Around one billion persons do not possess access to secured potable water and 150 million persons use surface water instead of potable water [1] [2]. In developing countries, a large part of the illnesses stays provoked by pathogens infected water [2]. Thus, furnishing secured potable water remains a huge dare to humans [2]. As a part of the present water treatment techniques, chlorination [3] [4] [5], ozonation [6], and ultraviolet (UV) technology remain frequently employed for killing pathogens and treating water [7] [8]. However, the two first methods form toxic disinfection by-products (DBPs) and the last one suffers from energy consumption [9] [10] [11]. To deal with such challenges, solar water disinfection (SODIS) was proposed as a cost-effective, point-of-use water treatment process, and especially practicable in developing countries [12] [13] [14]. Nonetheless, such a technique needs a comparatively longer period (from many hours to many days) for killing microbes following the intensity of accessible sunlight and water contamination [2]. Consequently, if more important disinfection rates in direct sunlight could be attained, it could certainly be a viable solution to the remaining disinfection techniques accessible [15] [16].

As an encouraging green method for ecological treatment and water remediation, photocatalysis has emerged during the past thirty years [2]. In photocatalytic disinfection technology, semiconductors with an appropriate optical bandgap may be employed as photocatalysts to produce reactive oxygen species (ROSs) in the occurrence of light for demobilizing microbes. Combining photocatalysis and SODIS methods seems to be a plausible concept to increase the kinetics of SODIS [2].

This work aims to focus on Escherichia coli especially its health impacts, exposure evaluation, and hazard reduction. This work firstly defines microbiologically E. coli bacteria and presents a brief history relating to their first discovery and following contagions. To understand E. coli’s behavior, a short description relating to their metabolism and physiology is given. As humankind is concerned by E. coli contagion, their clinical characteristics are discussed besides their subsistence in nature. A general examination concerning different techniques used for controlling such bacteria is finally presented.

The normal source of E. coli stays in the intestine of humans and other warm-blooded animals [36]. Even if E. coli will remain alive in nature, it does not seem to develop and will finally die [37]. Therefore, natural occurrence is considered a sign of faecal contamination [1]. There is proof that E. coli may, nevertheless, remain alive and multiply in tropical environments and so its value as an indication of faecal contamination in such regions is less certain [38] [39].

E. coli possesses significance in water bacteriology since it supplies a helpful sign of faecal infection and not on account of its intrinsic pathogenicity [40]. The philosophy is that, if E. coli is there, then possibly so could other pathogenic enteric microorganisms [41] [42]. Regardless of worries about the accuracy of E. coli as a sign of water safety, it remains the sole species that almost all routine samples are tested for [1] [43] [44].

Situations of E. coli pollution linked with infected under-treated water, and especially public potable water, have been communicated [45] [46] [47]. Contaminated cattle on farms are suspected to engender water infection with E. coli [48] [49] and irrigation water has been announced to be a pollution origin [50]. Researchers [51] established that E. coli O157: H7 was apt to remain alive for prolonged times in commercially bottled mineral water. As an illustration, after seeding water with 10³ E. coli O157: H7 cells/mL and storing samples at 15˚C, about 70 days passed before E. coli was not detectable in non-sterile mineral water, 49 days in sterile mineral water and 21 days in sterile distilled deionized water. Scientists [52] investigated subsistence of E. coli O157: H7 in well water microcosms utilizing water implied in a waterborne outbreak. Subsistence of the outbreak strain was similar to a wild-type E. coli strain under the identical circumstances. E. coli 0157 has been found in well water from four different sites in Scotland, UK [53]. Water samples were seeded with a lux-marked E. coli O157: H7 strain and stored at 15˚C. Following the water type, such microbes can be observed by culture for at least 65 days (end of monitoring) [1].

For E. coli 0157, the concentrations of chlorine (Cl₂) usually observed in water have been proved to be enough for its demobilization [63] [64]. Nonetheless, researchers [65] found that Cl₂ (5 mg/L) and ozone (O₃) at (22 - 24 mg/L) possess modest killing impact on E. coli O157: H7 [1].

