3.1.4 Anion (OH
À ) Deterring Cell Surface
The cell envelope, which comprises the cell membrane, cell wall, and associated cell
surface depositions, is a barrier that prevents the cytoplasm from the direct effect of
its environment. If cells must maintain a near-neutral cytoplasmic pH, the OH
À of
the alkaline environment should not freely ingress into the cell. The cell surface of
alkaliphiles contains acidic residues [106] that potentially repel the anions and
prevent the rise in the cytoplasmic pH. Moreover, the anionic cell surface can
capture H
+
, especially H
+ pumped out of the cell such as by the respiratory complex.
This ability of trapping H
+ might form a kind of H
+ reserve close to the membrane
surface which hypothetically reduces the difficulty of capturing H
+ from the bulk
alkaline environment. The H
+ trapped in the cell envelope can be retrieved such as
by the Na
+ /H
+ antiporters and ATP synthases and translocated to the cytoplasm,
which contributes to the pH homeostasis. These trapped protons may also neutralize
OH
À migrating to the cell membrane. Since the contribution of the cell envelope to
high pH adaptation is significant, it is relevant to discuss it in detail.
3.2 Protective Cell Envelope
The cell envelope, which consists of cell membrane and cell wall (plus the outer
membrane in Gram-negative bacteria), delineates the cell from its environment. This
structure plays a vital role in the cell survival primarily by maintaining its content,
while it allows controlled exchange of materials between the cell and its environment. Cells such as bacteria interact with their extracellular environment through
their cell envelope. Thus, adaptations of such kind of organisms to their habitats
often involve their cell envelope. There have been evidences that the cell envelopes
of alkaliphiles are part of the high pH adaptation assemblies. Some of the most
important studies that substantiate how cell envelope contributes to high pH adaptation of alkaliphiles are discussed below under the different components of the cell
envelope.
3.2.1 Cell Wall: Secondary Cell Wall Polymers
Among alkaliphiles, members of the genus Bacillus are the most studied. Like other
Gram-positive bacteria, the cell wall of Bacillus cells contains different polysaccharides. Most of these polysaccharides are covalently bonded to the peptidoglycan
(PG), which is the prominent cell wall scaffolding structure. Based on structural
properties, the cell wall polysaccharides of these organisms are categorized into
three groups: (1) teichuronic acids [107, 108], (2) teichoic acids [109, 110], and
(3) other polysaccharides which cannot be characteristically assigned to the other
two groups [111, 112]. These polysaccharides have been considered to play
98
G. Mamo
À ) Deterring Cell Surface
The cell envelope, which comprises the cell membrane, cell wall, and associated cell
surface depositions, is a barrier that prevents the cytoplasm from the direct effect of
its environment. If cells must maintain a near-neutral cytoplasmic pH, the OH
À of
the alkaline environment should not freely ingress into the cell. The cell surface of
alkaliphiles contains acidic residues [106] that potentially repel the anions and
prevent the rise in the cytoplasmic pH. Moreover, the anionic cell surface can
capture H
+
, especially H
+ pumped out of the cell such as by the respiratory complex.
This ability of trapping H
+ might form a kind of H
+ reserve close to the membrane
surface which hypothetically reduces the difficulty of capturing H
+ from the bulk
alkaline environment. The H
+ trapped in the cell envelope can be retrieved such as
by the Na
+ /H
+ antiporters and ATP synthases and translocated to the cytoplasm,
which contributes to the pH homeostasis. These trapped protons may also neutralize
OH
À migrating to the cell membrane. Since the contribution of the cell envelope to
high pH adaptation is significant, it is relevant to discuss it in detail.
3.2 Protective Cell Envelope
The cell envelope, which consists of cell membrane and cell wall (plus the outer
membrane in Gram-negative bacteria), delineates the cell from its environment. This
structure plays a vital role in the cell survival primarily by maintaining its content,
while it allows controlled exchange of materials between the cell and its environment. Cells such as bacteria interact with their extracellular environment through
their cell envelope. Thus, adaptations of such kind of organisms to their habitats
often involve their cell envelope. There have been evidences that the cell envelopes
of alkaliphiles are part of the high pH adaptation assemblies. Some of the most
important studies that substantiate how cell envelope contributes to high pH adaptation of alkaliphiles are discussed below under the different components of the cell
envelope.
3.2.1 Cell Wall: Secondary Cell Wall Polymers
Among alkaliphiles, members of the genus Bacillus are the most studied. Like other
Gram-positive bacteria, the cell wall of Bacillus cells contains different polysaccharides. Most of these polysaccharides are covalently bonded to the peptidoglycan
(PG), which is the prominent cell wall scaffolding structure. Based on structural
properties, the cell wall polysaccharides of these organisms are categorized into
three groups: (1) teichuronic acids [107, 108], (2) teichoic acids [109, 110], and
(3) other polysaccharides which cannot be characteristically assigned to the other
two groups [111, 112]. These polysaccharides have been considered to play
98
G. Mamo