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Low cost and rapid performance displayed by the whole-cell biosensor technology has led to increased interest in them. In addition bacterial biosensors are highly accurate, simple and easy to manufacture. Today, the technology of bioluminescent and fluorescent biosensors can, for example, be applied in the sensing of toxic metals and organic pollutants. In general whole-cell bacteria biosensors consists of a bacteria that has undergone genetic modification and carrying reporter gene. These bacteria, with the help of a promoter that responds to specific type of stimulation, encode the conspicuous phenotypes such as bioluminescence and fluorescence which is available for collection, analysis and interpretation (Sørensen, & Nybroe; cited in Development of new protocols for improved performance of whole-cell bacterial biosensors for determination of environmental quality ).
The toxicity of soil, atmosphere and water can therefore be assessed through biosensors. A system to continually monitor water consisting of mini-bioreactors in a two-stage setting was developed. This system is multi-channel where each of the channels has a different stress-responsive strain. In a system to assess the toxicity of gas, the biosensor is made to come into contact with the toxic gas in a sensing chamber through an immobilization technique while a soil biosensor was used to assess soil toxicity. In the latter soil system, recombinant bioluminescent bacteria were immobilized. The bacteria so constructed are activated at different levels of toxicity (Lichtfouse, Jan & Didier).
Determination of the total concentration of a pollutant in a sample has been used in pollution risk assessment. This particular assessment may include, traditionally, assessment of the whole product may be done while only a portion or very small amount is what is problematic to living organism. Thus an incidence of overestimating the risk by considering the total amount is likely (Harmsen). Bacterial sensor-reporters used in determination of bioaccessibility and bioavailability have been advantageous over biological and physico-chemical means which may alter fate of pollutants through biodegradation and reaching, for instance. The pollutants toxicity nature is associated to its bioavailability (Tecon, & JanRoelof).
In the constructing of bacterial-sensor reporter cell, the following parts are required. Principles of Genetic engineering can be used to combine together these parts. These parts include;
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DNA parts used in the making of circuit responsible for sensory and reporting (sensor-reporter circuitry). An example includes a case where sensing is through a single regulatory protein. The compound which is being examined is bound by this protein which also induces transcription of the reporter gene. Thus the signal is amplified.
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Component genes that is responsible for sensing and outputting (regulatory and reporter genes). The regulator for example may be separated from the sensor mechanism utilizing a protein (for instance a periplasmic receiver) and the signal is conveyed to the regulator mechanism which would induce the reporter after it is activated.
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Components for controlling the gene expression. These include ribosome binding sites, terminators, and promoters.
The conditions that are necessary for effective working and continual response of the biosensors include the following;
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Keeping the Microbe-based sensors (MBS) alive in an active state. This requirement imposes a difficulty in developing or producing them.
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Need for retention and development of specific optimal environment.
Most of the modern models consist of a reporter gene induced by the directing of a transcription factor from a DNA site such as promoter, and this means that the reporter protein synthesis is under the control of the transcription factor. The use of reporter protein depends on the application. While luciferases reporter proteins have been applicable at high sensitive areas in the bulk measurement of Micro-based sensors (MBS), eukaryotic luciferases has limitations because it requires permeabilization of cell membrane of bacteria and addition of substrate (7, 14).
The primary sensor protein and the transcription must be made to avail the specificity of the target to be detected. Protein synthesis may provide amplification of the signal sensed. In addition, RNA polymerase may be utilized in the interaction with the targeted perceptions transmitted by an internal effectors-binding domain. The sensory function may be provided by a sensory protein transmitting the perceived event through a signaling cascade.
Measuring of arsenic contamination in portable water can be done through use of biosensor cell through developed measures. Stocker, Denisa, Monika, Hauke, Jessika, Sylvia, Khurseed & JanRoelof used Green Fluorescent Protein (GFP), ²-galactosidase, and bacterial luciferase as reporter proteins, Escherichia coli and its resistance to arsenate and arsenite, which is natural. After exposing the sensor cells to test sample in aqueous form for 3 minutes incubation, light emission was then measured. Plain media was used to resuspend sensor cells, which could also be stored in frozen forms. For arsenite concentrations of 8 ¼g/L and above, visible blue color was noticed in ²-galactosidase-containing sample and a field testing achieved.
Elsewhere, the development of systems bioavailability and bioaccessibility assays that assess exposure of organisms to pollutants has been explored. Bioavailable portions of chemicals are those that are at the present freely available to cross from their place of inhabitance into the organism while bioaccessible ones only have potential for crossing into organism at this time. When the bioassays are immersed into the solution they detect the compound and report a signal.
A group of components that have been described as major pollutants of environment are the BTEX compounds found in crude oil-Benzene, Toluene, Ethylbenzene and Xylene. In addition to the carcinogenic effects of benzene, these chemicals affect respiratory and reproductive systems, may lead to arousal of blood disorder and affect the central nervous. These compounds may be ingested through drinking water.
