Journal of Biomaterials and Nanobiotechnology, 2011, 2, 156-162
doi:10.4236/jbnb.2011.22020 Published Online April 2011 (http://www.scirp.org/journal/jbnb)
Synthesis of AgNPs by Bacillus Cereus Bacteria
and Their Antimicrobial Potential
Anuradha Prakash1, Seema Sharma2*, Naheed Ahmad3, Ashok Ghosh4, Preety Sinha5
1Department of Zoology, A.N. College, Patna, India; 2 Department of Physics, A.N. College, Patna, India; 3 Department of Botany,
Patna University, Patna, India; 4 Department of Environment and Water Management, A.N. College, Patna, India; 5 Department of
Zoology, A.N. College, Patna, India.
Received December 26th, 2010; revised January 17th, 2011; accepted January 18th, 2011.
ABSTRACT
In the present work silver nanoparticles (AgNPs) were synthesized extracellularly by bacteria Bacillus cereus collected
from the riverine belt of Gangetic Plain of India. The microbes were isolated, screened and characterized by morpho-
logical and biochemical analyses. The silver resistant strain was exposed to different concentrations of silver salt (Ag-
NO3). UV-visible spectrum of the supernatant of cell culture showed absorbance peak of AgNPs at ~ 435 nm.The shape
and size of AgNPs were ascertained by High Resolution Transmission Electron Micrography (HRTEM), X-ray diffrac-
tion (XRD) and Energy Dispersive spectroscopy (EDS). Average size of the synthesized AgNPs was found to be in the
range of 10 - 30 nm with spherical shape. AgNPs were tested against antibacterial potential of some common human
pathogens.
Keywords: AgNPs, XRD, HRTEM, Antimicrobial
1. Introduction
Nanotechnology has become the new arena for future
technology which is mainly dependent on nanometals
and semiconductors. Nanoparticles synthesized by the
chemical processes had toxic effect hence there is a
growing need to develop environment friendly, cost ef-
fective and conveniently reproducible green methods of
nanoparticle synthesis. Diversity of microbes is immense
and this can be exploited purposefully for synthesis and
harvesting of different nanoparticles. A vast range of
organic and inorganic materials exist as colloids and na-
noparticles in soil, infiltration water and ground water.
The cell wall of bacteria is chemically and structurally
more complex to the surface of inorganic nanoparticles,
and it can adapt itself with the change in the environment.
Bioscience can be employed to explore medical sciences
in fighting pathogens [1], artificial implants [2], targeted
drug delivery [3] etc and understanding the role and ap-
plications of microorganisms for the remediation of
toxic and radionuclide contaminated sites and antibacte-
rial effects. Microorganisms that affect the reactivity and
mobility of metals can be used for detoxification and
remediation. Many organisms both prokaryotes and eu-
karyotes are known to produce many inorganic materials
either intracellularly or extracellularly [4]. These specific
characteristics of organisms particularly of microorgan-
isms can be used in the synthesis of nanoparticles. Metals
are present in the environment in trace amount which are
used by the organisms for their metabolic activities. Mi-
crobes have the capacity to alter their environment by
selective interaction with metals. The micro flora of
unique ecological niches yields microbial diversity which
can be exploited for various ends, including production
of nanoparticles. Biosynthesized nanoparticles find utili-
zation in the fields of bioremediation, biolabelling, bio-
sensors and many more [5]. It is reported that stable sil-
ver nanoparticles can be synthesized with controlled size
in suitable matrices such as micelles [6], organic small
molecules or microbes, [7] linear polymers [8], mesopo-
rous materials [9], dendritic polymers [10,11]. The large
scale synthesis of silver nanomaterials suffers from is-
sues such as polydispersity and stability, especially if the
reduction is carried out in aqueous media. Therefore the
extracellular biological synthesis of AgNPs can be an
attractive and ecologically friendly alternative method
for the large quantities because it offers the advantage of
easy downstream processing. Moreover, bacteria are easy
to handle and can be manipulated genetically without
much difficulty. The capability of producing crystalline
silver particles through microbes in nanosize, and with

Synthesis of AgNPs by Bacillus Cereus Bacteria and Their Antimicrobial Potential 157
controlled morphology is the basis of using this biologi-
cal method in the field of material science. Silver ions
promote bone growth and kill surrounding bacteria. They
are also reported to be nontoxic to human and most ef-
fective against bacteria, viruses and other eukaryotic mi-
cro-organisms at very low concentration and without any
side effects [12]. As most of the bacteria have developed
resistance to antibiotics, there is a need of an alternative
antibacterial substance [13]. AgNPs have an important
advantage over conventional antibiotics in that it kills all
pathogenic microorganisms, and no organism has ever
been reported to readily develop resistance to it [14]. The
silver nanoparticles synthesized by microbes can be used
in target oriented drug delivery, food industry, as an an-
tiseptic in waste water treatment, in curing and detecting
many diseases, in making electronic devices. In this work,
microbes were selected randomly and after several ex-
periments one of the strains identified as Bacillus cereus
was selected for synthesis of AgNPs and which were
found to be fairly dispersed and exhibited effective an-
timicrobial effect.
