of metallic ions to the cell wall of a fungus, observing that the ions were heterogeneously distributed according to specific binding sites (AshaRani et al. 2009).
Moreover, shape, size, and stability of the NPs are also determined by the binding
sites of the cell wall (Asmathunisha and Kathiresan 2013; Erasmus et al. 2014).
For some microorganisms, metallic ions are transferred into the cell via active
cellular pumps (ATP-dependent), followed by enzymes that reduce these ions and in
occasion cap them. Finally, capping proteins bind to NPs (Fig. 6.1) via open amine
groups and cysteine residues, neutralizing its surface charge. Capping proteins also
act on preventing the agglomeration and the alteration of NP properties playing an
important role as a site for bioconjugation with other molecules. The protein caps
provide stability to biologically synthesized NPs that are not otherwise found in
ENPs unless that surfactants are included in traditional methods, which are very
toxic (El-Deeb et al. 2013). Furthermore, the stability decreases the toxicity of the
NPs, making them more environmental friendly (Stark et al. 2015).
6.2.2 Extracellular Synthesis
There are two possible pathways for extracellular NPs synthesis. The first is similar
to the intracellular synthesis: ions pass through the cell membrane via active transporters, and then through reductive enzymes, the synthesis of the NPs is initiated.
The proteins bind to the NPs during the reductive process, capping and reducing
them through active sites. These NPs, after setting their size and form, are
transported outside the cell (Bansal et al. 2012). Thus, in some microorganisms,
the intracellular and extracellular synthesis of NPs can occur in the same cell
(Ramanathan et al. 2011) (Fig. 6.1). The second pathway involves the emission of
reducing proteins to the cell solution. This is a result of the whole change in pH in
presence of the metallic ions. Upon receiving this signal, the cell emits oxidoreductase enzymes that reduce ions and synthesize NPs. These or other proteins can cap
the NPs, adding stability among other properties as mentioned above. Thus, the cellfree supernatant from the microorganism culture contains the biomolecules responsible not only for biosynthesis of NPs but also of its dispersion (Huang et al. 2015).
6.3 Actinobacteria: General Features of the Group
and Promising Strains for Nanoparticle Biosynthesis
Actinobacteria is the most diverse group of the Bacteria domain. The phylum
includes Gram-positive or Gram-variable aerobes, facultative anaerobes, or anaerobes. Most strains are chemoorganotrophs, some with high G + C content and others
with low G + C content in their genomes (Ghai et al. 2012). The taxon includes
phenotypically diverse microorganisms with a wide variety of morphologies.
200
D. Costa et al.
Moreover, shape, size, and stability of the NPs are also determined by the binding
sites of the cell wall (Asmathunisha and Kathiresan 2013; Erasmus et al. 2014).
For some microorganisms, metallic ions are transferred into the cell via active
cellular pumps (ATP-dependent), followed by enzymes that reduce these ions and in
occasion cap them. Finally, capping proteins bind to NPs (Fig. 6.1) via open amine
groups and cysteine residues, neutralizing its surface charge. Capping proteins also
act on preventing the agglomeration and the alteration of NP properties playing an
important role as a site for bioconjugation with other molecules. The protein caps
provide stability to biologically synthesized NPs that are not otherwise found in
ENPs unless that surfactants are included in traditional methods, which are very
toxic (El-Deeb et al. 2013). Furthermore, the stability decreases the toxicity of the
NPs, making them more environmental friendly (Stark et al. 2015).
6.2.2 Extracellular Synthesis
There are two possible pathways for extracellular NPs synthesis. The first is similar
to the intracellular synthesis: ions pass through the cell membrane via active transporters, and then through reductive enzymes, the synthesis of the NPs is initiated.
The proteins bind to the NPs during the reductive process, capping and reducing
them through active sites. These NPs, after setting their size and form, are
transported outside the cell (Bansal et al. 2012). Thus, in some microorganisms,
the intracellular and extracellular synthesis of NPs can occur in the same cell
(Ramanathan et al. 2011) (Fig. 6.1). The second pathway involves the emission of
reducing proteins to the cell solution. This is a result of the whole change in pH in
presence of the metallic ions. Upon receiving this signal, the cell emits oxidoreductase enzymes that reduce ions and synthesize NPs. These or other proteins can cap
the NPs, adding stability among other properties as mentioned above. Thus, the cellfree supernatant from the microorganism culture contains the biomolecules responsible not only for biosynthesis of NPs but also of its dispersion (Huang et al. 2015).
6.3 Actinobacteria: General Features of the Group
and Promising Strains for Nanoparticle Biosynthesis
Actinobacteria is the most diverse group of the Bacteria domain. The phylum
includes Gram-positive or Gram-variable aerobes, facultative anaerobes, or anaerobes. Most strains are chemoorganotrophs, some with high G + C content and others
with low G + C content in their genomes (Ghai et al. 2012). The taxon includes
phenotypically diverse microorganisms with a wide variety of morphologies.
200
D. Costa et al.