Actinobacteria exhibit diverse metabolic properties, such as the production of
extracellular enzymes and specialized metabolites (Goodfellow and Fiedler 2010).
In fact, the order Actinomycetales produce about 45% of all microbial specialized
metabolites, with 80% of these compounds being produced by the Streptomyces
genus (Bérdy 2005). Actinobacteria also produce NPs, enzyme inhibitors,
phytotoxins, biopesticides, biosurfactants, probiotics, and enzymes involved in the
degradation of complex polymers (Manivasagan et al. 2013). The versatility in
specialized metabolite production makes actinobacteria important tools for pharmaceutical, medical, and biotechnological applications.
Actinobacteria play important role in remediation of toxic metals, through reduction of metal ions being promising organisms for synthesis of NPs. Consequently,
several strains have been isolated from extreme environments and screened for novel
bioactive compounds (Alvarez et al. 2017). Biogenic NPs are synthesized by
processes of metal reductions inherent to cellular detoxification mechanisms (Siddiqi
and Husen 2016). Microbial enzymes and/or specialized metabolites reduce the
metal ion in solution synthesizing NPs, which can be stabilized by capping proteins
(Fig. 6.1). Therefore, the use of microorganisms that present reductase activities is
very relevant for obtaining NPs. For instance, Alani et al. (2012) compared physicochemical properties of AgNPs synthesized intracellularly by a
thermoalkalotolerant Streptomyces sp. and the fungus Aspergillus fumigatus, concluding that the actinobacteria synthesized smaller and monodisperse NPs than
Aspergillus in a shorter time. This fact becomes relevant considering that among
microorganisms, fungi have been reported as one of the best NP producers. The
authors explain that different size distributions of NPs may be due to differences in
reductases produced by Streptomyces sp. and Aspergillus or the effects of other
proteins coating the NPs. Similarly, Ahmad et al. (2003b) demonstrated formation of
AuNPs was faster by Rhodococcus sp. (24 h) than the fungus Verticillium, when
cells of both microorganisms were suspended in an aqueous solution of chloroauric
acid. Moreover, the authors highlight the AuNPs synthesized using the fungus were
extremely polydisperse. Extracellular synthesis of AgNPs was also demonstrated in
a strain of Streptomyces hygroscopicus (Sadhasivam et al. 2010) and AuNP synthesis by a strain of Thermomonospora sp. (Hulkoti and Taranath 2014).
Some research studied the synergism of AgNPs in combination with antibiotics
such as amoxicillin and polymyxin B, founding higher bactericidal activity by
binding of AgNPs with antibiotics (Składanowski et al. 2017). For instance, Wypij
et al. (2017) obtained AgNPs using the acidophilic actinobacterial strain Streptomyces kasugaensis M338-M1
T . The AgNPs showed the maximum antimicrobial activity against E. coli, followed by B. subtilis and S. aureus. Further, the synergistic
effect of AgNPs in combination with commercial antibiotics (kanamycin, ampicillin,
and tetracycline) was also evaluated against bacterial isolates. The antimicrobial
efficacy of antibiotics was found to be enhanced in the presence of AgNPs.
The abovementioned research encouraged us to put the focus on a collection of
metal-resistant actinobacteria present in the “Laboratory of Biotechnology of
Actinobacteria” (PROIMI-CONICET) which bases their resistance to heavy metals
in reductase activities. For example, Streptomyces sp. MC1 exhibits chromate
6 Nanoparticles for New Pharmaceuticals: Metabolites from Actinobacteria
201
extracellular enzymes and specialized metabolites (Goodfellow and Fiedler 2010).
In fact, the order Actinomycetales produce about 45% of all microbial specialized
metabolites, with 80% of these compounds being produced by the Streptomyces
genus (Bérdy 2005). Actinobacteria also produce NPs, enzyme inhibitors,
phytotoxins, biopesticides, biosurfactants, probiotics, and enzymes involved in the
degradation of complex polymers (Manivasagan et al. 2013). The versatility in
specialized metabolite production makes actinobacteria important tools for pharmaceutical, medical, and biotechnological applications.
Actinobacteria play important role in remediation of toxic metals, through reduction of metal ions being promising organisms for synthesis of NPs. Consequently,
several strains have been isolated from extreme environments and screened for novel
bioactive compounds (Alvarez et al. 2017). Biogenic NPs are synthesized by
processes of metal reductions inherent to cellular detoxification mechanisms (Siddiqi
and Husen 2016). Microbial enzymes and/or specialized metabolites reduce the
metal ion in solution synthesizing NPs, which can be stabilized by capping proteins
(Fig. 6.1). Therefore, the use of microorganisms that present reductase activities is
very relevant for obtaining NPs. For instance, Alani et al. (2012) compared physicochemical properties of AgNPs synthesized intracellularly by a
thermoalkalotolerant Streptomyces sp. and the fungus Aspergillus fumigatus, concluding that the actinobacteria synthesized smaller and monodisperse NPs than
Aspergillus in a shorter time. This fact becomes relevant considering that among
microorganisms, fungi have been reported as one of the best NP producers. The
authors explain that different size distributions of NPs may be due to differences in
reductases produced by Streptomyces sp. and Aspergillus or the effects of other
proteins coating the NPs. Similarly, Ahmad et al. (2003b) demonstrated formation of
AuNPs was faster by Rhodococcus sp. (24 h) than the fungus Verticillium, when
cells of both microorganisms were suspended in an aqueous solution of chloroauric
acid. Moreover, the authors highlight the AuNPs synthesized using the fungus were
extremely polydisperse. Extracellular synthesis of AgNPs was also demonstrated in
a strain of Streptomyces hygroscopicus (Sadhasivam et al. 2010) and AuNP synthesis by a strain of Thermomonospora sp. (Hulkoti and Taranath 2014).
Some research studied the synergism of AgNPs in combination with antibiotics
such as amoxicillin and polymyxin B, founding higher bactericidal activity by
binding of AgNPs with antibiotics (Składanowski et al. 2017). For instance, Wypij
et al. (2017) obtained AgNPs using the acidophilic actinobacterial strain Streptomyces kasugaensis M338-M1
T . The AgNPs showed the maximum antimicrobial activity against E. coli, followed by B. subtilis and S. aureus. Further, the synergistic
effect of AgNPs in combination with commercial antibiotics (kanamycin, ampicillin,
and tetracycline) was also evaluated against bacterial isolates. The antimicrobial
efficacy of antibiotics was found to be enhanced in the presence of AgNPs.
The abovementioned research encouraged us to put the focus on a collection of
metal-resistant actinobacteria present in the “Laboratory of Biotechnology of
Actinobacteria” (PROIMI-CONICET) which bases their resistance to heavy metals
in reductase activities. For example, Streptomyces sp. MC1 exhibits chromate
6 Nanoparticles for New Pharmaceuticals: Metabolites from Actinobacteria
201