Using Bacteria and Plants for Arsenic Detection

Instead of using conventional man-made technologies, can you all imagine using bacteria and plants for arsenic detection?Let's take a look at this innovative bio-technology that has been developed.

Although biological monitoring may be a more sustainable way of monitoring arsenic, the decision of whether this method may be suitable or not depends on the purpose and the reseources available for a particular investigation.

One of the advantages of biological monitoring is the fact that there is a close association between biomonitors with the biological systems under study as it the biomonitor is normally part of the biological system.

Criteria for selecting good biological monitoring species include:

-the organism must be capable of accumulating metals in measurable amounts.

-the organism, or relevant parts of it, must be readily available both in terms of quantity and distribution so that unbiased sampling is possible.

-it should be available throughout the year or throughout the period of study, with relative ease of collection.

-Upon exposure, the organism should show a differential uptake or accumulation which allows us to determine the relative pollution levels and establish a relationship to deposition rate or air concentrations.


-the organism should not ingest metals from other sources, this is especially important for assesing airborne contamination.


-repeatability is essential

-reasonable cost of collection and analysis

Bacteria

Research has shown that bacteria can be used as a biosensor to identify and treat sites that are contaminated with arsenic. Although bacteria has been used to monitor nitrates in the past, the use of microbes to treat arsenic may also be feasible, according to scientists.

All cell-based organisms have the ability to detoxify arsenic compounds. The process involves a wide variety of proteins that will modify, transport and extrude the arsenic from the cell. In the presence of arsenic and through specific genetic mechanisms, the correct sequence of proteins can be synthesised and activated. Hence, the required proteins can be synthesized to activate the arsenic detoxification system.

Due to the fact that the correct sequence of genes is necessary to produce the desired protein, it is absolutely necessary that we understand the identity, specificity, and sensitivity of the genetic elements and their corresponding regulatory proteins when employing biosensors. In order to create a biosensor that can monitor arsenic, the arsenic-responsive DNA control sequences are linked to an additional gene which is called the reporter gene. This gene produces a protein that can produce an obvious response for easy detection as it produces a highly coloured material or fluorescent protein. With the advancement in genetic engineering, we can now produce microbes that generates a visible signal (i.e.fluorescent bright yellow), when it comes into contact with arsenic compounds.

Diagram showing how a reporter gene is made

These test tubes are filled with fluorescent proteins that are highly coloured

Genetically modified microbes were used in another recent study to develop a set of semi-quantitative assays for potable water. The investigators also developed an assay that produced a visible blue color with arsenite concentrations above 8 ppb.

The following illustrates some of the advantages and disadvantages of using this method:


Advantages:

- can detect arsenic down to ppb levels, in other words, it can measure only small concentrations of arsenic

-good potential for assaying arsenic

-environmentally friendly



Disadvantages:

-apply only to water assays

 - limited success rate

 - it is not clear whether the microbes are measuring all of the arsenic in a sample or just the bioavailable arsenic.

Plants

Compared with the use of microbes for arsenic detection, there are far less research involving the use of plants to detect arsenic. A recent experiment has been conducted on two water plants upon exposure to arsenic. It is found that there is a change in the colour pigments of the plants. In order to acquire more accurate results, it requires an incubation period of three days and quantified with a series of standards. Although this is a very sustainable and "low tech" assay, the results may be affected by other factors such as nutrient levels or microbial infection, which can generate the same pigment change as arsenic absorption.

If we are to consider using plants to monitor arsenic, it is necessary that we take into account of the factors that affect the efficiency of particulate capture and retention. This includes the size, shape, canopy structure and surface characteristics of the plants or plant organs as well as the degree of exposure. In addition to this, we also need to consider the relationship between the surface area and weight of the plant organs as these factors will affect the expression of the results and may cause the intepretation and comparison of the data to be very complicated.

Here are some of the advantages and disadvantages of using this method:


Advantages:

-its general ubiquity. Only in situations of extreme aerial contamination is vegetation likely to be sufficiently scarce to cause sampling problems. In other words, this detection method has a low risk for sampling problems to occur.

Disadvantages:

- coloration changes in plant systems may be due to factors other than arsenic detection.

-samples may vary between general herbage of several species to leaves, whole leafy shoots and bark of single species.

Recent breakthroughs for arsenic detection using bacteria

A student team from the University of Edinburgh has used genetically engineered bacteria to detect arsenic in water. In combination with a drop of pH indicator (far right), samples turn red (middle) in the presence of arsenic and yellow in its absence.

For more information ,feel free to read up more on : http://www.technologyreview.com/Biotech/18103/