4.1 Type of Microorganism and Cell Load
Nearly all organisms studied in treatment of concrete are those which precipitate
calcite. This efficiency of microorganisms can vary depending on the mechanism of
calcite induction, the organisms ability to raise the pH of the environment which in
turn increases the carbonate concentration, the amount and nature of
exopolysaccharide secreted by the organism, etc. Among the studies in this line, a
great deal of attention had been given to urease producing organisms which are
capable of precipitating high amounts of calcium. Arthrobacter nicotianae [125],
B. megaterium [233, 234], B. subtilis [233], Bacillus sp. [235], B. thuringiensis
[125, 233], Sporosarcina pasteurii [133, 236, 237], Deleya halophila [110],
Halomonas eurihalina [72], Kocuria flava [118], Lysinibacillus sphaericus [233],
Methylocystis parvum [223], Myxococcus xanthus [99], Proteus mirabilis [89],
Pseudomonas denitrificans [238], Sporosarcina ginsengisoli [131], and
Stenotrophomonas maltophilia [125] could be mentioned as examples. Abiotic
factors such as the type and amount of nutrients available and degree of aeration
are also known to influence calcite precipitation [239, 240].
The intended application dictates the type of microorganisms that have to be
considered. For instance, for concrete surface treatment applications, organisms that
can precipitate enough amount of calcium carbonate at reasonable pace and tolerate
the pH of the concrete may be good enough. But microorganisms planned for selfhealing concrete application need to be more resilient. At least in addition to
carbonate precipitation and tolerance to high pH, organisms envisioned for selfhealing application should be able to remain dormant preferably for a long time
(spore former) and withstand the dryness and the anoxic nature of the concrete. Not
only the type of microorganism but also the amount of cells used per unit volume
affects the level of calcite precipitation, which in turn influences the efficiency of the
treatment on concrete. Among urease producers, it has been shown that 10
6
–10
8
cells per ml resulted in enough amount of calcite precipitate [241]. The amount of
cells used influences the rate of urea hydrolysis more than the initial urea concentration. The mere increase of cell load from 10
6 to 10
8 cells/mL resulted in 30% more
CO 3
2À in solution [241]. However, it is worth to note that high rate of calcite
precipitation does not necessarily lead to the target effect on treated concrete
structures. For instance, in a two-step immersion system for surface treatment of
concrete (as described in Sect. 3.2), the bacteria that biodeposit calcite are only the
ones retained on the concrete surface [3, 190]. On the other hand, in a one-step
immersion system [187], potentially the entire amount of bacteria added to the
mixture act on the urea that leads to rapid decomposition which is accompanied
by fast CaCO 3 precipitation. However, this fast precipitation of calcite results in
loose CaCO 3 particles instead of a dense and cohesive CaCO 3 layer [124]. The loose
biodeposited matrix cannot serve the purpose and hence is not desirable. A similar
observation, fast precipitation of CaCO 3, has been made when using urease instead
of cells [242]. Another important issue related to microbial-induced calcite precipitation is the nature of the cell surface (cell wall and exopolysaccharides). It has been
known that bacterial cells serve as nucleation sites for CaCO 3 precipitation [133],
324
G. Mamo and B. Mattiasson
Nearly all organisms studied in treatment of concrete are those which precipitate
calcite. This efficiency of microorganisms can vary depending on the mechanism of
calcite induction, the organisms ability to raise the pH of the environment which in
turn increases the carbonate concentration, the amount and nature of
exopolysaccharide secreted by the organism, etc. Among the studies in this line, a
great deal of attention had been given to urease producing organisms which are
capable of precipitating high amounts of calcium. Arthrobacter nicotianae [125],
B. megaterium [233, 234], B. subtilis [233], Bacillus sp. [235], B. thuringiensis
[125, 233], Sporosarcina pasteurii [133, 236, 237], Deleya halophila [110],
Halomonas eurihalina [72], Kocuria flava [118], Lysinibacillus sphaericus [233],
Methylocystis parvum [223], Myxococcus xanthus [99], Proteus mirabilis [89],
Pseudomonas denitrificans [238], Sporosarcina ginsengisoli [131], and
Stenotrophomonas maltophilia [125] could be mentioned as examples. Abiotic
factors such as the type and amount of nutrients available and degree of aeration
are also known to influence calcite precipitation [239, 240].
The intended application dictates the type of microorganisms that have to be
considered. For instance, for concrete surface treatment applications, organisms that
can precipitate enough amount of calcium carbonate at reasonable pace and tolerate
the pH of the concrete may be good enough. But microorganisms planned for selfhealing concrete application need to be more resilient. At least in addition to
carbonate precipitation and tolerance to high pH, organisms envisioned for selfhealing application should be able to remain dormant preferably for a long time
(spore former) and withstand the dryness and the anoxic nature of the concrete. Not
only the type of microorganism but also the amount of cells used per unit volume
affects the level of calcite precipitation, which in turn influences the efficiency of the
treatment on concrete. Among urease producers, it has been shown that 10
6
–10
8
cells per ml resulted in enough amount of calcite precipitate [241]. The amount of
cells used influences the rate of urea hydrolysis more than the initial urea concentration. The mere increase of cell load from 10
6 to 10
8 cells/mL resulted in 30% more
CO 3
2À in solution [241]. However, it is worth to note that high rate of calcite
precipitation does not necessarily lead to the target effect on treated concrete
structures. For instance, in a two-step immersion system for surface treatment of
concrete (as described in Sect. 3.2), the bacteria that biodeposit calcite are only the
ones retained on the concrete surface [3, 190]. On the other hand, in a one-step
immersion system [187], potentially the entire amount of bacteria added to the
mixture act on the urea that leads to rapid decomposition which is accompanied
by fast CaCO 3 precipitation. However, this fast precipitation of calcite results in
loose CaCO 3 particles instead of a dense and cohesive CaCO 3 layer [124]. The loose
biodeposited matrix cannot serve the purpose and hence is not desirable. A similar
observation, fast precipitation of CaCO 3, has been made when using urease instead
of cells [242]. Another important issue related to microbial-induced calcite precipitation is the nature of the cell surface (cell wall and exopolysaccharides). It has been
known that bacterial cells serve as nucleation sites for CaCO 3 precipitation [133],
324
G. Mamo and B. Mattiasson