**1. Introduction**

Public health protection is of paramount importance that demands the rapid and accurate detection and quantitation of microorganisms in potable water and in various raw and processed foods to prevent undesirable outbreaks of microbial contamination. Water quality has been assessed for potable and recreational activities using culture-dependent quantification and sensing of fecal indicator bacteria (FIB), such as total coliforms, *Escherichia coli*, or Enterococci, an approach that is used as a reference standard in the evaluation of microbial safety of water [1]. The presence of FIBs in large numbers in freshwater, particularly *E. coli* and *Enterococcus*, has been associated with the emergence of waterborne illnesses [2, 3]. Children as young as five years are particularly susceptible to diarrheal infections, with over 800 children dying every day [4, 5]. Amongst coliform bacteria, *E. coli* is commonly regarded as an indicator of fecal pollution of water supplies [6, 7].

Waterborne diseases have been one of the major causes due to the consumption of contaminated water affecting seriously the public health of a humongous number of people in quick succession. In the 2014–2016 survey, the detection

rate of pathogenic bacteria was 79.3%, followed by pathogenic *E. coli* (5009 cases, 90%), *Vibrio* spp. (264 cases, 5%), *Shigella* spp. (67 cases, 1%), and *Salmonella* spp. (48 cases, 1%) [8]. The distribution of *E. coli* amongst Korean children suffering from diarrheagenic *E. coli* showed that enteropathogenic *E. coli* (EPEC) was the most common, followed by ETEC and enterohemorrhagic *E. coli* (EHEC) [8]. Of the pathogenic *E. coli*, enteropathogenic *E. coli* (EPEC) was the most common (39%), followed by enterotoxigenic *E. coli* (ETEC) (36%). In a separate study, children suffering from diarrhea were reported [9] in Utah, USA wherein the most commonly detected pathogens included toxigenic *Clostridium difficile* (16%) and diarrheagenic *E. coli* (15%) whereas Shiga toxin-producing *E. coli* were detected in 4% samples [9].

Between 2013 and 2016, a monocentric hospital-based investigation showed that *E. coli* was responsible for about 15% of child infection cases of severe enteritis and EPEC (54%) was the most dominant *E. coli* pathotype, followed by other pathogenic *E. coli* including Shiga toxin-producing *E. coli* [10]. And on the heels of that, in another waterborne outbreak of *E. coli* infection associated with the drinking of contaminated potable water at three different school premises in Korea was reported [11]. As a result of this outbreak, a total of 188 patients with severe gastrointestinal symptoms were reported. The EHEC and EPEC strains isolated from clinical fecal specimens and water samples from water purifiers and water basins respectively were confirmed by the pulsed field gel electrophoresis method [11]. It is warranted therefore to develop rapid and sensitive methods for the detection and quantitation of waterborne bacteria.

Coliforms, particularly *E. coli* is regarded as a primary fecal indicator [12] and indicate the contaminating presence of enteropathogenic bacteria in water and foods supplies [13]. Though these enteric bacteria are abundant in human and warm-blooded animal feces, an umpteen number of the *E. coli* strains have been reported as pathogens [14]. Despite the fact that the wild type of *E. coli* strain is not pathogenic, it could emerge as an infectious agent in immunologically vulnerable people. Furthermore, several *E. coli* O157:H7 outbreaks have been documented in both industrialized and developing economies, resulting in human mortality, notably amongst children under the age of five [15]. *E. coli* serogroup O157:H7 is the most common cause of hemorrhagic colitis in foodborne illness. *E. coli* serogroup O104:H4 was first discovered as an emerging strain in the 2011 German pandemic and was designated a microorganism of serious concern [16]. Perna et al. [17] reported that *E. coli* O157:H7 caused 75,000 cases of foodborne infections per year, of which 85 percent incidences were related to *E. coli* O157 infections [18, 19] with contaminated fruits, vegetables, and water is the principal sources of *E. coli* O157:H7 outbreaks [19].

Traditional microbiological detection techniques consume time as *E. coli* cells require to be isolated, cultivated, and identified using a sequence of biochemical tests [20]. For example, for identification and quantification of *E. coli* in water, the water samples are filtered using the membrane filtration method, followed by the counting of *E. coli* colonies using the plate count method [21]. Furthermore, such processes necessitate 24 to 48 hours to generate observable results and frequently require water samples to be transported to a central laboratory and trained employees to conduct the testing [22].

It is necessary to develop new approaches for detecting *E. coli* in contaminating food and water samples. Optical or impedimetric biosensor systems have evolved as an alternative to the traditional tools for *E. coli* detection, enabling selective, specific, and cost-effective solutions. DNA-based sensing approaches have played an essential role in the development of sensing for the detection of *E. coli*. Due to their rapidity and accuracy, sensing technologies such as the polymerase chain reaction

(PCR), loop associated isothermal method of amplification (LAMP), DNA-based biosensors, and CRISPR/Cas platforms have evolved over time for *E. coli* detection and have been applied in numerous applications in various industries, agriculture, and health care sectors.
