene in the cells. Toluene-adapted cells were able to export toluene from their