198
N. Y. Quintero
Bacteria are generally divided into two main groups, Gram-positive and Gramnegative, based on their Gram stain retention property; regarding the top eight of
the set of 83 microorganisms herein studied, ranks 1, 2, 4, 5, 7 and 8 correspond
to Gram-positive and ranks 3 and 6 to Gram-negative bacteria, i.e. there is no
correlation between U biosorption and Gram stain classification for these microorganisms (Quintero et al. 2017; Happel 1996). However, there are other studies
showing that Gram-positives bacteria absorb more U than negative ones [Quintero
and Restrepo, 2017], for the first have 40% more binding sites for trapping metal
ions than the second ones (Borrok et al. 2005). In relation to the particular top eight
microorganisms in Table 2, the number of binding-sites for U uptake has not yet
been determined (Quintero et al. 2017).
According to the top eight shown in Table 2 (Quintero et al. 2018), different
microorganisms belonging to the same genus (Bacillus for microorganisms 1, 2 and
3 and Pseudomonas for microorganisms 3 and 6) have good U uptake capacities.
However, this ability is not joined to the genus or species; in the current study,
there are populated genera by different number of bacteria and each bacterium
is characterised by pretty different UC; some examples of these genera and the
number of bacteria studied belonging to these genera, respectively are Bacillus (16);
Pseudomonas (9) and Streptomyces (25).
Likewise, strains belonging to the same species do not trap U in similar amounts,
although they share genetic similarity in the DNA (Quintero et al. 2017; Happel
1996); according to the literature, it is still not clear why these strains do not trap U
in quite similar amounts (Happel 1996). Likewise, different conditions used in the
harvesting from bacterial cultures could lead to different densities of binding sites
at the cell wall level.
Moreover, it has been proposed that bacterial growth phase and biomass pretreatment (including washing conditions and presence of inhibitors (Premuzic et
al. 1991) can exert some influence, i.e., if cells are in stationary or growing stage
rather than resting stage or if they have been washed with acidic solutions that could
increase their ability for trapping metal ions (Premuzic et al. 1991). Although there
is no such information for the microorganisms here studied, Pseudomonas sp. EPS5028 was exposed to inhibitors and its UC was increased (Marqués et al. 1991).
In addition, the nutrient composition in the growth media may influence the
bacterial growth and lead to variability in the number of functional groups onto the
cell walls (Quintero et al. 2017); additionally it has been proposed that extracellular
polymeric substances (EPS), i.e., macromolecules situated outside of cell walls
could enhance the UC, in some strains of bacteria (Ates 2015). In this study, the top
eight microorganisms were cultured in different nutrient solutions, being difficult
to assess the influence of EPS in their high efficiency as U trappers (Quintero
et al. 2017). However, the two first bacteria in Table 2 (Quintero et al. 2018), B.
licheniformis and B. mucilaginosus secrete EPS or capsules made of extracellular
polysaccharides (Yi and Lian 2012; Yi and Yao 2012). Still, additional work at the
molecular level is needed to understand the chemical properties responsible for the
specificity of the cell wall U interactions found in bacteria (Borrok et al. 2005).
N. Y. Quintero
Bacteria are generally divided into two main groups, Gram-positive and Gramnegative, based on their Gram stain retention property; regarding the top eight of
the set of 83 microorganisms herein studied, ranks 1, 2, 4, 5, 7 and 8 correspond
to Gram-positive and ranks 3 and 6 to Gram-negative bacteria, i.e. there is no
correlation between U biosorption and Gram stain classification for these microorganisms (Quintero et al. 2017; Happel 1996). However, there are other studies
showing that Gram-positives bacteria absorb more U than negative ones [Quintero
and Restrepo, 2017], for the first have 40% more binding sites for trapping metal
ions than the second ones (Borrok et al. 2005). In relation to the particular top eight
microorganisms in Table 2, the number of binding-sites for U uptake has not yet
been determined (Quintero et al. 2017).
According to the top eight shown in Table 2 (Quintero et al. 2018), different
microorganisms belonging to the same genus (Bacillus for microorganisms 1, 2 and
3 and Pseudomonas for microorganisms 3 and 6) have good U uptake capacities.
However, this ability is not joined to the genus or species; in the current study,
there are populated genera by different number of bacteria and each bacterium
is characterised by pretty different UC; some examples of these genera and the
number of bacteria studied belonging to these genera, respectively are Bacillus (16);
Pseudomonas (9) and Streptomyces (25).
Likewise, strains belonging to the same species do not trap U in similar amounts,
although they share genetic similarity in the DNA (Quintero et al. 2017; Happel
1996); according to the literature, it is still not clear why these strains do not trap U
in quite similar amounts (Happel 1996). Likewise, different conditions used in the
harvesting from bacterial cultures could lead to different densities of binding sites
at the cell wall level.
Moreover, it has been proposed that bacterial growth phase and biomass pretreatment (including washing conditions and presence of inhibitors (Premuzic et
al. 1991) can exert some influence, i.e., if cells are in stationary or growing stage
rather than resting stage or if they have been washed with acidic solutions that could
increase their ability for trapping metal ions (Premuzic et al. 1991). Although there
is no such information for the microorganisms here studied, Pseudomonas sp. EPS5028 was exposed to inhibitors and its UC was increased (Marqués et al. 1991).
In addition, the nutrient composition in the growth media may influence the
bacterial growth and lead to variability in the number of functional groups onto the
cell walls (Quintero et al. 2017); additionally it has been proposed that extracellular
polymeric substances (EPS), i.e., macromolecules situated outside of cell walls
could enhance the UC, in some strains of bacteria (Ates 2015). In this study, the top
eight microorganisms were cultured in different nutrient solutions, being difficult
to assess the influence of EPS in their high efficiency as U trappers (Quintero
et al. 2017). However, the two first bacteria in Table 2 (Quintero et al. 2018), B.
licheniformis and B. mucilaginosus secrete EPS or capsules made of extracellular
polysaccharides (Yi and Lian 2012; Yi and Yao 2012). Still, additional work at the
molecular level is needed to understand the chemical properties responsible for the
specificity of the cell wall U interactions found in bacteria (Borrok et al. 2005).