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 Table of Contents  
REVIEW ARTICLE
Year : 2020  |  Volume : 4  |  Issue : 4  |  Page : 280-284

Genetic makeup and associated virulence posed by the enteropathogenic Escherichia coli and the Enterotoxigenic Escherichia coli pathotypes


Department of Life Sciences, School of Environment and Life Sciences (SELS), Independent University, Bangladesh (IUB), Dhaka 1229, Bangladesh

Date of Submission11-May-2020
Date of Acceptance27-Jun-2020
Date of Web Publication30-Dec-2020

Correspondence Address:
Dr. Rashed Noor
Department of Life Sciences (DLS), School of Environment and Life Sciences (SELS), Independent University, Bangladesh (IUB), Plot 16, Block B, Bashundhara, Dhaka 1229
Bangladesh
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/bbrj.bbrj_70_20

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  Abstract 


The enteropathogenic Escherichia coli (EPEC) is known to trigger diarrhea in infants, whereas the enterotoxigenic E. coli (ETEC) accounts for the children's diarrhea and the travelers' diarrhea. Transmission of the pathogenic bacteria usually occurs in a fecal–oral route usually originating from the poultry items. Thus, the study relating these E. coli pathotypes to the required virulence factors would be of great interest for the welfare of mass public health. Although the reports on the food-oriented pathogenic E. coli so far are actually uncountable, the present review especially concentrated on the genetics of virulence factors required for the pathogenesis by EPEC and ETEC based on the information given by the previous literature. The review focused on the expressional regulation of the components required for the EPEC pathophysiological impact on humans. The necessary studies correlating the genome with the expression of the virulence factors have been well discussed.

Keywords: Enteropathogenic Escherichia coli, Escherichia coli (enterotoxigenic Escherichia coli), genetic makeup, public health, virulence


How to cite this article:
Maniha SM, Noor R. Genetic makeup and associated virulence posed by the enteropathogenic Escherichia coli and the Enterotoxigenic Escherichia coli pathotypes. Biomed Biotechnol Res J 2020;4:280-4

How to cite this URL:
Maniha SM, Noor R. Genetic makeup and associated virulence posed by the enteropathogenic Escherichia coli and the Enterotoxigenic Escherichia coli pathotypes. Biomed Biotechnol Res J [serial online] 2020 [cited 2021 Dec 8];4:280-4. Available from: https://www.bmbtrj.org/text.asp?2020/4/4/280/305648




  Introduction Top


Contamination of food by pathogenic  Escherichia More Details coli (especially with E. coli O157 and O104) is widely known for a long time all over the world.[1],[2] Indeed, E. coli has long been known as a commensal bacterium colonizing the gastrointestinal tract of infants in the very early life, but some may account for diarrhea, with all fatality and morbidity.[3] The diarrheagenic E. coli (DEC) can be abundant in nature (i.e., the transmission of pathogenic E. coli from infected human or animal feces to new hosts via the environmental reservoirs such as hands, water, and soil), the animal reservoirs, the food processing places, and within the contaminated food matrices, based on huge variations of surface molecular determinants acting as surface colonization factors (CFs) including the extracellular polysaccharides, extracellular DNA, and the surface proteins.[3],[4],[5],[6] The so-called farm-to-table continuum from environmental, animal, or human sources is a major cause of foodborne illness. Extensive researches have so far been executed on the understanding the E. coli borne foodborne outbreaks as well as the food-contamination prevention measures.[5],[6],[7] E. coli commensals including the nonpathogenic E. coli strains serve as a fecal bacteria indicator as their presence links with the elevated possibility of enteric pathogens associated with diarrheal disease.[3]

The major foodborne diarrheagenic pathotypes of E. coli depending on the infection strategy and the onset of disease symptoms include (1) enteropathogenic E. coli (EPEC), (2) Shiga toxin-producing E. coli/enterohemorrhagic E. coli (STEC/EHEC), (3) Shigella/enteroinvasive E. coli, (4) enteroaggregative E. coli, and (5) enterotoxigenic E. coli (ETEC).[3] EPEC and ETEC pathotypes have been detected as the common pathogens responsible for moderate-to-severe diarrhea, especially in the low- and middle-income countries.[4] EPEC commonly triggers diarrhea in infants, while EHEC strains (especially the O157:H7 strain) are significant which cause diarrhea, hemorrhagic colitis, and hemolytic–uremic syndrome.[7] The main difference between the EHEC pathotype over the EPEC that the former strain can produce Shiga toxin (stx genes).[6] The EPEC pathotype is further divided into two subtypes: typical EPEC (tEPEC) consisting of the large virulence plasmid encoding the bundle-forming pilus (BFP) (encoded by bfp) and an adherence factor whereas atypical EPEC (aEPEC) lacks these; nevertheless, they can possess multiple virulence genes, can be exhibited adherence and attaching and effacing (A/E) lesion formation, can be disrupted tight junctions, and can be coclassified with the extraintestinal pathogenic E. coli (ExPEC) and the ETEC pathotypes.[6]

