1. Introduction
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.
2. Microbiological Viewpoint and Natural History
2.1. Microbiological Viewpoint
In most cases, E. coli are Gram-negative, rod shaped (2.0 - 6.0 mm in length and 1.1 - 1.5 mm wide bacilli) bacteria with rounded ends (Figure 1) [1] [15]. The real form of such microbes could, nonetheless, change from spherical (cocci) cells to elongated or filamentous rods [17] [18]. E. coli are non-spore forming, and are generally motile through the action of peritrichous flagella. E. coli are facultatively anaerobic and generate gas from fermentation of carbohydrates, as seen by acid and gas formation from lactose at 37˚C and 44˚C. Most E. coli produce a positive ortho-nitrophenyl-β-D-galactoside (ONPG) reaction, showing β-galactosidase activity. The methyl red reaction is also positive for E. coli showing mixed acid fermentation of glucose; however, the Voges-Proskauer reaction (acetoin production) is negative. E. coli generate indole, yet are not able to hydrolyze urea or develop in Møller’s KCN broth (depicting an incapability to develop in the existence of cyanide). In addition, formation of hydrogen sulfide (H2S) is not usually clear when E. coli are cultured on triple sugar iron (TSI) agar or Kligler’s iron agar (KIA). E. coli as well do not induce gelatin liquefaction via gelatinase activity. The majority of strains decarboxylate lysine, utilize sodium acetate, yet do not develop on Simmons’ citrate agar, where citrate is the only carbon source [1].
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Figure 1. Typical microscope image of E. coli cells processed with Microbe Tracker [15].
Numerous E. coli cells are capsulated or microcapsulated and such capsules are constituted of acidic polysaccharides [1]. Mucoid strains of E. coli generate extracellular slime consisting either of a polysaccharide of certain K antigen specificities, or a usual acid polysaccharide (frequently reported as M antigen) formed of colanic acid [19]. E. coli display fimbriae (or pili) of changing structure and antigenic specificity and since such fimbriae are hydrophobic, they furnish host- or organ-specific adhesion features.
Many E. coli serogroups are familiar and the plurality is non-pathogenic; nonetheless, several groups could provoke dangerous diarrhoeal disease, sometimes with fatal outcome. E. coli is of faecal origin and is almost exclusively detected in the digestive tract of warm-blooded animals, especially human beings. Therefore, observation of E. coli in drinking water is employed as an indicator of human or animal excreta pollution, and is known as the coliform index [20].
The most famous and well-investigated E. coli strain is enterohaemorrhagic (EHEC) E. coli O157: H7 [1] [21]. Members of the “O157” serogroup possess the usual somatic (cell surface) O antigen, while the flagellar H antigen is employed to define the specific serotype. E. coli O157: H7 is seen as one of the most problematic and pathogenic serotypes, and is frequently synonymously referred to as EHEC. From 1982 to 2002, E. coli O157: H7 was notified in 49 states of the USA and related to 73,000 illnesses [22]. Such serotype manifests extended subsistence in water at low temperatures [23] [24]. Subsistence was indeed depicted to expand beyond 8 months in a farm water gutter, and such microbes were then capable to colonize cattle. Importantly, swimming in polluted water has as well led to outbreaks of contagion [25] [26].
Some less frequently faced strains of E. coli could be found in nature and drinking water reservoirs, and could as well provoke diarrhoeal illnesses (like dehydrating diarrhoea and traveler’s diarrhea [27] ) via changing routes [1]. The incubation period for disease depends on strain and this is mostly related to the changing pathogenic routes revealed. In most cases, the incubation period is 1 - 2 days, even so could expand to 5 days. Even if the pathogenic character of E. coli has been recognized for a long time, its character as an enteric pathogen has not long ago been reinforced via the manifestation of E. coli O157: H7 and the relationship of such strain with haemorrhagic enteritis and haemolytic uremic syndrome (HUS) [28].
E. coli are linked with a set of human infections, following circulation from the intestines of patients who have an underlying problem [1]. As an illustration, urinary tract infections (UTIs) attributed to E. coli frequently happen following direct diffusion from the rectum to the urethra. Infections at other body sites commonly grow by haematogenous diffusion (through the blood stream), as typified by appearance of meningitis in young babies. E. coli are as well a frequent reason for postoperative wound contagion, where direct infection of the wound (when the bowel was opened) could grow, or indirect infection by faecal infection of patient fingers. E. coli can as well infect patients via colonized members of the health care team, as well as other patients [1].
