High hydrostatic pressures greater than 200 MPa have been used for
sterilizing microorganisms for preserving food stuffs as a non-thermal
processing alternative [27–29]. Exponentially growing E. coli cells are
much more pressure-sensitive than stationary-phase E. coli, and the
loss of viability is accompanied by irreversible disruption of the cell
membrane, as measured by uptake of propidium iodide and loss of
osmotic response [30,31]. Casadei et al. showed that the heating profiles
of differential scanning calorimetry for lipids extracted from E. coli cells
clearly correlated with membrane fluidity index, expressed by the ratio
of (unsaturated fatty acids + cyclopropane fatty acid)/saturated fatty
acid, and that cells with fluid cell membranes are more resistant to
high pressures of 200–400 MPa [32]. Contrary to this observation,
Ulmer et al. demonstrated that Lactobacillus plantarum living cells
with a liquid-crystalline (fluid) membrane are more sensitive to a pressure
of 200 MPa than those with a gel-phase membrane, in terms of reduction
in HorA multidrug resistant transporter activity and the loss of
viability [33]. In their analysis, the phase state of the membrane was
determined by means of Fourier transform-infrared spectroscopy and
Laudan fluorescence spectroscopy. The authors showed that irreversible
pressure denaturation of HorA is faster if pressure is applied to
proteins embedded in a liquid-crystalline membrane compared to denaturation
in a gel-phase membrane. This is in agreement with our results
showing that the cell membrane of deep-sea piezophiles is rigid
and imperturbable to high pressure [22] (see below). Accordingly,
how the membrane phase or fluidity affects membrane protein functions
and cell viability may differ with respect to different organisms
and experimental systems, and hence there is no straightforward
direction.