Indeed, studying the pathotypes of the diarrhea causing E. coli (DEC) in terms of their virulence factors and the genetic basis of their pathogenesis would aid the researchers to pinpoint the molecular mechanism of disease onset, which, in turn, may aid to the development of appropriate treatment strategy or even for the development of vaccines especially against ETEC. The present review discussed the genetic basis of the EPEC and ETEC virulence together with their pathogenicity on the basis of the reports published so far.


  Sources of Escherichia coli Pathotypes and Associated Pathophysiological and Genetic Studies Top


Food animals and birds, particularly in poultry, may be the potential reservoirs for the diarrheagenic E. coli pathotypes (including EPEC and ETEC) which possess virulence traits for causing fatal pathogenicity among humans.[6],[8] The study on the E. coli pathotypes (from chickens, cattle, and pigs) conducted by Comery et al. (in 2013) unraveled a huge portion of EPEC; and 18 of the 450 E. coli isolates as aEPEC, mostly from chicken samples, showing multiple virulence genes including eae (the gene product is necessary for intimate attachment to epithelial cells) and bfp (encoding the type IV bundle-forming pili BfpB, an outer-membrane lipoprotein and member of the secretin protein superfamily by EPEC), the formation of adherence and A/E lesion, disrupted tight junctions.[6],[7],[9],[10],[11],[12] This is to be noted that the isolates were coclassified with the ExPEC and ETEC pathotypes too.[6] Besides, the functional existence of the eae gene was also detected in the EHEC strains which were noticed to be capable of intimate attachment and microvillus effacement.[12]

Besides the EPEC strains, another important facet on the diarrheagenic E. coli underlies on the STEC in animals and birds (broiler chickens) as can be exemplified by an outbreak of acute diarrhea in poultry birds in India in 2007.[13] The study covered the isolation of E. coli from rectal swabs, intestinal contents, heart blood, and spleen of the poultry birds that had died due to acute diarrhea; and the expression of the important virulence genes including stx1, stx2, (encoding Shiga toxin, the most potent bacterial toxin), eaeA and hlyA (α-hemolysin gene, mostly detected in EHEC) conducted by multiplex PCR assay both from the STEC and the EPEC isolates.[13],[14] Isolation of EPEC strains from the poultry samples (42 E. coli isolates were resolved from 25 chickens, 2 ducks, and 15 pigeons from the work of Wani et al. in 2004) in association with the virulence gene expression has also been reported by other groups in India.[15],[16] Shiga toxin is common in Shigella dysenteriae 1 and to some extent in some serogroups of E. coli like O157:H7 (called as Stx-producing E. coli or STEC).[14]

The study on the E. coli pathotypes has indeed a long history; the severity by these strains has been understood since more than four decades.[3],[4],[5],[6],[17],[18],[19],[20] One interesting study showed that the endoscopically directed biopsies of tissue collected from the naturally infected children had revealed the presence of EPEC pathotype (1) infecting the small intestine, (2) adhering to the epithelial cell surfaces resulting in the condensation of actin, and (3) effacement of microvilli.[11],[19] Studies on the EPEC-infected epithelial cell monolayers disclosed analogous cytopathic changes, which, in turn, led to the identification of two discrete but synchronized manners of the EPEC pathogenesis: (1) formation of adherent microcolonies, i.e., the localized adherence (LA), and (2) the distinguishing modifications of the cytoskeleton beneath the attached bacteria, i.e., the attaching and effacing phenotype, which is currently known as the attaching and effacing (A/E) lesions on the intestinal cells.[6],[11]

Genetic makeup of enteropathogenic Escherichia coli and the subsequent pathogenesis