As aforesaid, some strains of E. coli provoke diarrhoea after faecal-oral diffusion from humans and animals [1]. The next sections will be dedicated mostly to diarrhoeal contamination emerging from the different pathogenic types of E. coli and will define the epidemiology and clinical characteristics as well as virulence factors and their underlying genetic pathways [29].
2.2. Natural History
Escherichia coli was originally discovered in 1885 and called “Bacterium coli” [30] by Dr. Theodor Escherich, a German paediatrician [1]. He discovered the bacterium during investigations of the intestinal flora of infants. After that, the bacterium was established to possess pathogenic features implying extraintestinal contamination [1]. Up until 1919, the name “Bacterium coli” was largely employed and then Castellani and Chalmers [31] described the genus Escherichia and established the type species E. coli [1].
There are at least six principal diarrheagenic pathovars of E. coli (two different pathovars are related to UTIs and neonatal meningitis) and each type integrates some form of initial attachment to the host cell with following harmful results, either via the elaboration of a toxin, or direct action [32]. Such E. coli types comprise the already mentioned enterohaemorrhagic (EHEC), along with enterotoxigenic (ETEC), enteroinvasive (EIEC), enteropathogenic (EPEC), enteroaggregative (EAEC) and diffuse adherent E. coli (DAEC). Each specific type provokes diarrhoeal disease by numerous routes and each disease manifests with many clinical symptoms [1].
3. Metabolism and Physiology, and Clinical Characteristics
3.1. Metabolism and Physiology
Most strains of E. coli have the potential to ferment lactose, and the occurrence of lactose will as well display a positive ONPG reaction via the activity of β-galactosidase [1]. E. coli forms indole from the amino acid tryptophan via action of the enzyme tryptophanase, and this is a special property of E. coli from other enteric bacteria [33].
E. coli are incapable to hydrolyze urea and as well do not form gelatinase [1]. E. coli do not develop in Møller’s KCN broth due to development inhibition by cyanide. H2S generation is usually absent when E. coli are grown on TSI and KIA. These media are utilized to reveal fermentation of specific carbohydrates and via integration of sodium thiosulfate and iron allow H2S detection. E. coli does not deaminate phenylalanine, while most strains can decarboxylate lysine and use sodium acetate. E. coli do not grow on Simmons’ citrate agar, which includes citrate as the sole carbon source. The biochemical properties of the Escherichia genus are listed in Table 1, and those of E. coli are given in Table 2.
3.2. Clinical Characteristics
A century ago, Muir and Ritchie [34] were the premier to define the pathogenic
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Table 1. Biochemical properties of Escherichia genus [1].
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Table 2. Biochemical properties of E. coli [1].
features of Bacterium coli related to contaminations of the intestine and urinary tract, some cases of summer diarrhoea (cholera nostras), infantile diarrhoea and food poisoning [1] [34]. The pathogenicity of Bacterium coli was defined as: Bact. coli is a normal inhabitant of the intestine of man and other animals. In certain circumstances it acquires pathogenicity, and may cause local or general infection. It is a frequent cause of acute and chronic infection of the urinary tract, and may give rise to an acute or chronic cholecystitis” [1].
Nowadays, E. coli is rated as a harmless member of the normal microbiota of the human inhabiting the distal end of the intestinal tract [1]. The organism is usually acquired at birth or via the faecal oral route from the mother and also from nature. The serotypes of E. coli that provoke contaminations are listed in Table 3.
E. coli remains the most prevalent reason for acute UTIs and urinary tract sepsis [35]. E. coli could as well rise neonatal meningitis and sepsis, and as well abscesses in several organ systems. E. coli can as well occasion acute enteritis in humans, as well as animals and is a reason for “traveler’s diarrhea”, dysenterylike disease touching humans and haemorrhagic colitis frequently known as “bloody diarrhea”. Numerous oral challenge investigations have been performed with E. coli serogroups to evaluate infection doses. The findings of such investigations propose that levels of 105 - 1010 EPEC organisms, 108 - 1010 ETEC and 108 cells of EIEC have to be ingested to bring about diarrhoea and infection. Such numbers will naturally change with the age and sex of the recipient as well as the acidity of stomach. For EHEC, the infective dose that is apt to provoke contamination is less than 100 [1].
