scrapped off their surrounding EPS (EPS-depleted cells) their O 2 evolution was
significantly reduced on desiccation treatment. Similar to the EPS-depleted cells,
another Nostoc commune KU002 had only small amount of EPS naturally present on
cells, which was found to be sensitive to desiccation stress. These findings suggest
the role of EPS in stress tolerance during desiccation and freeze thawing.
6.6
Cell Membrane Associated Changes
Low temperature exposure primarily affects the fluidity of lipid bilayer of the
bacterial cell membrane, which upon cold shock becomes rigid and impairs membrane associated functions such as transport, energy generation, and cell division
(Shivaji and Prakash 2010). At low temperature, the fluidity of bacterial cell
membrane decreases, and the maintenance of optimum membrane fluidity becomes
crucial for survival. To adapt at low temperature, it is crucial for the bacterium to
restore membrane function by increasing the fluidity of the membrane. Bacteria
modulate membrane fluidity using various strategies such as by altering the size and
charge of the polar head groups, by changing the proportion of short and long chain
fatty acids, by changing the extent of fatty acid desaturation, by changing the
proportion of cis and trans fatty acids, and by changing the composition of
carotenoids (Shivaji and Prakash 2010).
A predominance of C 15:0 was found in the fatty acid profile of food-borne
pathogen Listeria monocytogenes when cells were grown at 5
C, while two coldsensitive mutants were found to be deficient to synthesize C 15:0 and another
branched chain fatty acid C 17:0 . It is known that a switchover in the synthesis from
iso to anteiso fatty acid in bacteria is the CoA ester of 2-methylbutyric acid that is
derived from isoleucine. It was postulated that cold sensitivity of mutants might be
due to inability to produce 2-methylbutyric acid (Annous et al. 1997). In an Antarctic
strain of Pseudomonas syringae, decrease in membrane fluidity with concomitant
increase in the amount of saturated and trans monounsaturated fatty acids was
evidenced. However, the cti gene was found to be constitutively expressed in the
same organism irrespective of growth temperature implying that production of cistrans isomerase in this organism was post-transcriptionally regulated (Kiran et al.
2005).
Polyunsaturated fatty acid and fatty acyl lipid desaturases are essential for the
acclimation of cyanobacteria to low temperature (Murata and Wada 1995; Wada and
Murata 1990; Chintalapati et al. 2004). Cyanobacteria respond to low temperature
by increasing the level of the polyunsaturated fatty acid (C 18:3 (9, 12, 15) ) at the
expense of mono- and di-unsaturated fatty acids (C 18:1(9) ) and C18:2 (9, 12), respectively (Shivaji and Prakash 2010). In Gram-negative bacteria little is known about
the impact of low temperature on outer membrane. Outer membrane major component of Gram-negative bacteria is lipopolysaccharide. This has three components: a
polysaccharide that acts as an antigen, a hydrophobic membrane anchor known as
lipid A, and a core oligosaccharide that connects the antigen polymer to lipid A. The
lipid A moiety of LPS is of interest since cold shock alters the de novo synthesis of
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P. K. Mishra et al.
significantly reduced on desiccation treatment. Similar to the EPS-depleted cells,
another Nostoc commune KU002 had only small amount of EPS naturally present on
cells, which was found to be sensitive to desiccation stress. These findings suggest
the role of EPS in stress tolerance during desiccation and freeze thawing.
6.6
Cell Membrane Associated Changes
Low temperature exposure primarily affects the fluidity of lipid bilayer of the
bacterial cell membrane, which upon cold shock becomes rigid and impairs membrane associated functions such as transport, energy generation, and cell division
(Shivaji and Prakash 2010). At low temperature, the fluidity of bacterial cell
membrane decreases, and the maintenance of optimum membrane fluidity becomes
crucial for survival. To adapt at low temperature, it is crucial for the bacterium to
restore membrane function by increasing the fluidity of the membrane. Bacteria
modulate membrane fluidity using various strategies such as by altering the size and
charge of the polar head groups, by changing the proportion of short and long chain
fatty acids, by changing the extent of fatty acid desaturation, by changing the
proportion of cis and trans fatty acids, and by changing the composition of
carotenoids (Shivaji and Prakash 2010).
A predominance of C 15:0 was found in the fatty acid profile of food-borne
pathogen Listeria monocytogenes when cells were grown at 5
C, while two coldsensitive mutants were found to be deficient to synthesize C 15:0 and another
branched chain fatty acid C 17:0 . It is known that a switchover in the synthesis from
iso to anteiso fatty acid in bacteria is the CoA ester of 2-methylbutyric acid that is
derived from isoleucine. It was postulated that cold sensitivity of mutants might be
due to inability to produce 2-methylbutyric acid (Annous et al. 1997). In an Antarctic
strain of Pseudomonas syringae, decrease in membrane fluidity with concomitant
increase in the amount of saturated and trans monounsaturated fatty acids was
evidenced. However, the cti gene was found to be constitutively expressed in the
same organism irrespective of growth temperature implying that production of cistrans isomerase in this organism was post-transcriptionally regulated (Kiran et al.
2005).
Polyunsaturated fatty acid and fatty acyl lipid desaturases are essential for the
acclimation of cyanobacteria to low temperature (Murata and Wada 1995; Wada and
Murata 1990; Chintalapati et al. 2004). Cyanobacteria respond to low temperature
by increasing the level of the polyunsaturated fatty acid (C 18:3 (9, 12, 15) ) at the
expense of mono- and di-unsaturated fatty acids (C 18:1(9) ) and C18:2 (9, 12), respectively (Shivaji and Prakash 2010). In Gram-negative bacteria little is known about
the impact of low temperature on outer membrane. Outer membrane major component of Gram-negative bacteria is lipopolysaccharide. This has three components: a
polysaccharide that acts as an antigen, a hydrophobic membrane anchor known as
lipid A, and a core oligosaccharide that connects the antigen polymer to lipid A. The
lipid A moiety of LPS is of interest since cold shock alters the de novo synthesis of
194
P. K. Mishra et al.