Cheswick et al. [66] examined the performance of Cl₂ disinfection for the first time over a range of disinfection conditions employing flow cytometry to furnish new insights into disinfection methods. Demobilization was followed for pure culture bacteria (E. coli) and pathogens in real treated water from operational water treatment works (WTWs). A dose dependent increase in demobilization rate (k) was noted for both test matrices, with values of 0.03 - 0.26 and 0.32 - 3.14 L/mg min for the WTW bacteria and E. coli, respectively. Following 2 min, E. coli was decreased by 2 log for all Cl₂ doses (0.12 - 1.00 mg/L). For the WTW filtrate microbes, following 2 min log reductions were between 0.54 and 1.14 with augmenting Cl₂ injection, reaching between 1.32 and 2.33 after 30 min. A reduction in disinfection performance was detected as temperature decreased from 19˚C to 5˚C for both microbial populations. Concerning chlorination at varying pH (pH 6, 7, 8), membrane demolition was more significant at higher pH. This was not consistent with the higher disinfection performance observed at lower pH when culture based methods are utilized to estimate microbial reductions [66].

With chloramines, E. coli O157: H7 is known to possess a CT level (C = disinfectant concentration (mg/L); T = time in min) of around 9.2 mg-min/L. Such value is required to reach a 4 log₁₀ degree of demobilization [67]. Elevated temperatures have been proved to be efficient in killing E. coli [68] [69]. Regardless of this, EHEC has been observed to be fit to develop across a wide temperature span [1].

Radiofrequency power and UV light subjection have been notified to block E. coli [70]. Scientists [71] depicted that at 12 J/m², a 6-log reduction in culturability of E. coli O157: H7 happened. Nonetheless, resistant strains of E. coli O25: K98: NM requested bigger degrees of UV light for demobilization [1].

Researchers [72] tested potential merits of consecutive disinfection to dominate E. coli under circumstances of potable water distribution systems. They treated biofilms developed in polycarbonate and cast-iron reactors with Cl₂, chlorine dioxide (ClO₂) and monochloramine single or in integration with UV. Most important killing took place with the integration treatments with UV. Most importantly, chloramine was proved to be efficient in neutralizing E. coli in the effluent, yet not in the biofilm. The influence of Cl₂ on the growth phase of E. coli, in matter of its vulnerability to Cl₂, has established Cl₂ to be less performant when E. coli is in the stationary phase (juxtaposed with initial lag and exponential growth phase). This has been considered as a significant thought in wastewater treatment technique with changing solids retention times [1] [73].

The growing demand to decrease Cl₂ use and dominate (DBPs) augmented the search of fresh procedures in wastewater disinfection. For example, organic peracids are more and more attracting attention in killing pathogens as an encouraging alternative to Cl₂ and Cl₂-founded chemicals. Pironti et al. [74] evaluated the antimicrobial characteristics towards E. coli and Staphylococcus aureus of a fresh organic peracid, permaleic acid (PMA) juxtaposed with the reference peracetic acid (PAA). PMA presented a 10- and 5-fold reduction in the microbial inhibitory concentration level toward E. coli and S. aureus respectively, juxtaposed to PAA. Trials demonstrated higher performance of PMA concerning wastewater and treated wastewater disinfection at low dosages. PMA was more performant than PAA to avoid the regrowth of planktonic cells of S. aureus and E. coli. Therefore, PMA may be utilized as a potential alternative to the presently employed disinfection agents [74].

Even if pulsed UV (PUV) technique is adopted commercially for disinfection inside the food packaging industry, it is not used in the water/wastewater field [75]. Fitzhenry et al. [75] estimated the performance of PUV for disinfecting water disinfection below flow-through conditions. They employed E. coli, S. aureus and Listeria innocua to examine the capacity for photoreactivation and/or dark repair post PUV flow-through disinfection. Bacterial demobilization via flow-through PUV is a function of energy output with E. coli showing greatest sensitivity to PUV application (5.3 log₁₀ demobilization following application at 1539 mJ/cm², output in UV range < 300 nm); L. innocua presented the highest PUV resistance (3.0 log₁₀ demobilization following application at 1539 mJ/cm², output in UV range < 300 nm) below identical treatment circumstances. Greater photoreactivation took place at lower PUV outputs for both S. aureus and E. coli following flow-through PUV treatment. Therefore, exposure of inactivated bacteria to natural light, immediately post flow-through PUV treatment, must be averted to decrease photoreactivation. The LPUV proved demobilization of all microbes below the limit of detection (1 colony-forming unit (CFU)/mL) and inhibited the presence of photoreactivation.

