Multiphase Bioreactor Design

Figure 5.2 Relationship between

solvent Log P value and residual

activity measured in aqueous-organic

solvent reaction mixtures for steroid

∆^1 -dehydrogenation by free ( ) and

immobilised (∆) Arthrobacter simplex

and for naphthalene oxidation by free

( ) and immobilised ()

Pseudomonas putida. Redrawn from

Harrop et al. (1992).

At a fundamental level, a considerable amount of work has addressed the issue of how
micro-organisms respond when exposed to an organic solvent and the features that
characterise “solvent-tolerant” strains. The toxic effect of solvents is now known to be
due to their interaction with the cytoplasmic membrane: the degree of toxicity being
proportional to the solvent concentration in the membrane itself which, in turn, is
proportional to the solvent Log P value. Solvent-resistant bacteria typically possess a
number of physiological mechanisms to counteract the presence of solvents in the
cytoplasmic membrane (de Bont, 1998). These mechanisms include the rapid synthesis of
trans-unsaturated fatty acids to alter membrane fluidity and the active pumping of
solvents out of the membrane.

Choice of Phase Ratio

The amount of organic phase present in the medium is commonly characterised by the
phase volume ratio (organic volume/aqueous volume). The phase volume ratio affects
both the reactor concentration of water-soluble and poorly water-soluble organic
components and the degree of reduction or elimination of substrate or product inhibition.
The ease or otherwise of pH control at different phase volume ratios is a particular issue

Multiphase bioreactor design 128