Biosynthesis of Metal Nanoparticles Using Bacteria

Among the milieu of natural resources, prokaryotic bacteria have been known to interact with metals and are explored for biotechnological applications such as ore leaching and metal recovery. One of the reasons for the “bacterial preference” for NP synthesis is their relative ease of manipulation. Bacteria isolated from different habitats and nutritional modes have been employed for the synthesis of metal NPs either intracellularly or extracellularly. Among the different biological entities employed for the biosynthesis of NPs, bacteria have received the most attention and are preferred due to their ability to withstand high concentrations of heavy metal ions, ease of culturing, manipulation of genetic make-up, and downstream processing (Parikh et ah, 2011).

Most of the research in the field of bacteria mediated biosynthesis has been concentrated on the synthesis of noble metal NPs that dates back to the 1970s, when edomicrobium-like budding bacteria was shown to accumulate lacelike gold-deco- rated structures, and it was postulated that in the near future, the "Midas-gene” could be isolated, cloned, and over-expressed for the fast synthesis of gold NPs. Today, after about two decades of research in the area, a repertoire of bacterial species isolated from different families in the bacterial classification system have been reported for the synthesis of gold NPs. This ability of bacteria to synthesize NPs was attributed to the presence of potential anionic sites on the cell wall which includes teichoic phosphodiester groups, free carboxylic groups of the peptidoglycan layer, and the sugar hydroxyl groups from wall polymers and amide groups of the peptide chains that bind and reduce gold ions to their Au° forms (Rayman and MacLeod, 1975; Mandal et al., 2006). Most of the bacterial species isolated for the synthesis of gold NPs are on isotropic spherical or quasi-spherical NPs. But bacterial isolates like Escherichia coli (Park et al., 2010), Lactobacilli (Nair and Pradeep. 2002), Rhodopseudomonas capsulate (He et al., 2008), Bacillus licheniformis (Kalishwaralal et al., 2009), and Shewanella cdgae (Konishi et al., 2007) have been reported for the synthesis of anisotropic particles ranging from triangular or hexagonal plates to nanowires and nanocubes.

In addition to gold NPs, the ability of bacterial systems to synthesize NPs from the platinum group of metals (PGM) including palladium and platinum NPs (PtNPs) has also been reported. Although the reports for this group are scarce, this shows the potential of bacterial species to synthesize a range of metal NPs with a wide size range (Hennebel et al., 2011). Besides PGM, bacteria have also been shown to survive in high concentrations of tellurium and selenium ions and have evolved cellular mechanisms to convert the highly toxic ionic states of these metalloids to their zero- valent forms.

Interestingly, the cellular mechanism for the bioremediation of these metalloids has been well studied with reports of reductase enzymes present in metalloid resistant bacteria mediating the reduction of selenate/selenite and tellurate/tellurite (Stolz et al„ 2006). Even more interesting is the fact that some of the organisms reducing these ions have show'n the ability to control NP shapes, including, for example, selenium nanorods by Pseudomonas alacliphila (Zang et ah, 2011) and nano-rosettes of tellurium by B. beveridgei (Baesman et ah, 2009). Recently, the even more interesting ability of Enterobacter sp. was shown towards the synthesis of mercury NPs. This ability of bacteria to stabilize mercury NPs is important because elemental mercury is known to be volatile (Sinha et ah, 2011).

In addition to the biosynthesis of a range of metals and metalloids like gold, platinum, selenium, and tellurium NPs, the biosynthesis of silver NPs (AgNPs) encompasses a large population of bacterial species with the first reports only dating back to 1992 where silver-resistant Pseudomonas stutzeri was shown to accumulate AgNPs (Slawson et ah, 1992). Unlike gold, in which the resistance mechanism was only recently reported, ionic silver is known to be toxic to bacterial cells and genes conferring silver resistance have been studied and reported for bacterial survival in high silver concentration environments (Parikh et ah, 2008). Although silver biosynthesis is well studied, most studies report isotropic NPs except Pseudomonas stutz.eri AG259, where a few triangular plates w'ere found to accumulate in the periplasmic space of the bacterium (Klaus et ah, 1999). Among the first reports of intracellular semiconductor NP synthesis, E. coli, when incubated with cadmium chloride (CdCl,) and sodium sulfide (Na,S) spontaneously, formed CdS semiconductor nanocrystals (NC) which showed that the formation of NCs was markedly affected by physiologic parameters. The entry into the stationary phase increased the yield by 20-fold (Sw'eeney et ah, 2004). In line with these observations, it was found that Clostridium thermoaceticum precipitates CdS at the cell surface as well as in the medium when exposed to CdCl, in the presence of cysteine hydrochloride as a source of sulfide in the growth medium (Cunningham and Lundie, 1993).