Studies on EPEC virulence gene regulation linked the environmental stimulators, the stress signals, bacterial metabolism, and finally the host-associated signal transduction.[9],[17],[18] Upon fecal–oral exposure to the contaminated surfaces or food products (mostly transmitted by person-to-person), the subsequent onset of enteric diseases is generally caused by two groups of E. coli: nontoxigenic and noninvasive EPEC (frequently causing infantile diarrhea and occasionally the sporadic diarrhea in adults), and some of the Shiga toxin-producing E. coli, i.e., STEC/EHEC, possessing a cluster of virulence genes a chromosomal pathogenicity island termed as the locus of enterocyte effacement (LEE).[9] Indeed, the EPEC and EHEC pathotypes are known to share a virulence strategy encompassing the formation of A/E lesions on the intestinal cells, which is usually illustrated by (1) the firm adherence of the bacteria to the intestinal epithelium, (2) subsequent damage of the host's absorptive microvilli, and (3) finally the formation of actin pedestal structures at the site of bacterial attachment.[4]

A/E lesion formation requires the LEE pathogenicity island (LEE, the most important virulence factor), which encodes a type III secretion system containing the bacterial outer membrane protein intimin (encoded by the eae gene). Intimin binds Tir, which is inserted into the host intestinal cell membrane, mediating bacterial attachment to the host cells and thus serving as a bridge between the bacterial cell and the host cell to permit the passage of the effector molecules imparting pathophysiological changes on the host.[9] Ler is the master regulator of the LEE, while other major virulence regulators for EPEC pathogenesis include H-NS and the other nucleoid-associated proteins including GrlA and PerC [Figure 1].
Figure 1: Transcriptional/translational regulation of Escherichia coli (enteropathogenic Escherichia coli) locus of enterocyte effacement and pEAF genes (adopted from Platenkamp and Mellies, 2018). The environmental signals are transmitted through the phosphorelay systems, sRNAs, stringent responses, quorum-sensing responses, and envelope stress responses which influence the transcription of the locus of enterocyte effacement and pEAF. The LEE1 operon (encoding Ler) is regulated by (1) the nucleoid-associated proteins: Ler, H-NS, Hha, Fis, and IHF; (2) the pEAF-encoded regulator PerC, (3) locus of enterocyte effacement-encoded GrlA/GrlR, (4) the quorum-sensing (QS) factors QseA, BipA, and GrvA. RelA, CpxR, and QseA regulate the per operon on the pEAF; and PerA promotes bfp transcription and auto-activates per gene

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The plasmid pEAF contains genes required for the LA phenotype which are found on the 69-kb EPEC adherence factor (EAF).[11] The plasmid-encoded regulators are known to be the perABC that is also termed bfpTVW.[21] The plasmid also specifies the BFP of the bacteria.[11] A very early study showed that the disruption of the EAF locus demolished the LA phenotype as well and reduced the virulence of the mutant.[11],[22] The perA gene mediates the transcription of bfp and the perC gene is known to mediate the expression of proteins encoded on the locus of LEE.[7],[9] The regulation of EPEC LEE and pEAF has been shown in Figure 1. Ler, the LEE-encoded regulator, is a part of the Per-mediated regulatory cascade, which upregulates LEE1 (regulated by the nucleoid-associated proteins Ler, H-NS, Hha, Fis, and IHF), LEE2, LEE3, and LEE4 promoters.[7] Indeed, the ler locus is known to encode a regulator for the expression of the virulence genes both for EPEC and EHEC O157:H7 and hence Ler is considered as the global regulator of virulence gene expression in both the pathotypes.[7] The signal transduction for mediating the transcription of the LEE and pEAF has been described to be triggered by certain environmental signals such as the envelope stress responses, the quorum-sensing factors (QseA, BipA, and GrvA), by the Hfq-dependent small RNAs (sRNAs including MgrR, RyhB, and McaS affect grlRA), and by the stringent ligands RelA or CpxR through the phosphorelay systems.[9] RyhB has been shown to directly repress the translation of grlRA by base-pairing to a shorter RNA sequence to curb the translation, and McaS has been found to act indirectly to suppression the expression of repress grlRA.[17]

Environmental signals and enteropathogenic Escherichia coli pathogenesis

Environmental signals such as the nutrient depletion, the bacterial growth conditions, as well as the envelope stress responses have been found to impart a profound impact on the expression of the EPEC virulence genes. The two-component regulatory system CPxR has been shown to downregulate escD and LEE4 [Figure 1]. Variations in the growth conditions have been reported to affect GrlA-mediated control of the LEE.[9] The per gene has been shown to be transcribed in response to environmental signals, which, in turn, modulated the expression of the LEE PAI and other genes.[9],[22],[23] The presence of tryptophan has been found to possess a positive effect on the virulence trait as well. Besides, the elevated concentration of indole may hinder the expression of LEE5 [Figure 1]. Interestingly, the rpoE gene-encoded σE-mediated stress response and the two-component CpxRA system have also been noticed to be activated in course of the expression of the EPEC virulence genes.[24] As shown in Figure 1, the rpoE gene product has been shown to repress the transcription/translation of espG and the LEE1.