2. Experimental Details
2.1. Culture and Isolation
The soil samples were collected from different sites of
the wetland area, rich in microfauna of North Bihar of
India along the river Ganges. Serial dilutions nutrient
agar plates were made and pure culture was isolated after
the requisite period of incubation (Figure 1).The identi-
fication and characterization of the culture was per-
formed on morphological and biochemical basis. One of
the most resistant strain Bacillus cereus was selected for
further experiment. It was grown in 250 ml MGYP (glu-
cose 1%, malt extract 1%, yeast extract 0.3%, peptone
0.5%) medium in 500 ml Erlenmeyer flask. The flasks
were incubated at 37.5˚C on a rotary shaker set at 100
rpm for 24 h.
2.2. Synthesis of AgNPs
AgNO3 was added to the innoculum and put on shaker
for 24 h. One of the set of culture was treated as control
for the experiment (without the silver salt). AgNO3 was
added in different concentration ranging from 50, 100,
500, 1000, 1500, 20000 ppm respectively. After 24 h, this
culture was filtered through Whatman filter paper no. 1
(medium retention, flow rate and porosity which are fre-
quently used for clarifying liquids in biological experi-
ments) and the cell free supernatant was observed on
UV-VIS spectrophotometer with wavelength range of
200-600nm.The absorbance maxima of supernatant was
taken at different time intervals after adding AgNO3. The
culture was then centrifuged at 5000 rpm for 15 minutes
to recover the synthesized nanoparticles in the aliquot
and was washed with distilled water 3 - 4 times to avoid
any interference of the media in the characterization of
the nanoparticles. Then the nanoparticles were allowed to
dry and made into fine powder for their characterization
through X-ray diffraction, Electron Diffraction Spec-
troscopy and Transmission Electron Microscopy.
2.3. Characterization of AgNPs
2.3.1. X- Ray Diffraction Analysis
The X-ray Diffraction (XRD) measurements of drop-
coated films of AgNPs on glass substrate were recorded
in a wide range of Bragg angle 2θ at a scanning rate of
2°min–1 carried out on a Philips PW 1830 instrument that
was operated at a voltage of 40 kV and a current of 30
mA with Cu Kα radiation (λ = 1.5405 Å).
2.3.2. TEM and Electron Diffraction Analysis
High Resolution Transmission Electron Microscopy
(HRTEM) was performed by TECHNAI G20-STWIN
(a)
(b)
Figure 1. (a) Photograph of Bacillus cereus, the bacterial
strain (100x magnification); (b) Photograph of Bacillus ce-
reus (enlarged section).
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Synthesis of AgNPs by Bacillus Cereus Bacteria and Their Antimicrobial Potential 158
(200 KV) machine with a line resolution 2.32 (in ˚A).
These images were taken by drop coating AgNPs on a
carbon-coated copper grid. Energy Dispersive Absorp-
tion Spectroscopy photograph of AgNPs were carried out
by the HRTEM equipment as mentioned above.
2.4. Antimicrobial Activity Test for AgNPs
After characterization of AgNPs their antibacterial effect
was checked by disc diffusion or Kirby-Bauer method
[15] on some common pathogens of human, E.coli and
Streptococcus. This method is used to determine suscep-
tible and resistant values [16].
3. Results and Discussions
3.1. Synthesis of Silver Nanoparticles
The results of this experiment i.e. extracellular synthesis
of AgNPs were observed. The colour of the aliquot cha-
nged to deep orange after 24 h of agitation with AgNO3
(Figure 2 which further blackened in 72 h. The black
colour of the culture confirms the extracellular reduction
of silver salt to AgNPs, however the bacterial strain
treated with de-ionized water retained its original colour
[17].
The absorbance scan taken by UV-VIS spectropho-
tometer showed a sharp plasmon peak at ~ 435 nm (Fig-
ure not shown here), which confirms silver in nanoscale
range. The size, shape and distribution of the nanomate-
rial affect the magnitude, peak and width of the spectrum
band. As the colour of the culture changed in course of
synthesis, its absorbance was recorded after regular in-
tervals of time (24 h, 48 h, 72 h, 7days, 15days and 30
days). The suspension was stored for about one month to
observe the stability of the synthesized nanocrystallites.
The silver nanocrystallites showed the peak at wave-
length 430 - 440 nm range.
