3.2.2 Lipopolysaccharides
The nature of the Gram-negative and Gram-positive bacteria cell surfaces is different. Gram-negative cells lack SCWPs and do not seem to benefit from the high pH
adaptation role of these structures. However, it seems that the outer membrane of
Gram-negative bacteria plays more of the protection role. This membrane of Gramnegative bacteria contains lipopolysaccharides (LPS) which are exposed to the outer
surface of the cells. Although, little is done on its involvement in high pH adaptation,
it seems that it may function the same way as SCWPs of Gram-positive alkaliphiles.
Indeed, structural analysis of the haloalkaliphilic strain, Halomonas pantelleriensis
lipopolysaccharide O-chain revealed that it has a unique repeating unit, 4-O-((S)-1carboxyethyl)-D-GlcA residue [118]. This repeating unit contains carboxyl groups
which make the polymer highly negatively charged. Further, chemical, NMR, and
MS study results show that the LPS of this haloalkaliphilic strain are very rich in
carboxylate groups [118]. A similar observation of highly carboxylated LPS is
reported from another Gram-negative bacteria, H. magadiensis. A protective buffering effect of this negatively charged LPS has been suggested [119], which is
expected to be similar to that of SCWPs, repelling the OH
À by the anions of the
LPS and neutralization by the trapped cations.
3.2.3 S-Layer Proteins
The cell envelopes of Gram-positive alkaliphiles are also known to have a special
proteinaceous layer known as cell surface layer (S-layer) [120, 121]. S-layers are
composed of identical (glyco) protein structures that form lattices on the bacterial
cell surfaces. This layer is sometimes referred to as “nonclassical” SCWPs; however,
based on compositional and structural analysis, Schäffer and Messner [122]
suggested that it belongs to the third cell wall group of Araki and Ito [111].
Alkaliphilic cells express a range of S-layer proteins. For instance, 17 S-layer
homology (SLH) domain-containing proteins, including S-layer protein A (SlpA),
are identified in the genome sequence of B. pseudofirmus OF4 [123]. The contribution of S-layer to high pH adaptation has been assessed through mutational studies
using B. pseudofirmus OF4 cells which produce SlpA both at neutral and alkaline
conditions. Mutants that lack SlpA grow more slowly at pH 11 than the wild-type
cells, especially when the Na
+ concentration was low [85]. On the other hand, the
wild-type cells expressing SlpA grow slower at neutral condition than at high pH
[85]. Although it seems that the expression of SlpA at neutral pH reduces growth
efficiency, those facultative organisms expressing SlpA in the neutral range could
benefit if sudden alkalinization happens. The results of the mutational studies
indicate that the presence of SlpA on the cell surface has a high pH adaptive
advantage. Like other cell wall proteins from alkaliphiles, SlpA has low (4.36)
isoelectric point (pI) which is mainly due to its fewer arginine and lysine content.
Like TUA and TUP, the relatively abundant negatively charged residues of SlpA
favor H
+ accumulation and deter OH
À penetration [54, 55, 85, 124, 125].
100
G. Mamo
The nature of the Gram-negative and Gram-positive bacteria cell surfaces is different. Gram-negative cells lack SCWPs and do not seem to benefit from the high pH
adaptation role of these structures. However, it seems that the outer membrane of
Gram-negative bacteria plays more of the protection role. This membrane of Gramnegative bacteria contains lipopolysaccharides (LPS) which are exposed to the outer
surface of the cells. Although, little is done on its involvement in high pH adaptation,
it seems that it may function the same way as SCWPs of Gram-positive alkaliphiles.
Indeed, structural analysis of the haloalkaliphilic strain, Halomonas pantelleriensis
lipopolysaccharide O-chain revealed that it has a unique repeating unit, 4-O-((S)-1carboxyethyl)-D-GlcA residue [118]. This repeating unit contains carboxyl groups
which make the polymer highly negatively charged. Further, chemical, NMR, and
MS study results show that the LPS of this haloalkaliphilic strain are very rich in
carboxylate groups [118]. A similar observation of highly carboxylated LPS is
reported from another Gram-negative bacteria, H. magadiensis. A protective buffering effect of this negatively charged LPS has been suggested [119], which is
expected to be similar to that of SCWPs, repelling the OH
À by the anions of the
LPS and neutralization by the trapped cations.
3.2.3 S-Layer Proteins
The cell envelopes of Gram-positive alkaliphiles are also known to have a special
proteinaceous layer known as cell surface layer (S-layer) [120, 121]. S-layers are
composed of identical (glyco) protein structures that form lattices on the bacterial
cell surfaces. This layer is sometimes referred to as “nonclassical” SCWPs; however,
based on compositional and structural analysis, Schäffer and Messner [122]
suggested that it belongs to the third cell wall group of Araki and Ito [111].
Alkaliphilic cells express a range of S-layer proteins. For instance, 17 S-layer
homology (SLH) domain-containing proteins, including S-layer protein A (SlpA),
are identified in the genome sequence of B. pseudofirmus OF4 [123]. The contribution of S-layer to high pH adaptation has been assessed through mutational studies
using B. pseudofirmus OF4 cells which produce SlpA both at neutral and alkaline
conditions. Mutants that lack SlpA grow more slowly at pH 11 than the wild-type
cells, especially when the Na
+ concentration was low [85]. On the other hand, the
wild-type cells expressing SlpA grow slower at neutral condition than at high pH
[85]. Although it seems that the expression of SlpA at neutral pH reduces growth
efficiency, those facultative organisms expressing SlpA in the neutral range could
benefit if sudden alkalinization happens. The results of the mutational studies
indicate that the presence of SlpA on the cell surface has a high pH adaptive
advantage. Like other cell wall proteins from alkaliphiles, SlpA has low (4.36)
isoelectric point (pI) which is mainly due to its fewer arginine and lysine content.
Like TUA and TUP, the relatively abundant negatively charged residues of SlpA
favor H
+ accumulation and deter OH
À penetration [54, 55, 85, 124, 125].
100
G. Mamo