He et al. [76] suggested a solar-light-driven magnetic photocatalyst, reduced-graphene-oxide/Fe,N-TiO₂/Fe₃O₄@SiO₂ (RGOFeNTFS), for the photocatalytic disinfection of various strains of bacteria: gram-negative E. coli and Salmonella typhimurium, and gram-positive Enterococcus faecalis (Figure 2). Gram-positive E. faecalis was observed to be more vulnerable to photocatalytic disinfection and showed a higher leakage of intracellular components than the two gram-negative bacteria. The interactions between the bacteria and RGOFeNTFS were examined for Zeta potential, hydrophilicity and scanning electron microscopy (SEM). The opposite surface charges of the bacteria (negative Zeta potential) and RGOFeNTFS (positive Zeta potential) contribute to their interactions. With a more negative Zeta potential (than E. coli and E. faecalis), S. typhimurium interacts more strongly with RGOFeNTFS and is mostly attacked by ^•OH near the photocatalyst surface. With less negative Zeta potentials, E. coli and E. faecalis interact less strongly with RGOFeNTFS, and compete for the dominant reactive species (^{• O 2 −} ) in the bulk solution. Thus, the co-existence of bacteria significantly inhibits the photocatalytic disinfection of E. coli and E. faecalis, but insignificantly for S. typhimurium. Furthermore, photocatalytic disinfection employing RGOFeNTFS is

promising for dealing with real sewage, and various bacteria are killed simultaneously [76].

Suggesting narrow-band mercury-free light sources, like light emitting diodes (LEDs) and excilamps, has motivated investigation on killing pathogens via dual-wavelength light radiation [77]. Indeed, dual-wavelength light radiation is considered as a developed instrument for improving microbial demobilization in water in view of potential synergistic effect. Matafonova and Batoev [77] focused on its pathways under dual-wavelength light exposure and discussed some related references in terms of yes-or-no synergy. They suggested three fundamental demobilization pathways, which work in the estimated spectrum ranges I (190 - 254 nm), II (250 - 320 nm) and III (300 - 405 nm) and furnish a synergistic effect when combined. Such pathways implicate proteins damage and deoxyribonucleic acid (DNA) repair suppression (I), direct and indirect DNA damage (II) and generation of ROSs via endogenous photosensitizers (III), like porphyrins and flavins (Figure 3). A synergy under dual-wavelength light irradiation simultaneously or sequentially takes place if coupling two wavelengths of different ranges (I + II, I + III, II + III) in order to trigger various demobilization pathways. New progresses of dual-wavelength light strategy in photodynamic therapy can be applied for water disinfection. They open perspectives for using the sources of near-UV and visible radiation and making the disinfection techniques more energy- and cost-effective. In such context, the synergistically efficient dual-wavelength combinations II + III and the combinations within the extended to 700 nm range III (near-UV + VIS) seem to be encouraging for

developing fresh advanced oxidation processes for disinfection of real turbid waters [77].

In rural communities, developing communities with low quality centralized water distribution, portable water purification devices are requested to furnish potable water. As mentioned above, filtration, UV light, or chemical injections present tools to eliminate microorganisms from water. Montenegro-Ayo et al. [78] suggested a small portable photoelectric point-of-use device that uses a commercial teacup from which TiO₂ nanotube photoanodes were formed in-situ. With a small rechargeable battery powered 365 nm LED, the apparatus was found to reach 5-log demobilization of E. coli in 10 s and 2.6-log of Legionella in 60 s of treatment in model water samples (Figure 4). Dealing with natural water attained a 1-log bacteria demobilization after 30 s thanks to matrix effects.

For killing pathogens, hydrodynamic cavitation seems to be an encouraging technology. Sun et al. [79] investigated the disinfection properties of an advanced hydrodynamic cavitation reactor (ARHCR) in pilot scale. They examined the impacts of different flow rates (1.4 - 2.6 m³/h) and rotational speeds (2600 - 4200 rpm) on killing E. coli. A disinfection rate of 100% was attained in only 4 min for 15 L of simulated effluent under 4200 rpm and 1.4 m³/h, with energy efficiency at 0.0499 kWh/L. The morphological changes in E. coli were studied by SEM (Figure 5). The ARHCR may conduct to serious cleavage and surface damages to E. coli. They suggested a likely damage route of the ARHCR, comprising both the hydrodynamical and sonochemical impacts (Figure 6).