There have been MBS developed for the BTEX such as the one using TodST sensor-regulatory proteins P todX promoters and the one using XylR and the P u promoters. Dissolved chemical concentrates may provide a model for the reactions that take place between the MBS and other components in the soil-water concentrates except in instances where metabolic interference takes places. Metabolic interferences may occur for example in cases where sensor-reporter cells may be limited to access the compound solution.
A polycyclic aromatic hydrocarbon (PAHs) which consists of more than a hundred components that are poorly soluble in water may be found in grilled meat, cigarettes, and may accumulate in tissues of animals. The number of aromatic rings and the chemical nature determine rates of biodegradation of PHs and this means that there is necessity to determine the bioavailability and bioaccessibility accurately in a varying environment. The knowledge of the degradation genetic details of PAHs in water has influenced the formulation of bacterial MBS and the designing has mostly favored Naphthalene.
MBS for Multi-target biosensor and toxic organic compounds
There is possibility of designing biosensors that target more than one target components. This is through use of bacteria such as Escherichia coli which has the possibility of acting as a host strain for sensor-reporter models and construction of variable reporter strains. This is because the bacteria have an easily controllable and maintainable growth. Construction of various models for different target components may entail testing of sample against different sensors. To determine whether MBS is reporting satisfactorily, there may be need to use an inducer of specific concentration added to the samples (Tecon & Beggah).
Phenols and their derivatives are compounds used as disinfectants, fungicides and herbicides that may be toxic and cause health damage over a long duration of exposure. Bacterial degradants of phenols have been developed. In this case, DmpR, TfdR, as the regulatory proteins and the P o P DII promoters have been applied. Modification of the sensor domain in the DmpR category made possible development of the more sensitive and models that can detect on broader ranges.
Polychlorinated biphenyls (PCBs) and oils
Although PCBs have been indicated as possible causers of negative health, lack of known bacterial systems for sensing them has hampered development of MBS systems for these compounds. One solution to this difficulty is the use of co-induction technique. This involves activation with uncharacterized proteins for example in the development of pUTK60. Other ideas explored includes use of PCB metabolites 3-Chlorobenzoate to detect PCBs in a bio-sensor-reporter and detection of hydroxylated PCBs in human serum using the HbpR system (Boldt, Sørensen, Karlson, Mølin, Ramos). Use of Pseudomonas oleovorans, AlkS regulatory protein as a control in producing bacterial luciferase and P alkB promoter (Sticher, Jaspers, Stemmler, Harms, , Zehnder, & van der Meer) have been used to build systems for alkane detection.
With the environmental pollution control slowly becoming a necessity and its awareness being expanded from governmental activities to a more publicized activity even in the production and manufacturing sector, what could be expected is a brighter future in the development of biosensors in the areas where difficulties and challenges have been identified.
There is need for continued research to determine the way forward to this kind of technology that is advantageous in some way, compared to the biological and physico-chemical means of assessing toxicity. In addition, evolution of technology presents an opportunity for more positive development of newer and better forms of biosensors. The possibility of expansion of application of biosensors through mutation to result through improved systems is an opportunity for development of better and improved forms.
In conclusion, interest in whole-cell biosensor technology due to their low cost and rapid performance. There are systems that have been developed to access environmental pollution and toxicity. The biotechnological techniques are used in the combination of particular components of these systems which would also require stringent conditions to function properly and continuously. The systems offer advantages over biological and physico-chemical means which may alter fate of pollutants through biodegradation and reaching, for instance, and which may offer inaccuracy determination of toxicity through overestimating.
Another advantage is that they are more accurate in determination of toxicity. Development of biosensors is favored also by the fact that mutations may be evolved to result in more better-for instance more sensitive, and diverse applications.
Work Cited
Boldt, T.S.; Sørensen, J.; Karlson, U.; Mølin, S.; Ramos, C. Combined use of different Gfp reporters for monitoring single-cell activities of a genetically modified PCB degrader in the rhizosphere of alfalfa. FEMS Microbiol. Ecol. 2004, 2, 139-148.
Development of a Set of Simple Bacterial Biosensors for Quantitative and Rapid Measurements of Arsenite and Arsenate in Potable Water. 2003. Web.
Development of new protocols for improved performance of whole-cell bacterial biosensors for determination of environmental quality . Web.
Harmsen, J. Measuring bioavailability: From a scientific approach to standard methods. J. Environ. Qual. 2007, 5, 1420-1428.
Lichtfouse Eric, Jan Schwarzbauer and Didier Robert. Environmental Biosensors Using Bioluminescent Bacteria.
Sørensen, J. and Nybroe, O. Reporter genes in bacterial inoculants monitor in vivo life conditions and functions in soil. In: Nucleic acids and proteins in soil (Eds. Smalla, K. and Nannipieri, P.). New York: Springer (2006).
Sticher, P.; Jaspers, M.C.M.; Stemmler, K.; Harms, H.; Zehnder, A.J.B.; van der Meer, J.R. Development and characterization of a whole-cell bioluminescent sensor for bioavailable middle-chain alkanes in contaminated groundwater samples. Appl. Environ. Microbiol. 1997, 10.
Tecon and Jan Roelof. Robin Bacterial Biosensors for Measuring Availability of Environmental Pollutants. 2008. Web.
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