WHO consultation on ETEC and Shigella burden of disease, Geneva, 6–7th April 2017: Meeting report

The colonization factor CS6 of enterotoxigenic Escherichia coli contributes to host cell invasion

Enterotoxigenic Escherichia coli (ETEC) is one of the main causes of diarrhea in children and travelers in low-income regions. The virulence of ETEC is attributed to its heat-labile and heat-stable enterotoxins, as well as its colonization factors (CFs). CFs are essential for ETEC adherence to the intestinal epithelium. However, its invasive capability remains unelucidated. In this study, we demonstrated that the CS6-positive ETEC strain 4266 can invade mammalian epithelial cells. The invasive capability was reduced in the 4266 ΔCS6 mutant but reintroduction of CS6 into this mutant restored the invasiveness. Additionally, the laboratory E. coli strain Top 10, which lacks the invasive capability, was able to invade Caco-2 cells after gaining the CS6-expressing plasmid pCS6. Cytochalasin D inhibited cell invasion in both 4266 and Top10 pCS6 cells, and F-actin accumulation was observed near the bacteria on the cell membrane, indicating that CS6-positive bacteria were internalized via actin polymerization. Other cell signal transduction inhibitors, such as genistein, wortmannin, LY294002, PP1, and Ro 32-0432, inhibited the CS6-mediated invasion of Caco-2 cells. The internalized bacteria of both 4266 and Top10 pCS6 strains were able to survive for up to 48 h, and 4266 cells were able to replicate within Caco-2 cells. Immunofluorescence microscopy revealed that the internalized 4266 cells were present in bacteria-containing vacuoles, which underwent a maturation process indicated by the recruitment of the early endosomal marker EEA-1 and late endosomal marker LAMP-1 throughout the infection process. The autophagy marker LC3 was also observed near these vacuoles, indicating the initiation of LC-3-associated phagocytosis (LAP). However, intracellular bacteria continued to replicate, even after the initiation of LAP. Moreover, intracellular filamentation was observed in 4266 cells at 24 h after infection. Overall, this study shows that CS6, in addition to being a major CF, mediates cell invasion. This demonstrates that once internalized, CS6-positive ETEC is capable of surviving and replicating within host cells. This capability may be a key factor in the extended and recurrent nature of ETEC infections in humans, thus highlighting the critical role of CS6.

Field evaluation of a simple and rapid diagnostic test, RLDT to detect Shigella and enterotoxigenic E. coli in Indian children

The diagnostic assays currently used to detect Shigella spp. (Shigella) and enterotoxigenic Escherichia coli (ETEC) are complex or elaborate which make them difficult to apply in resource poor settings where these diseases are endemic. The simple and rapid nucleic acid amplification-based assay "Rapid LAMP-based Diagnostic Test (RLDT)" was evaluated to detect Shigella spp (Shigella) and enterotoxigenic Escherichia coli (ETEC) and determine the epidemiology of these pathogens in Kolkata, India. Stool samples (n = 405) from children under five years old with diarrhea seeking care at the hospitals were tested, and 85(21%) and 68(17%) by RLDT, 91(23%) and 58(14%) by quantitative PCR (qPCR) and 35(9%) and 15(4%) by culture, were positive for Shigella and ETEC, respectively. The RLDT showed almost perfect agreement with qPCR, Kappa 0.96 and 0.89; sensitivity 93% and 98%; specificity 100% and 97% for Shigella and ETEC, respectively. While RLDT detected additional 12% Shigella and 13% ETEC than culture, all culture positives for Shigella and ETEC except one each were also positive by the RLDT, sensitivity 97% and 93% respectively. RLDT is a simple, sensitive, and rapid assay that could be implemented with minimum training in the endemic regions to strengthen the disease surveillance system and rapid outbreak detection.

Epidemiology of Enterotoxigenic Escherichia coli among Children and Adults Seeking Care at Hospitals in Two Geographically Distinct Rural Areas in Bangladesh

Enterotoxigenic Escherichia coli (ETEC) infections undeniably continue to have substantial morbidity and mortality in younger children; however, limited data are available on the disease burden of older children and adults and on ETEC epidemiology by geographical location at the subnational level. Facility-based surveillance over the years was established to identify patients with ETEC diarrhea in two geographically distinct areas in rural Bangladesh, Chhatak in the north and Mathbaria in the southern coastal area. ETEC was highly prevalent in both areas, while the proportions, toxin types and colonization factors varied by location, season and age groups. Children < 5 years old and adults between 20 and 60 years old were at the highest risk of ETEC diarrhea which required urgent care. This study underscores the importance of capturing subnational and seasonal variations in ETEC epidemiology. ETEC vaccine developers and public health stakeholders may need to target adults between 20 and 60 years of age in addition to young children as new vaccines currently under development become licensed and introduction begins.