Antibiotic-resistant bacteria (ARB) form a dangerous threat to public health [80]. As a low energy consumption and environmentally-friendly technology, electrochemistry seems to be appropriate for killing ARB. Liu et al. [80] investigated the suitability of electrochemical disinfection (ED) for demobilizing ARB (E. coli K-12 LE392 resistant to kanamycin, tetracycline, and ampicillin) and the regrowth probability of the treated ARB. They depicted that 5.12-log ARB reduction was reached within 30 min of applying molybdenum carbide as the anode

and cathode material under a voltage of 2.0 V. No ARB regrowth was noted in the cathode chamber after 60 min of incubation in unselective broth, showing that the technique in the cathode chamber was more efficient in demobilizing ARB permanently. The pathways underlying the ARB demobilization were verified founded on intercellular ROSs measurement, membrane integrity detection, and genetic damage assessment. Higher ROSs generation and membrane permeability were detected in the cathode and anode groups (p < 0.001) compared to the control group (0 V). Furthermore, the DNA was more likely to be damaged throughout the ED application. Such investigation establishes once again that ED is an encouraging process for disinfecting water to avert the diffusion of ARB (Figure 7).

Xia et al. [81] presented a new process founded on piezoelectric catalytic persulfate (PS) activation for water advanced disinfection. They synthesized and used silver modified barium titanate (Ag-BTO) as a piezoelectric catalyst to activate PS under ultrasonic (US) vibration for demobilizing E. coli. The suggested US/Ag-BTO/PS method reached a 6.2 log demobilization within 5 min and 20 min, respectively, for E. coli at culturable state and viable but nonculturable state (Figure 8). The important performance of E. coli reduction was related to the successive formation of hydroxyl radicals and sulfate radicals via PS activation by piezo-catalytically formed electrons and superoxide radicals. A synergism between ultrasonication and radical oxidation was observed by employing the

US/Ag-BTO/PS method for E. coli demobilization. The ultrasonication disrupted cell membrane of E. coli, accelerated the permeation of the radicals and enhanced the following inner metabolic dysfunction and enzyme oxidation by radicals [81].

Concerning E. coli diarrhoeal contagions, fluid and electrolyte correction is obligatory [1]. Extraintestinal E. coli contagions are, yet, treated with antibiotics. If such procedure of treatment is used, it must be admitted that E. coli reveal intrinsic resistance to benzylpenicillin. E. coli remain usually vulnerable to the antibiotics (ampicillin, tetracycline, aminoglycosides, trimethoprim and the cephalosporins). Nonetheless, due to the broad diffusion proof of antibiotic resistance being acquired by plasmid transfer, there is growing numbers of E. coli resistant to streptomycin and tetracycline. For such cause, antibiograms must be realized, mostly for epidemiological objectives. Employing antibiotics in dealing with E. coli 0157 remains controversial because of the trouble of growing numbers of E. coli 0157 with antibiotic resistance. Most important is the augmenting propagation of extended-spectrum β-lactamase (ESBL) producing E. coli [82]. ESBL generating E. coli show resistance to the majority of the b-lactam antibiotics, comprising penicillins, monobactams and most cephalosporins. The explanation for this stays the occurrence of plasmid-encoded enzymes, which hydrolyze the β-lactam antibiotics [1]. Such microbes are more and more involved in provoking hospital, as well as community-acquired contagions and when existing, tend to be treated with carbapenems, since these remain active against many ESBL-E. coli [83]. However, resistance of E. coli to carbapenems is as well as emerging [84] [85] [86].

Antibiotics that have historically been active against E. coli have comprised sulfonamides, tetracyclines, aminoglycosides (comprising gentamicin and amikacin), chloramphenicol, semi-synthetic penicillins, cephalosporins, β-lactamase-inhibitor combinations, carbapenems and fluoroquinolones [87] [88]. Notwithstanding all such antibiotics being standard therapies for E. coli, there has been much worry concerning the fast expansion of resistance [89] [90]. Fundamental pathways of resistance against such antibiotics have comprised exclusion and efflux of the agents from the bacterial cell, acquisition of resistance genes comprising those that generate enzymes fit to decompose β-lactamas, carbapenemases, and aminoglycoside. As aforesaid, extended-spectrum β-lactamases (ESBLs) are plasmid-mediated enzymes that may decompose recent analogues of cephalosporins. It has been assessed in the UK that of all 10% of E. coli bacteraemias are linked with ESBL strains and 90% of these are CTX-M type [91].

Relating to antibiotic resistance amongst environmental isolates, scientists [92] fixed the minimum inhibitory concentrations for 241 E. coli isolates recuperated from water, sediment and biofilms in an intensive agricultural watershed (Elk Creek, British Columbia, Canada) between 2005 and 2007. Such isolates had an elevated frequency of resistance to tetracycline, ampicillin, streptomycin and nalidixic acid [1].