The mechanism for the synthesis of AgNPs by bacteria
is not exactly deciphered till date but several possible
ways of synthesis is being explained. The possible che-
mical reactions in the culture medium may be as follows:
C
6H12O6 →CH3-CO-COOH
(Glucose) (Pyruvate)
NaHCO
3 ↔ Na+ +
3
HCO
HCO
3 ↔ OH– + CO2
AgNO
3 +(OH)2 →Ag2O + H2O +N2
Ag(OH)
2→ Ag+↓ + H2O
According to this concept, the transmembrane proton
gradient effected by respiratory, adenosine trip- hos-
phatase, or bacteriorhodopsin activity is used to extrude
Na+ from the bacterial cells by means of a Na+ - H+ anti-
porter, resulting in the creation of an inwardly directed
transmembrane Na+ gradient. The chemical and electrical
components of this gradient, either separately or in com-
bination, can then be used.
Figure 2. Photograph of change in colour of aliquot after
addition of AgNO3 during different time period (from left to
right): control, 24 hrs, 48 hrs and 72 hrs.
to drive the intracellular accumulation of nutrients. The
proton gradient can power active transport indirectly,
through the formation of sodium ion gradient in micro-
organism. In this reaction, Sodium symport (mentioned
above) is an important process in cells and is used in
sugar and amino acid uptake. ATP binding employ spe-
cial substrate binding proteins, which are attached to
membrane lipids on the external face of gram positive
bacteria (B.cereus). A proton gradient can power active
transport indirectly and most of the bacteria which have
electrokinetic potential readily attract the cations and this
probably acts as an initiator for the biosynthesis of na-
noparticles [18].
The mechanism of transformation of oxidase to reduc-
tase and vice- versa due to change in pH might have tak-
en place at two levels, at cell membrane level, or in the
membrane of endoplasmic reticulum (Figure 3). Oxidase
gets activated at lower pH whereas reductase gets acti-
vated at higher pH in the cell membrane [19,20] which
makes the molecular oxygen available for the transfor-
mation by the tautomerization of quinones. The molecu-
lar oxygen released by both processes is utilized in the
conversion of silver to silver oxide. NADH serves as an
electron carrier and transfers electron to receptors. The
electrons flow from carriers with more negative reduc-
tion potential to those with more positive potentials and
eventually combine with O2 and H2 to form water.
X-ray Diffraction (XRD) pattern (Figure 4) shows in-
tense Bragg’s reflections that can be indexed on the basis
of the fcc structure of silver [19]. The XRD pattern thus
obtained clearly shows [111], [200], [220] planes, and
exhibit that the synthesized AgNPs by the Bacillus cer-
eus were crystalline in nature. The values agree well with
those reported for silver (face centric cubic) by the Joint
Committee on Powder Diffraction Standards File No.
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Synthesis of AgNPs by Bacillus Cereus Bacteria and Their Antimicrobial Potential 159
Figure 3. Schematics for the biosynthesis of AgNPs using Bacillus Cereus.
04-0783. The diffraction peaks were found to be broad
around their bases indicating that the silver particles are
in nanosizes. The peak broadening at half maximum in-
tensity of the X-ray diffraction lines is due to a reduction
in crystallite size, flattening and microstrains within the
diffracting domains. Scherrer’s equation for broadening
resulting from a small crystalline size, the mean, effec-
tive or apparent dimension of the crystalline composing
the powder is,
hk11 2
Pckos

 ,
where θ is the Bragg angle and λ is the X-ray wavelength,
β is the breadth of the pure diffraction profile in radians
on 2θ scale and k is a constant approximately equal to
unity and related both to the crystalline shape and to the
way in which β is defined. The best possible value of k
has been estimated as 0.89. The particle sizes of all the
samples in our study have been estimated by using the
above Scherer’s equation and were found to be ~15 nm
for the strongest peak. TEM technique was employed to
visualize the size and shape of the silver nanoparticles
formed.
Figure 5(a) and b shows the typical bright-filed TEM
images of the synthesised silver nanoparticles. It is ob-
served from this image that the nanoparticles are isolated
and are surrounded by a layer of organic matrix at some
places, which acts as capping agent for the silver nano-
particles. The difference in size may possibly be due to
the fact that the nanoparticles are being formed at differ-
ent times. Most of the silver nanoparticles are spherical
in shape, and are in the range of 10 - 30 nm in size which
is in close agreement with the particle size calculated
Figure 4. Room temperature X-ray diffraction pattern of
AgNPs.