Antibody induction in mice by liposome-displayed recombinant enterotoxigenic Escherichia coli (ETEC) colonization antigens

BACKGROUND

Enterotoxigenic Escherichia coli (ETEC) strains cause infectious diarrhea and colonize host intestine epithelia via surface-expressed colonization factors. Colonization factor antigen I (CFA/I), a prevalent ETEC colonization factor, is a vaccine target since antibodies directed to this fimbria can block ETEC adherence and prevent diarrhea.

METHODS

Two recombinant antigens derived from CFA/I were investigated with a vaccine adjuvant system that displays soluble antigens on the surface of immunogenic liposomes. The first antigen, CfaEB, is a chimeric fusion protein comprising the minor (CfaE) and major (CfaB) subunits of CFA/I. The second, CfaEad, is the adhesin domain of CfaE.

RESULTS

Owing to their His-tag, recombinant CfaEB and CfaEad, spontaneously bound upon admixture with nanoliposomes containing cobalt-porphyrin phospholipid (CoPoP), as well as a synthetic monophosphoryl lipid A (PHAD) adjuvant. Intramuscular immunization of mice with sub-microgram doses CfaEB or CfaEad admixed with CoPoP/PHAD liposomes elicited serum IgG and intestinal IgA antibodies. The smaller CfaEad antigen benefitted more from liposome display. Serum and intestine antibodies from mice immunized with liposome-displayed CfaEB or CfaEad recognized native CFA/I fimbria as evidenced by immunofluorescence and hemagglutination inhibition assays using the CFA/I-expressing H10407 ETEC strain.

CONCLUSION

These data show that colonization factor-derived recombinant ETEC antigens exhibit immunogenicity when delivered in immunogenic particle-based formulations.

Novel inhibitors that target bacterial virulence identified via HTS against intra-macrophage survival of Shigella flexneri

Shigella flexneri is a facultative intracellular pathogen that causes shigellosis, a human diarrheal disease characterized by the destruction of the colonic epithelium. Novel antimicrobial compounds to treat infections are urgently needed due to the proliferation of bacterial antibiotic resistance and lack of new effective antimicrobials in the market. Our approach to find compounds that block the Shigella virulence pathway has three potential advantages: (i) resistance development should be minimized due to the lack of growth selection pressure, (ii) no resistance due to environmental antibiotic exposure should be developed since the virulence pathways are not activated outside of host infection, and (iii) the normal intestinal microbiota, which do not have the targeted virulence pathways, should be unharmed. We chose to utilize two phenotypic assays, inhibition of Shigella survival in macrophages and Shigella growth inhibition (minimum inhibitory concentration), to interrogate the 1.7 M compound screening collection subset of the GlaxoSmithKline drug discovery chemical library. A number of secondary assays on the hit compounds resulting from the primary screens were conducted, which, in combination with chemical, structural, and physical property analyses, narrowed the final hit list to 44 promising compounds for further drug discovery efforts. The rapid development of antibiotic resistance is a critical problem that has the potential of returning the world to a "pre-antibiotic" type of environment, where millions of people will die from previously treatable infections. One relatively newer approach to minimize the selection pressures for the development of resistance is to target virulence pathways. This is anticipated to eliminate any resistance selection pressure in environmental exposure to virulence-targeted antibiotics and will have the added benefit of not affecting the non-virulent microbiome. This paper describes the development and application of a simple, reproducible, and sensitive assay to interrogate an extensive chemical library in high-throughput screening format for activity against the survival of Shigella flexneri 2457T-nl in THP-1 macrophages. The ability to screen very large numbers of compounds in a reasonable time frame (~1.7 M compounds in ~8 months) distinguishes this assay as a powerful tool in further exploring new compounds with intracellular effect on S. flexneri or other pathogens with similar pathways of pathogenesis. The assay utilizes a luciferase reporter which is extremely rapid, simple, relatively inexpensive, and sensitive and possesses a broad linear range. The assay also utilized THP-1 cells that resemble primary monocytes and macrophages in morphology and differentiation properties. THP-1 cells have advantages over human primary monocytes or macrophages because they are highly plastic and their homogeneous genetic background minimizes the degree of variability in the cell phenotype (1). The intracellular and virulence-targeted selectivity of our methodology, determined via secondary screening, is an enormous advantage. Our main interest focuses on hits that are targeting virulence, and the most promising compounds with adequate physicochemical and drug metabolism and pharmacokinetic (DMPK) properties will be progressed to a suitable in vivo shigellosis model to evaluate the therapeutic potential of this approach. Additionally, compounds that act via a host-directed mechanism could be a promising source for further research given that it would allow a whole new, specific, and controlled approach to the treatment of diseases caused by some pathogenic bacteria.

