Graphene
is the most widely researched new material. It is an allotrope of carbon and
due to special properties the material is being tested out within the fields of
consumer and medicinal electronics. While these applications have received
considerable attention developments relating to microbiology are taking place.
This short review article considers bacterial staining and anti-bacterial
activity, as two of the most promising future developments.
An essay by Tim Sandle
An essay by Tim Sandle
Graphene
is a material derived from carbon and it has unique physicochemical properties.
Graphene is formed where graphite is taken and atom thick layers are sliced
away. The resultant structure is a single-layer of carbon atoms linked in a hexagonal
chicken-wire pattern. Within the structure each of the atoms share a cloud of
electrons moving freely about the surface. The material is light, transparent,
strong and very conductive (Allen et al, 2010).
Compared to other carbon allotrope, such as fullerenes, carbon nanotubes and
graphite, graphene exhibits many exceptional physical and chemical properties. Graphene
related materials are of great interest in the field of biomedicines and
applications are underway in biosensing and drug delivery.
Graphene as a
Gram-stain alternative
New
research, from the University of Illinois at Chicago, suggests that graphene
can be used for performing Gram-stains, as part of microbial identification. A
review of the properties of graphene shows it can detect variances to cell
vibration when a cell comes into contact with the material. So far the tests
undertaken with graphene have related to cancer. Here atomic vibration differs
depending upon whether the cell is a cancer cell or a normal cell. This happens
because the cancer cell’s hyperactivity leads to a higher negative charge, and
this causes a higher level of protons to be released. This difference can be
detected, helping medical technologists to identify cancerous growth.
Assessing
the variances in vibration is possible using an established laboratory method
called Raman spectroscopy (a spectroscopic technique used to observe
vibrational, rotational, and other low-frequency modes in a system).
According
to lead researcher Vikas Berry, who is the associate professor and head of
chemical engineering, who led the research along with Ankit Mehta, assistant
professor of clinical neurosurgery in the UIC College of Medicine: “We may be
able to use it with bacteria to quickly see if the strain is Gram-positive or
Gram-negative…We may be able to use it to detect sickle cells.” Berry made this
remark to Controlled Environments magazine (Anon, 2017).
In
a parallel development, one research group have created a graphene sensor for
Escherichia coli. This involved fabricating a flexible substrate onto which a sensor
device with O-ring is fitted. Once contact takes place with the suspected
organism, Raman spectra is used to indicate the presence (Basu et al, 2014).
As
all microbiologists know, the Gram-stain is the key test for distinguishing
between two groups of bacteria based on cell wall morphologies (Sandle, 2014). The
Gram stain procedure distinguishes between Gram positive and Gram negative
groups by coloring these cells red or violet. Gram staining is a common
technique used to differentiate two large groups of bacteria based on their
different cell wall constituents. The Gram stain procedure distinguishes
between Gram positive and Gram negative groups by coloring these cells red or
violet. Gram positive bacteria stain violet due to the presence of a thick
layer of peptidoglycan in their cell walls, which retains the crystal violet
these cells are stained with. Alternatively, Gram negative bacteria stain red,
which is attributed to a thinner peptidoglycan wall, which does not retain the
crystal violet during the decolouring process (Sandle, 2004).
While
the Gram-stain technique is well described it is sometimes prone to error. This
can relate to the types of organisms, the age of the cultures, or due to errors
made by the person performing the test (such as over decoloursation). A method
based on graphene would error proof. Whether such a method becomes commercially
available will depend on development costs.
Graphene as an
antibacterial agent
As
well as using graphene as a potential diagnostic tool, graphene can also be
used as an anti-bacterial measure (where liposome-embedded graphene reduces the
growth capability of bacteria). Research by Zappacosta and colleagues showed
that graphene aqueous dispersion is stable for several days and demonstrates
significant antibacterial activity against both Gram-positive (Staphylococcus aureus) and Gram-negative
(Escherichia coli) strains, with a
reduction in the growth of S. aureus
and E. coli as high as 60 and 78%,
respectively.
In
a similar application, researchers have looked at graphene-iodine
nano-composites, formed via electrostatic interactions between positively
charged graphene derivatives and triiodide anions, as anti-bacterial agents
(Some et al, 2015). With this, the antibacterial
potential of these graphene-iodine composites against Klebsiella pneumonia, Pseudomonas aeruginosa, Proteus mirobilis,
Staphylococcus aureus, and Escherichia
coli has been demonstrated. The success against the organisms relates to
the inherent cytotoxicity of the nanocomposite, specifically through electron
transfer interaction from microbial membrane to graphene.
Furthermore,
scientists are studying graphene oxide with the aim of creating
bacteria-killing catheters and medical devices. Here coating surgical tools
with this carbon-based compound could kill bacteria, reducing the need for
antibiotics, decreasing the rates of post-operative infections and speeding
recovery times. This is with graphene oxide, which is a form of graphene with
molecular oxygen incorporated into it. This compound protects against infection
by destroying bacteria before it gets inside the body. In terms of the process
the graphene oxide wraps around the bacteria, puncturing its membrane. A broken
membrane prevents the bacteria from growing and often kills it.
Studies
conducted at the Università Cattolica del Sacro Cuore in Rome indicate that the
compound is most effective when paired with salt. Getting the salt balance
correct is important. With too little salt and then the graphene oxide is
unable to wrap around the bacteria; and with too much salt and the graphene aggregates,
failing to puncture the bacteria's membrane. In order to destroy both Gram
positive and Gram negative bacteria a 300 nanometer sheet of graphene oxide
solution must be mixed with low molarity (<10 mM) calcium chloride is
required (Anon, 2015).
Summary
These
two related research strands (for differential microbiology and as an
antibacterial agent) signal that graphene, the so-called ‘wonder material’ of
our age, is set to make a significant impact upon microbiology. As with the
development of any novel method, progress will be slow. However, the research
results reported to date suggest that graphene is set to make a major
contribution to microbiology.
References
Allen,
M. J., Tung, V.C. and R. B. Kaner, R.B. Honeycomb carbon: a review of graphene,
Chem. Rev., 2010, 110, 132
Anon.
Biophysical Society report “Towards a “green” antimicrobial therapy: Study of
graphene nanosheets interaction with human pathogens”, 2015 (http://tinyurl.com/zzgsofu)
Anon.
First Use of Graphene to Find Cancer Cells, Controlled Environments, 2017 (http://www.cemag.us/news/2017/02/first-use-graphene-find-cancer-cells)
Basu,
P.K., Indukuri, D., Keshavan, S., Bhat, N. (2014) Graphene based E. coli sensor
on flexible acetate sheet, Sensors and
Actuators B Chemical 190:342-347 (https://www.researchgate.net/publication/256926101_Graphene_based_E_coli_sensor_on_flexible_acetate_sheet)
Sandle,
T. (2004) Gram’s Stain: History and Explanation of the Fundamental Technique of
Determinative Bacteriology’, IST Science
and Technology, No. 54, pp3-4
Sandle,
T. (2014). ‘Microbial Identification: Laboratory Techniques and Methods. In
Chesca, A. (Ed.) Methods for Diseases:
Diagnostic with Applicability in Practice, Lambert Academic Publishing,
Germany, pp15-26
Some,
S., Sohm, J., Kim, J. et al Graphene-Iodine
Nanocomposites: Highly Potent Bacterial Inhibitors that are Bio-compatible with
Human Cells, Scientific Reports 6,
Article number: 20015 (2016). doi:10.1038/srep20015
by Dr. Tim Sandle
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