Synthesis of AgNPs by Bacillus Cereus Bacteria and Their Antimicrobial Potential 160
(a)
(b)
Figure 5. TEM photographs of AgNPs from Bacillus cereus
at different magnifications.
from the XRD profile. A few agglomerated silver nano-
particles were also observed in some places, thereby in-
dicating possible sedimentation at a latter time (after 12
weeks). The TEM image suggests that the particles are
polydispersed and are mostly spherical in shape. Selected
area electron diffraction (SAED) spots that corresponded
to the (from inside to outside of the central ring) [111],
[200], [220], [311] and [222] planes of the face-centered
cubic (fcc) structure of elemental silver are clearly seen
in Figure 6(a). HRTEM image shows (Figure 6(b)) the
d spacing of 2.02 Å, which matches with the [200] crys-
tallographic plane of AgNPs. Figure 7 shows the Energy
Dispersive Absorption Spectroscopy photograph of
AgNPs. All the peaks of Ag are observed and are as-
signed. Peaks for Cu and C are from the grid used and
the peaks for S, P and N correspond to the protein cap-
ping over the AgNPs. It is reported earlier that proteins
can bind to nanoparticles either through free amine
groups or cysteine residues in the proteins [20] and via
(a)
(b)
Figure 6. (a) Selected area diffraction pattern of AgN-Ps; (b)
HRTEM image showing characteristics spacing for [200]
plane.
the electrostatic attraction of negatively charged car-
boxylate groups in enzymes present in the cell wall of
bacteria [21] and therefore, stabilization of the AgNPs by
protein is a possibility. The amide linkages between
amino acid residues in proteins give rise to the well-
known signatures in the infrared region of the electro-
magnetic spectrum and have been shown by the FTIR-
spectrum [22]. In future, it would be important to under-
stand the biochemical and molecular mechanism of the
synthesis of nanoparticles by the cell filtrate in order to
achieve better control over size and polydispersity of the
nanoparticles.
3.2. Antibacterial Properties of Bio-Synthesized
AgNPs
The bactericidal effect of AgNPs is well established;
however the mechanism is only partially understood. It
has been reported that ionic silver strongly interacts with
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Synthesis of AgNPs by Bacillus Cereus Bacteria and Their Antimicrobial Potential 161
Table 1. Data showing zone of inhibition of streptococcus, E. coli and some standard antibiotics in different concentration of
AgNPs .
Zones of inhibition (diameter, in cm)
No of observa-
tions
Concentration of
AgNPs (in ppm) Pathogens Antibiotics
E.Coli StreptococcusChloremphenicolWymox Erythromycin Oxytetracyclin
1 100 1 - 1.7 1 - 2 1 - 2.1 1 - 1.75 1 - 1.7 1 - 2.4
2 50 1 - 1.5 1 - 1.7 1 - 1.3 1 - 1.5 1 - 1.2 1 - 1.6
3 20 1 - 1.2 1 - 1.6 1 - 1.4 1 - 1.4 1 - 1.6 1 - 1.20
Figure 7. Energy Dispersive Absorption Spectroscopy photograph of AgNPs.
thiol group of vital enzymes and inactivates them [23,
24]. Experimental evidence suggests that DNA loses its
replication ability once the bacteria have been treated
with silver ions [25]. The antibacterial effect of nanopar-
ticles can be attributed to their stability in the medium as
a colloid, which modulates the phosphotyrosine profile
of the bacterial proteins and arrests bacterial growth. The
effect is dose dependent and is independent of acquisi-
tion of resistance by the bacteria against antibiotics. An-
tibacterial effect of the synthesized AgNPs was tested
with a gram negative and gram positive bacteria E. Coli
and Streptococcus in varying strength of nanoparticles
colloid. It was observed that the lowest concentration up
to 50 ppm was sufficient to inhibit bacterial growth.
Zones of inhibition were measured after 24 - 48 hrs of
incubation (Table 1). Zones of inhibition were almost of
circular shape therefore the inhibitory zones were meas-
ured in diameter (cm). The comparative stability of discs
containing Wymox, Chloremphenicol, Ampicilin, Oxytet-
racycline was also made which exhibited reduced growth
of microbes. It was observed that the zone of inhibition
formed due to silver nanoparticles was more prominent
as compared with the inhibition zones of the antibiotics.
4. Conclusions
In this study, extracellular synthesis of AgNPs has been
shown from silver-resistant Bacillus sp. isolated from the
riverine belt of Gangetic Plain of India. XRD analysis
showed that the nanoparticles were crystalline and metal-
lic in nature. HRTEM analysis showed that most of the
particles were spherical in shape with size ~15 nm. This
extracellular bacterial synthesis of AgNPs has many ad-
vantages over the chemically derived nanoparticles and
might be an excellent means of developing an ecologi-
cally friendly protocol.
5. Acknowledgements
The authors wish to acknowledge University Grants
Commission, New Delhi, India for the financial support
under the major research project scheme.

Synthesis of AgNPs by Bacillus Cereus Bacteria and Their Antimicrobial Potential 162
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