Waterborne and related diseases not only deteriorate the environment and cause pollution, but they are also the reason of increasing rates in morbidity and mortality. Though efforts are continuously being made to preserve water safety, waterborne outbreaks are still reported
worldwide. Diseases related to water cause more than 3.4 million deaths each year (Berman, 2009).
Water is a "Universal Solvent," as it is able to dissolve almost everything (Anne Marie Helmenstine, 2019). This makes water essential for the proper functioning of the Earth's ecosystem. All water-related problems are a major public health concern all around the world. The foremost source of water pollution are the industries. Industries use freshwater to carry the wastes from different sources into rivers, lakes, and oceans. Water pollution has a negative impact on all living creatures and consequently the food chain as well. It can negatively affect the daily use of water from drinking to bathing. To get the proper solution to all these water-related problems, the treatment of water is necessary. Therefore, for the proper analysis and control of water quality, the detection of the pollutants and toxic contaminants is a major step. To detect any type of contaminant, a wide variety of tools, as well as advanced analytical methods, are required. Specialized, appropriate, specific, and powerful diagnostic tools have been developed to detect various contaminants in water (Ramírez-Castillo, 2015), as well as chemical analysis techniques are also of great importance in determining specific substances, but most of these tools and the techniques are limited in analysing a complex sample that contains contaminants in a huge variety. Also, they are time-consuming, require lengthy protocols, and rely on very expensive instrumentation.
In recent years, the progress of biosensor has unlocked an extraordinary perspective to the onsite, easy, and money-saving monitoring of water quality (Chaubey A, 2002). In order to analyse contaminants of water, the innovative idea of biosensors is appropriate and convenient. By electrochemical means, an immense proportion of enzymatic biosensors are operated. Towards the target analyte, high sensitivity is the benefit of Enzymatic Biosensor (Eggins, 2013). Nevertheless, due to enzyme deactivation, it also faces drawbacks like time consumption, expensive protocols of enzyme purification and immobilization; as well as less life span and deprived stability (Jaffrezic-Renault, 2011).
As compared to Enzymatic Biosensor, Microbial Biosensor is more beneficial as it has more simplicity associated with biocatalyst production, predominantly when huge quantities are needed. To a large variety of analytes, microbial biosensors are more flexible and tactful (Jaffrezic-Renault, 2011). Mainly Microbial Biosensors have been explored as water quality monitoring devices, and presently few prototypes used as water toxicity sensors are also commercialized. Microbes that remain alive under high alkaline, acidic, high temperature, and saline conditions give ways to great perspectives on water monitoring for industrial process waste monitoring (D'souza, 2001).
E.coli has the honour of being the most extensively studied microbial organism due to its various important roles in fields related to biotechnology, industrial sciences, medicine, and biological sciences; and in recent years, it has gained quite a positive reputation in development of Microbial Biosensors. Being a model organism for laboratories, almost every aspect of E.coli, from its genome to its protein-making capability, has been exploited (Lee, 2009). In bacterial metabolism, oxygen acts as the hydrogen acceptor in the TCA cycle under the aerobic condition for growth and respiration (Hadjipetrou, 1966). However, under anaerobic conditions, an artificial hydrogen acceptor can be utilized by the bacterial metabolism as well. This artificial hydrogen acceptor can be a nitrate, sulphate, benzoquinone, or ferricyanide. We used ferricyanide as a hydrogen acceptor because after accepting the hydrogen, it gets reduced into ferrocyanide, and this whole reaction is accompanied by a change in colour (Pujol-Vila, 2015). Bacteria are termed immortal because they do not die a natural death, but they only die because of abiotic factors like temperature, pH, and insufficient nutrient supply (Bickerstaff, 1997).
Several assays indirectly assess the activity of microbial cells by quantitatively analysing changes to the bacterial metabolic system (Buffi, 2011), specifically to the electron transport chain. The electron transport chain is selected because it maintains the basic integrity of the cell. There are various aspects of analysing the microbial metabolism, and it can be analysed by measuring inhibition of respiration, by qualitatively assessing the accumulation of ferrocyanide in ferricyanide mediated bioassays or by quantifying bioluminescence. The incubation of bacteria that can utilize ferricyanide as an alternate terminal electron acceptor is referred to as ferricyanide-mediated respirometry assay (FM-RES assay) (Catterall, 2010). Our experiment revolves around the fact that in the presence of a heavy metal such as copper, lead, mercury, chromium, cadmium, etc., this metabolic conversion of ferricyanide into ferrocyanide altogether stops. The bacteria are unable to reduce ferricyanide; thus, no ferrocyanide is produced. We are using this principle to detect the presence of heavy metals in water.