EHEC is the E. coli serogroup that is probably of most clinical concern. Such microbes are recognized to provoke HUS, a case that is distinguished by acute renal failure and generally happens in children under the age of 5 years old. An
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Table 3. Serogroups and illness associations of E. coli [1].
evaluated 10% of those contaminated with EHEC 0157 may develop HUS, haemolytic anaemia or thrombocytopaenia. Around 5% of cases of EHEC develop haemorrhagic colitis, which could progress into HUS, in which mortality rates may be as high as 10%. The pathogenic pathways of the major pathogenic groups of E. coli are summarized by Percival and Williams [1].
4. Subsistence in Nature
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].
4.1. Subsistence in Water and Epidemiology
All enterovirulent E. coli are obtained directly or indirectly from human or animal carriers [1]. Danger from potable water, for that reason, only follows faecal pollution of the supply. Considering the vulnerability of E. coli to Cl2 and other killing agents, even if the microbes infect the supply, sufficient chlorination as a rule efficiently eliminates any health hazard [54].
There are no standards for E. coli 0157 (EHEC) inside the 1980 European Drinking Water Inspectorate Directive [1]. In 1997, the Environment Group of the previous Scottish Office of Agriculture, Environment and Fisheries Department authorized the Water Research Council to assume an investigation inspecting present proof for waterborne transmission of E. coli 0157 [1]. Such investigation concluded that there was no proof that E. coli 0157 was more enduring in nature or more reluctant to water treatment technologies contrasted with non-pathogenic E. coli detected in the human gastrointestinal tract [1].
Worries subsist concerning the possible part that biofilms play in saving enterovirulent E. coli. The very elevated infectious injections needed for all enterovirulent E. coli, other than EHEC, propose that such potential pathway of transmission is improbable as a hazard. The lower infectious injection of EHEC dose possibly augments the danger of contamination from biofilms in water; however, there have been no epidemics or occasional cases of EHEC involved with adequately disinfected water supplies [1].
Fresh investigation has proved that E. coli may persist in aquatic environments, even if the elements that participate in subsistence are not well grasped. Higher predominance of E. coli 0157 has surely been observed throughout warmer periods [55]. Recreational water exposure, comprising use of swimming pools, has been implied as an origin of E. coli O157: H7 [25] [56], yet details concerning E. coli O157: H7 in natural waters and potable water stays restricted, which is mainly related to the reality that frequently only small levels (below the sensitivity limits of detection) of such bacterium take place [1].
In the Netherlands, E. coli O157 was isolated (employing a particular enrichment procedure) in 2.7% of 144 private wells, regardless of such samples satisfying the requested potable water standards [57]. In Canada, after a 2-year investigation realized in the Oldman River watershed, 0.9% of surface water samples (n = 1483) were infected with E. coli O157: H7 [49]. In river water, E. coli O157: H7 has as well been isolated from the Oldman River Basin in Southern Alberta, Canada [58]. E. coli O157 was detected in 33 surface water samples in Baltimore, USA. Nonetheless, E. coli 0157 was only observed in low levels of <1 cells per 100 mL of raw water [59].
4.2. Enterotoxigenic (ETEC)
Person to person diffusion of ETEC seems to be scarce and most diffusion of occasional illness is via food and water sources [1]. Because of ETEC, numerous waterborne outbreaks have been registered. One very huge outbreak touched more than 2000 staff and visitors to an American National Park in Oregon in the summer of 1975 [60]. ETEC were separated from 20 (16.7%) of 120 rectal swabs inspected. There was a strong association between disease and potable park water in park staff and visitors (p < 0.00001). The only group in which there was no correlation with potable water was one comprising visitors on 7-9 July 1975, when chlorination of the water supply was carefully supervised. Water came from a shallow spring that was polluted by a sewage overflow some 650 m uphill from the spring. The supply was assumed to be chlorinated; however, there was no methodical control of Cl2 concentrations throughout the distribution system. Different known outbreak touched 251 passengers and 51 crew on a Mediterranean cruise [61]. ETEC was separated from 13 of 22 passengers and 6 of 13 crew sampled. Faecal coliforms were separated from tap water and potable tap water was the single hazard element related to disease in a case-control study. There were numerous disorders in the ship’s water system, comprising probable malfunctioning chlorination and malfunctioning includes letting bilge water into the water containers. Researchers [62] notified three outbreaks of ETEC contamination linked with cruise ships. All three outbreaks were related to consuming drinks with ice cubes on board the ship and two were as well linked with potable unbottled water. Water bunkered in overseas ports was the probable origin of the pollution and this water should be treated before usage [1].