Sulfate-reducing bacteria synthesized magnetic iron sulfide (FeS) NPs about 20 nm in size on their surfaces which were separated from the solution by a high gradient field of 1 Tesla. Bacterially produced FeS is an adsorbent for a wide range of heavy metals and some anions. Furthermore, magnetite is a common product of bacterial iron reduction and could be a potential physical indicator of biological activity in geological settings (Watson et ah, 1999). Acinetobacter, a non-magnetotactic bacterium. was employed for magnetite NP synthesis. In prior studies, biosynthesis of magnetite was found to be extremely slow (often requiring 1 week) under strictly anaerobic conditions. Acinetobacter spp. were capable of magnetite synthesis by reaction with suitable aqueous iron precursors under fully aerobic conditions. Importantly, the extracellular magnetite NPs showed excellent magnetic properties

TABLE 4.1

Metallic Nanoparticles Synthesized by Different Bacteria

Microorganism

Type of Nanoparticle

Location

Size Range (nm)

Reference

Pseudomonas stutzeri

Ag

Intracellular

-200

Klaus et al. (1999)

MorganeUa sp.

Ag

Extracellular

20-30

Parikh et al. (2008)

Plectonema

boryanum

(Cyanobacteria)

Ag

Intracellular

  • 1-10
  • 1-100

Lengke et al. (2007)

Escherichia coli

CdS

Intracellular

2-5

Sweeney et al. (2004)

Clostridium

thermoaceticum

CdS

Intracellular and extracellular

Cunningham and Lundie (1993)

Actinobacter spp.

Magnetite

Extracellular

10-40

Bharde et al. (2005)

ShewaneUa algae

Au

Intracellular. pH 7 Extracellular. pH 1

  • 10-20
  • 50-500

Konishi et al. (2004)

Rhodopseudomonas

capsulata

Au

Extracellular. pH 7 Extracellular. pH 4

  • 10-20
  • 50-400

Shiying et al. (20075

Escherichia coli DH5a

Au

Intracellular

25-33

Liangwei et al. (2007)

Tliermomonospora

sp.

Au

Extracellular

8

Ahmad et al. (2003)

Rhodococcus sp.

Au

Intracellular

5-15

Ahmad et al. (2003a)

Klebsiella

pneumoniae

Ag

Extracellular

5-32

Shahverdi et al. (2007)

Pseudomonas

aeruginosa

Au

Extracellular

15-30

Husseiney et al. (2007)

ShewaneUa

oneidensis

Uranium (IV)

Extracellular

Marshall et al. (2007)

Lactobacillus spp. from yoghurt

Ag and TiO,

Extracellular

  • 10-25
  • 10-70

Jha and Prasad (2010)

Lactobacillus

sporogens

ZnO

Extracellular

5-15

Prasad and Jha (2000)

S. thermophilus

Se(0)

Intracellular

50-500

Eszenyi et al. (2011)

Lactobacillus spp.

TiO,

Extracellular

8-35

Jha et al. (2011)

Lactobacillus spp.

Ag and Au

Intracellular

Nair and Pradeep (2002)

Lactobacillus

fermentum

Ag

Extracellular

11.2

Sintubin et al. (2009)

(Bharde et al., 2005). Bacteria synthesized in a variety of metallic NPs with different size is tabulated in Table 4.1.

Although the field of biosynthesis has been much explored, achieving shape control is still one of the biggest challenges with very few reports entailing shape control (Ramanathan et al., 2011). Even in the case where anisotropic shapes have been achieved, it only reports the outcomes of exposure to heavy metal ions to bacteria, without making any deliberate efforts to control the bacterial growth kinetics to achieve shape control. Another interesting aspect is that, although a wide range of genera has been reported for the biosynthesis of metal NPs, in all cases, typically only a few species of those particular genera have shown the ability to biosynthesize NPs.

Thus, to survive in environments containing high levels of metals, organisms have adapted by evolving mechanisms to cope with them. This mechanism mainly involves altering the chemical nature of the toxic metal so that it no longer causes toxicity, resulting in the formation of NPs of the metal concerned. Thus NPs formation is the “by-product” of a resistance mechanism against a specific metal, and this can be used as an alternative way of producing them.

 
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