Despite the fact that S-layer proteins exist in Gram-positive and Gram-negative
bacteria [126], studies made so far on high pH adaptation role of S-layer proteins
have been restricted to Gram-positive bacteria. Hence, relatively, little is known
about the contribution of Gram-negative S-layer proteins to high pH adaptations.
Scanning electron microscopy analysis of the surface of a Gram-negative bacterium,
Pseudomonas alcaliphila, revealed that cells grown at pH 10 have rougher surface
than those grown at pH 7 [127]. This might show the possibility that Gram-negative
strains also make some surface depositions (S-layer proteins) to thrive in high pH
environments. However, this needs specific experimental evidences. Further studies
on other S-layer proteins of Gram-positive bacteria and probably other cell surface
deposited proteins of alkaliphilic Gram-negative and Gram-positive bacteria may
improve our understanding of these interesting proteins contribution to high pH
adaptation.
3.2.4 Cell Membrane
The other component of the cell envelope that contributes to high pH adaptation is
the cell membrane. The contribution of the outer membrane of Gram-negative
bacteria is briefly discussed above in Sect. 3.2.1. In addition to serving as an anchor
to negatively charged polymers, the cell membrane of alkaliphiles has shown a
stunning difference in composition when compared to that of non-alkaliphiles. Even
a difference is noted between the membrane of obligate and facultative alkaliphiles.
For instance, a comparative analysis of the membrane fatty acid composition
revealed that the unsaturated fatty acids account for 20% and up to 3% of the total
phospholipid fatty acids of the obligate and facultative alkaliphilic Bacillus strains,
respectively, when grown in alkaline condition [128]. Similarly, the membrane
composition of an organism can vary with the pH of the growth medium. The
membrane of facultative alkaliphiles grown in pH 7.5 medium was almost free of
unsaturated fatty acids, while the unsaturated fatty acid content rises to about 3%
when these cells were grown in pH 10.5 medium [128]. In another study, Yersinia
enterocolitica cells were grown at pH 9 and pH 5, and the analysis of the fatty acid
content of the cells revealed that the unsaturated fatty acid content was higher when
it was grown at pH 9 and significantly decreased for cells grown at pH 5 [129]. A
similar observation of high percentage of unsaturated membrane lipid has been
reported for different alkaliphiles [130].
The rise in the content of unsaturated fatty acid seems correlated to the fatty acid
desaturase (an enzyme that forms carbon double bonds in fatty acids) activity
(Fig. 6). The membrane of obligate alkaliphiles has very high fatty acid desaturase
activity, while the membranes of facultative strains do not have detectable activity
[131]. As shown in Fig. 6, desaturase mediated reaction consumes oxygen. Aono
et al. [132] have shown that the oxygen uptake rate of membrane vesicles of Bacillus
lentus C-125 (which is later named B. halodurans C-125) grown at pH 9.9 is more
than double than that of neutral grown. Part of this oxygen consumption may be
related to the formation of unsaturated fatty acid bonds. However, this is yet to be
experimentally proven. In general, very little is known about the role of desaturase in
Challenges and Adaptations of Life in Alkaline Habitats
101
bacteria [126], studies made so far on high pH adaptation role of S-layer proteins
have been restricted to Gram-positive bacteria. Hence, relatively, little is known
about the contribution of Gram-negative S-layer proteins to high pH adaptations.
Scanning electron microscopy analysis of the surface of a Gram-negative bacterium,
Pseudomonas alcaliphila, revealed that cells grown at pH 10 have rougher surface
than those grown at pH 7 [127]. This might show the possibility that Gram-negative
strains also make some surface depositions (S-layer proteins) to thrive in high pH
environments. However, this needs specific experimental evidences. Further studies
on other S-layer proteins of Gram-positive bacteria and probably other cell surface
deposited proteins of alkaliphilic Gram-negative and Gram-positive bacteria may
improve our understanding of these interesting proteins contribution to high pH
adaptation.
3.2.4 Cell Membrane
The other component of the cell envelope that contributes to high pH adaptation is
the cell membrane. The contribution of the outer membrane of Gram-negative
bacteria is briefly discussed above in Sect. 3.2.1. In addition to serving as an anchor
to negatively charged polymers, the cell membrane of alkaliphiles has shown a
stunning difference in composition when compared to that of non-alkaliphiles. Even
a difference is noted between the membrane of obligate and facultative alkaliphiles.
For instance, a comparative analysis of the membrane fatty acid composition
revealed that the unsaturated fatty acids account for 20% and up to 3% of the total
phospholipid fatty acids of the obligate and facultative alkaliphilic Bacillus strains,
respectively, when grown in alkaline condition [128]. Similarly, the membrane
composition of an organism can vary with the pH of the growth medium. The
membrane of facultative alkaliphiles grown in pH 7.5 medium was almost free of
unsaturated fatty acids, while the unsaturated fatty acid content rises to about 3%
when these cells were grown in pH 10.5 medium [128]. In another study, Yersinia
enterocolitica cells were grown at pH 9 and pH 5, and the analysis of the fatty acid
content of the cells revealed that the unsaturated fatty acid content was higher when
it was grown at pH 9 and significantly decreased for cells grown at pH 5 [129]. A
similar observation of high percentage of unsaturated membrane lipid has been
reported for different alkaliphiles [130].
The rise in the content of unsaturated fatty acid seems correlated to the fatty acid
desaturase (an enzyme that forms carbon double bonds in fatty acids) activity
(Fig. 6). The membrane of obligate alkaliphiles has very high fatty acid desaturase
activity, while the membranes of facultative strains do not have detectable activity
[131]. As shown in Fig. 6, desaturase mediated reaction consumes oxygen. Aono
et al. [132] have shown that the oxygen uptake rate of membrane vesicles of Bacillus
lentus C-125 (which is later named B. halodurans C-125) grown at pH 9.9 is more
than double than that of neutral grown. Part of this oxygen consumption may be
related to the formation of unsaturated fatty acid bonds. However, this is yet to be
experimentally proven. In general, very little is known about the role of desaturase in
Challenges and Adaptations of Life in Alkaline Habitats
101