This work firstly defined microbiologically E. coli bacteria and presented a brief history relating to their first discovery and following contagions. To understand E. coli’s behavior, a short description relating to their metabolism and physiology is given. As humankind is concerned by E. coli contagion, their clinical characteristics are discussed besides their subsistence in nature. A general examination concerning different techniques used for controlling such bacteria is finally presented. The main points drawn from this work may be listed as below:

1) In terms of health effects (i.e., the occurrence of disease, degree of morbidity and mortality): a) the pathogenic E. coli serotypes are categorized following their route of producing symptoms (The contagious dose of most pathogenic E. coli is elevated, ranging from 10⁵ to 10¹⁰ organisms) [1]. b) The six groups are enteropathogenic (EPEC), enterotoxigenic (ETEC), verocytotoxigenic (VTEC; comprising enterohaemorrhagic, EHEC), enteroinvasive (EIEC), enteroaggregative (EAEC) and diffusely adherent. c) All pathogenic E. coli provoke diarrhea at different levels of gravity. d) EPEC: traveler’s diarrhoea, enteritis in infants; ETEC: traveler’s diarrhoea, diarrhoea, vomiting and fever; VTEC: Shigella-like dysentery (stools with blood and mucus) and haemolytic uremic syndrome (HUS); diffusely adherent: diarrhoea in children. e) E. coli induces urinary tract infections (UTIs) and may provoke sepsis and meningitis in neonates. f) Pathogenic E. coli strains lead to the majority of childhood diarrhoea. g) The level of morbidity and mortality changes following the strain and the host’s properties. In poor countries, diarrhoeal illness is much more probably to conduct to the dangerous disease and death. In rich countries, even if childhood diarrhoea is less serious, contagion with verocytotoxigenic E. coli may lead to HUS and thrombiotic thrombocytopaenia purpura. Such circumstances could create acute kidney failure and death [1].

2) Conventional water treatments employ high energy-intense mercury lamps or chlorine injection, which remain neither a feasible nor economically viable methods for vulnerable populations in developing areas [78]. Such competitive techniques may be overcome by a more affordable and off-grid method like a device that is founded on TiO₂ photoelectrocatalytic disinfection concepts. Applying photoelectrocatalytic processes in scaled-down and portable devices authorizes performant water disinfection when using an off-grid point-of-use system. Employing LED sources guarantees working with low energy consumption. This furnishes interesting alternatives to traditional disinfection treatments [78].

3) A pilot-scale ARHCR was tested for water disinfection, and a new probable disinfection route of the ARHCR was proposed comprising hydrodynamical and sonochemical effects. The ARHCR could be used as an encouraging alternative or complementary tool for water disinfection as well as other process intensifications. More research on the disinfection route, structural optimization, and scale up remains required in the future [79].

The Research Deanship of University of Ha’il, Saudi Arabia, has funded this research through the Project RG-20 113.

The authors declare no conflicts of interest.

D., Ghernaout, Elboughdiri, N. and Lajimi, R. (2022) E. coli: Health Impacts, Exposure Evaluation, and Hazard Reduction. Open Access Library Journal, 9: e8860. https://doi.org/10.4236/oalib.1108860

ARB Antibiotic-Resistant Bacteria

ARHCR Advanced Hydrodynamic Cavitation Reactor

CFU Colony-Forming Unit

DAEC Diffuse adherent E. coli

DBPs Disinfection by-Products

DNA Deoxyribonucleic Acid

EAEC Enteroaggregative E. coli

ED Electrochemical Disinfection

EHEC Enterohaemorrhagic E. coli O157: H7

EIEC Enteroinvasive E. coli

EPEC Enteropathogenic E. coli

ESBL Extended-spectrum β-Lactamase

ETEC Enterotoxigenic E. coli

HUS Haemolytic Uremic Syndrome

k Demobilization rate (k)

KIA Kligler’s Iron Agar

LEDs Light Emitting Diodes

ONPG Ortho-Nitrophenyl-β-D-Galactoside

PAA Peracetic Acid

PMA Permaleic Acid

PPRI Photo-Produced Reactive Intermediate

PUV Pulsed Ultraviolet

RGOFeNTFS Reduced-Graphene-Oxide/Fe,N-TiO₂/Fe₃O₄@SiO₂

ROSs Reactive Oxygen Species

SEM Scanning Electron Microscopy

SODIS Solar Water Disinfection

TSI Triple Sugar iron

US Ultrasonic

UTIs Urinary Tract Infections

UV Ultraviolet

VTEC Verocytotoxigenic E. coli

WTW Water Treatment Works