  Enterotoxigenic Escherichia coli Pathotype Top


ETEC, toxigenic but not invasive, is actually the most common causes of bacterial diarrheal cases (with the onset of acute watery nonbloody diarrhea with abdominal cramps in infants and adults), especially in the developing countries, and also it has been noticed that a major proportion of travelers are likely to suffer from diarrhea due to the infection by this E. coli pathotype.[25],[26] Although the direct person-to-person spread does not take place in case EPEC dissemination, infections are mostly acquired through the consumption of fecally contaminated foodstuff or water resulting in severe morbidity and mortality.[27],[28] In general, the pathogenesis of ETEC pathotypes is known to involve its attachment to the intestinal mucosa mediated by fimbrial appendage protein called colonization factor antigen 1 (CFA/1) together with the action of two classes of enterotoxins: heat-stable toxins (STa and STb) and heat-labile toxins (LT-I and LT-II).[29],[30],[31] The heat-stable toxin STa has mostly been noticed to be associated with the ETEC diarrhea, either alone or associated with LT, and has long been known to be accountable for severe syndrome over the LT-only ETEC strains.[31] However, it is to be noted that till date, no licensed vaccine to combat the ETEC diarrhea has been awarded to the human civilization.[25],[32]


  Enterotoxigenic Escherichia coli Virulence Factors and Pathogenesis Top


As discussed above, the ETEC bacteria are unique in the production of the CFA or the coli surface antigen (CS) adhesins, and the enterotoxins including LT and heat-stable type 1b toxin, i.e., STa.[25] Both LT and STa are known to activate the cystic fibrosis transmembrane regulator causing in ion secretion resulting in watery diarrhea. Then, by means of CFs, the ETEC pathotype attaches to the intestinal epithelium.[33] It is to be noted that most ETEC-specific virulence factors are plasmid-encoded and so far more than 30 CFs have been reported.[33]

Indeed, the CFA and the CS adhesins are known to mediate the bacteria to attach to host cell receptors (GM1) followed by the colonization along the small intestines, whereas the enterotoxins (LT and STa) are likely to heighten the intracellular cyclic AMP or cGMP levels in the host epithelial cells, which, in turn, results in the hypersecretion of water and fluids, ultimately causing the onset of watery diarrhea.[25],[34] Truly, the GM1-binding LT is the key virulence factor of ETEC diarrhea.[25] Another important point is to note that the assessment of the burden of ETEC illness together with the relevant traits of the corresponding strains infecting either children or the travelers is often challenging due to (1) the relative intricacy of the laboratory methods for the identification of the enterotoxin-producing strains and (2) due to the variations in the CFs.[35],[36] Besides, an optimal combination of genetic factors is required for survival, virulence, and transmission of the ETEC pathotypes as has been shown in the study conducted by Sahl et al. in 2017.[33] Besides, significant genomic diversity has been seen within the ETEC isolates from a research whereby 94 previously uncharacterized ETEC isolates were studied and 28 distinct sequence types classified those ETEC isolates into three phylogenomic groups.[33]


  Conclusion Top


Besides beef and others, the most common source for meeting the demand of meat by the humans all over world is chicken; unfortunately, for decades, chickens have been identified as the main reservoir of an array of foodborne pathogenic bacteria, especially with the large fraction of Escherichia coli pathotypes. The study of the association between sources of such virulent strains of E. coli and the impact on the individuals poses the potential of maintaining food safety and hence such research bears the enormous public health implications. As discussed in the present review, the fatality and the mortality posed by EPEC and the ETEC pathotypes are of long-term public health concern. The review clearly demonstrated the genetic makeup of the major pathotype of infecting E. coli strain, which, in turn, may help the epidemiologist to understand the link between the host defense mechanism in concert with the associated pathogenesis. The information provided here would useful to design effective drugs dedicated to hinder the genes coupled with the expression of the virulence factors. However, further research on the antimicrobial resistance surveillance specifically linking to the chicken-borne EPEC and ETEC mitigation would be of public health benefit in mass scale.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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