4.3. Enterohaemorrhagic (EHEC)
EHEC are observed in the intestines of numerous animal species, comprising cattle [1]. Contamination of persons may go after direct faecal-oral diffusion from infected animals or other persons, or be linked with pollution of food or water. Several outbreaks have pursued the consumption of beef products, especially undercooked beef burgers or salad products. Numerous outbreaks of EHEC related to recreational water contact and consuming drinking water have been reported. Serotype O157: H7 stays the most regularly announced EHEC strain in Europe and North America and the single strain related to outbreaks of potable water-related illness. Nonetheless, EHEC strains other than O157 are more and more being seen as reasons for outbreaks due to foodborne and person-to-person transmission [1].
The first outbreak of contamination related to E. coli O157: H7, which was robustly associated with the consumption of potable water, took place in Burdine Township (Missouri, USA) between 15 December 1989 and 20 January 1990 [46]. Of a population of 3126, a total of 243 persons developed illness and of these, 86 developed bloody diarrhoea, 36 were hospitalized and four died [1]. In a case-control investigation founded on 53 cases, the sole crucial element was that contaminated persons drank more cups of urban water per day than other persons. The water supply to the city came from two deep-ground water sources, and two major water breaks took place on the 23 and 26 December, following the beginning of the outbreak, yet prior to its principal peak.
4.4. Enteroinvasive (EIEC)
Most sickness is suggested to be food- or waterborne, even if individual to individual diffusion as well happens. Nonetheless, at most one outbreak of invasive EIEC because water has been noticed in the literature and this was some 50 years ago [1].
5. Antimicrobial Monitoring
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 (•
) 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
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Figure 2. Scanning electron microscopy (SEM) images of (a) E. coli; (b) S. typhimurium; (c) E. faecalis after 60 min of photocatalytic disinfection using RGOFeNTFS [76].
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
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Figure 3. Conceptual model of sunlight inactivation pathways in viruses and bacteria. For direct mechanisms, the photon is absorbed by a chromophore at the site of damage (orange star). For indirect pathways, the photon is absorbed by a sensitizer (Sens), and damage (orange star) occurs at a different site. Green shapes represent proteins. PPRI = photo-produced reactive intermediates [77].
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).
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Figure 5. Scanning electron micrographs of E. coli cells before ((a) 0 min (×10,000) and (b) 0 min (×40,000)) and after ((c) 10 min (×10,000) and (d) 10 min (×40,000)) the ARHCR treatment at 3800 rpm and 1.4 m3/h [79].
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
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Figure 6. Likely disinfection mechanism of the ARHCR: (a) hydrodynamical and (b) sonochemical effects [79].
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
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Figure 7. Variations of the current, potential and pH levels in the two-chamber apparatus below various voltages (1.2, 1.5, and 2.0 V) throughout electrochemical disinfection (ED) with Mo2C employed as the anode and cathode. (a): Current and potential below 1.2 V voltage; (b): Current and potential below 1.5 V voltage; (c): Current and potential below 2.0 V voltage; (d): pH levels in the two-chamber apparatus below various voltages (1.2, 1.5, and 2.0 V) [80].
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Figure 8. (a) Fluorescence microscopic images and (b) Scanning electron microscopy (SEM) images of E. coli K-12 untreated E. coli and after the treatment in the US/Ag-BTO and US/Ag-BTO/PS systems for 5 min.
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].
6. Conclusions
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].
Acknowledgements
The Research Deanship of University of Ha’il, Saudi Arabia, has funded this research through the Project RG-20 113.
Abbreviation
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