evolution, there must have been some means to remove Ca
2+ from the cell in order
to prevent precipitation of cellular constituents. The ability to control Ca
2+ content,
or at least intracellular free Ca
2+ concentration, may thus be a fundamental attribute
of all cells. A wide array of mechanisms participate in attaining such a homeostatic
condition. Calcium regulation in bacterial cells comprises influxes and effluxes
dependent on active and passive transport mechanisms. Due to the fact that normal
intracellular calcium concentrations are usually up to 10
3 times lower than extracellular concentrations, passive transport usually accounts for calcium influx
(Norris et al. 1996). A net Ca
2+ concentration gradient across the cell membrane
is formed which by present-day values would be of the order of 10
À3 to 10
À4 M.
This gradient has to be maintained by continuous exclusion of Ca
2+ from the cell.
The removal of Ca
2+ by active extrusion requires energy to pump the Ca
2+ against
the electrochemical gradient. The metabolic apparatus that serves this function
involves Ca
2+ protein-based and non-proteinaceous channels, Ca
2+ antiporters
(Ca
2+ /2H
+
, Ca
2+ /Na
+
), and ATP-dependent Ca
2+ pumps (Norris et al. 1996;
Hammes and Verstraete 2002). ATP-driven calcium efflux appears to play a pivotal
role in microbial calcium regulation (Naseem et al. 2009). Another way to remove
Ca
2+ is to bind it into a harmless form by Ca
2+ chelating materials.
The possibility to form calcium minerals might be also seen as a tool to remove
this cation, but a very few experimental studies are available to support it. Anderson
et al. (1992) demonstrated that the ability of the soil bacterium Pseudomonas
fluorescens to synthetize extracellular calcite in a calcium-stressed environment
enabled the organism to regulate its cytosolic calcium content maintaining it at
levels compatible with cell survival. The authors measured the calcium content in
different cellular fractions of a P. fluorescens culture grown in a medium added
with 10-mM calcium, during growth. As growth progressed, no significant increase
in cytoplasmic calcium was recorded. After 16 h of growth, extracellular deposition
of a calcium precipitate was observed which continued through to stationary phase
and did not appear to have an inhibitory influence on cellular multiplication. Most
of the calcium was sequestered in this precipitate, which was calcite with hexagonal
crystal habits. No extracellular residue was evident if the culture medium was not
inoculated, indicating that the metabolism of P. fluorescens was necessary for the
formation of the calcite precipitate. While during the exponential phase of growth,
the cytoplasmic calcium content appeared to be regulated by mechanism(s) other
than biotransformation of the metal as an insoluble residue, calcite bioprecipitation
could ensure a more efficient utilization of energy at the stationary phase of growth.
The P. fluorescens crystalline CaCO 3 was then involved, according to the authors,
in calcium homeostasis and allowed the organism to survive in the calcium-stressed
medium by contributing to the maintenance of a low intracellular Ca
2+ concentration. The authors concluded that the maintenance of innocuous levels of cytosolic
calcium by the extracellular deposition of crystalline CaCO 3 is an uncommon
occurrence, but we could suspect a larger occurrence and conservation of such a
mechanism among bacteria. It is worth nothing that biomineralization as a metabolic detoxification process induced by increases in intracellular [Ca
2+ ] was
proposed by Simkiss in 1977 (see Sect. 5.3.2.2).
128
B. Perito and G. Mastromei
2+ from the cell in order
to prevent precipitation of cellular constituents. The ability to control Ca
2+ content,
or at least intracellular free Ca
2+ concentration, may thus be a fundamental attribute
of all cells. A wide array of mechanisms participate in attaining such a homeostatic
condition. Calcium regulation in bacterial cells comprises influxes and effluxes
dependent on active and passive transport mechanisms. Due to the fact that normal
intracellular calcium concentrations are usually up to 10
3 times lower than extracellular concentrations, passive transport usually accounts for calcium influx
(Norris et al. 1996). A net Ca
2+ concentration gradient across the cell membrane
is formed which by present-day values would be of the order of 10
À3 to 10
À4 M.
This gradient has to be maintained by continuous exclusion of Ca
2+ from the cell.
The removal of Ca
2+ by active extrusion requires energy to pump the Ca
2+ against
the electrochemical gradient. The metabolic apparatus that serves this function
involves Ca
2+ protein-based and non-proteinaceous channels, Ca
2+ antiporters
(Ca
2+ /2H
+
, Ca
2+ /Na
+
), and ATP-dependent Ca
2+ pumps (Norris et al. 1996;
Hammes and Verstraete 2002). ATP-driven calcium efflux appears to play a pivotal
role in microbial calcium regulation (Naseem et al. 2009). Another way to remove
Ca
2+ is to bind it into a harmless form by Ca
2+ chelating materials.
The possibility to form calcium minerals might be also seen as a tool to remove
this cation, but a very few experimental studies are available to support it. Anderson
et al. (1992) demonstrated that the ability of the soil bacterium Pseudomonas
fluorescens to synthetize extracellular calcite in a calcium-stressed environment
enabled the organism to regulate its cytosolic calcium content maintaining it at
levels compatible with cell survival. The authors measured the calcium content in
different cellular fractions of a P. fluorescens culture grown in a medium added
with 10-mM calcium, during growth. As growth progressed, no significant increase
in cytoplasmic calcium was recorded. After 16 h of growth, extracellular deposition
of a calcium precipitate was observed which continued through to stationary phase
and did not appear to have an inhibitory influence on cellular multiplication. Most
of the calcium was sequestered in this precipitate, which was calcite with hexagonal
crystal habits. No extracellular residue was evident if the culture medium was not
inoculated, indicating that the metabolism of P. fluorescens was necessary for the
formation of the calcite precipitate. While during the exponential phase of growth,
the cytoplasmic calcium content appeared to be regulated by mechanism(s) other
than biotransformation of the metal as an insoluble residue, calcite bioprecipitation
could ensure a more efficient utilization of energy at the stationary phase of growth.
The P. fluorescens crystalline CaCO 3 was then involved, according to the authors,
in calcium homeostasis and allowed the organism to survive in the calcium-stressed
medium by contributing to the maintenance of a low intracellular Ca
2+ concentration. The authors concluded that the maintenance of innocuous levels of cytosolic
calcium by the extracellular deposition of crystalline CaCO 3 is an uncommon
occurrence, but we could suspect a larger occurrence and conservation of such a
mechanism among bacteria. It is worth nothing that biomineralization as a metabolic detoxification process induced by increases in intracellular [Ca
2+ ] was
proposed by Simkiss in 1977 (see Sect. 5.3.2.2).
128
B. Perito and G. Mastromei