Field evaluation of a simple and rapid diagnostic test, RLDT to detect Shigella and enterotoxigenic E. coli in Indian children

The diagnostic assays currently used to detect Shigella spp. (Shigella) and enterotoxigenic Escherichia coli (ETEC) are complex or elaborate which make them difficult to apply in resource poor settings where these diseases are endemic. The simple and rapid nucleic acid amplification-based assay "Rapid LAMP-based Diagnostic Test (RLDT)" was evaluated to detect Shigella spp (Shigella) and enterotoxigenic Escherichia coli (ETEC) and determine the epidemiology of these pathogens in Kolkata, India. Stool samples (n = 405) from children under five years old with diarrhea seeking care at the hospitals were tested, and 85(21%) and 68(17%) by RLDT, 91(23%) and 58(14%) by quantitative PCR (qPCR) and 35(9%) and 15(4%) by culture, were positive for Shigella and ETEC, respectively. The RLDT showed almost perfect agreement with qPCR, Kappa 0.96 and 0.89; sensitivity 93% and 98%; specificity 100% and 97% for Shigella and ETEC, respectively. While RLDT detected 12% more Shigella and 13% more ETEC than culture, all culture positives for Shigella and ETEC except one each were also positive by the RLDT, sensitivity 97% and 93% respectively. RLDT is a simple, sensitive, and rapid assay that could be implemented with minimum training in the endemic regions to strengthen the disease surveillance system and rapid outbreak detection.

Cloning, improved expression and purification of invasion plasmid antigen D (IpaD): an effector protein of enteroinvasive Escherichia coli (EIEC)

The widespread increase in broad-spectrum antimicrobial resistance is making it more difficult to treat gastrointestinal infections. Enteroinvasive Escherichia coli is a prominent etiological agent of bacillary dysentery, invading via the fecal-oral route and exerting virulence on the host via the type III secretion system. IpaD, a surface-exposed protein on the T3SS tip that is conserved among EIEC and Shigella, may serve as a broad immunogen for bacillary dysentery protection. For the first time, we present an effective framework for improving the expression level and yield of IpaD in the soluble fraction for easy recovery, as well as ideal storage conditions, which may aid in the development of new protein therapies for gastrointestinal infections in the future. To achieve this, uncharacterized full length IpaD gene from EIEC was cloned into pHis-TEV vector and induction parameters were optimized for enhanced expression in the soluble fraction. After affinity-chromatography based purification, 61% pure protein with a yield of 0.33 mg per litre of culture was obtained. The purified IpaD was retained its secondary structure with a prominent α-helical structure as well as functional activity during storage, at 4°C, -20°C and -80°C using 5% sucrose as cryoprotectants, which is a critical criterion for protein-based treatments.

The Diversity of Escherichia coli Pathotypes and Vaccination Strategies against This Versatile Bacterial Pathogen

Escherichia coli (E. coli) is a gram-negative bacillus and resident of the normal intestinal microbiota. However, some E. coli strains can cause diseases in humans, other mammals and birds ranging from intestinal infections, for example, diarrhea and dysentery, to extraintestinal infections, such as urinary tract infections, respiratory tract infections, meningitis, and sepsis. In terms of morbidity and mortality, pathogenic E. coli has a great impact on public health, with an economic cost of several billion dollars annually worldwide. Antibiotics are not usually used as first-line treatment for diarrheal illness caused by E. coli and in the case of bloody diarrhea, antibiotics are avoided due to the increased risk of hemolytic uremic syndrome. On the other hand, extraintestinal infections are treated with various antibiotics depending on the site of infection and susceptibility testing. Several alarming papers concerning the rising antibiotic resistance rates in E. coli strains have been published. The silent pandemic of multidrug-resistant bacteria including pathogenic E. coli that have become more difficult to treat favored prophylactic approaches such as E. coli vaccines. This review provides an overview of the pathogenesis of different pathotypes of E. coli, the virulence factors involved and updates on the major aspects of vaccine development against different E. coli pathotypes.

Both LTA and LTB Subunits Are Equally Important to Heat-Labile Enterotoxin (LT)-Enhanced Bacterial Adherence

There is increasing evidence indicating that the production of heat-labile enterotoxin (LT) enhances bacterial adherence within in vitro and in vivo models. However, which subunit plays the main role, and the precise regulatory mechanisms remain unclear. To further elucidate the contribution of the A subunit of LT (LTA) and the B subunit of LT (LTB) in LT-enhanced bacterial adherence, we generated several LT mutants where their ADP-ribosylation activity or GM1 binding ability was impaired and evaluated their abilities to enhance the two LT-deficient E. coli strains (1836-2 and EcNc) adherence. Our results showed that the two LT-deficient strains, expressing either the native LT or LT derivatives, had a significantly greater number of adhesions to host cells than the parent strains. The adherence abilities of strains expressing the LT mutants were significantly reduced compared with the strains expressing the native LT. Moreover, E. coli 1836-2 and EcNc strains when exogenously supplied with cyclic AMP (cAMP) highly up-regulated the adhesion molecules expression and improved their adherence abilities. Ganglioside GM1, the receptor for LTB subunit, is enriched in lipid rafts. The results showed that deletion of cholesterol from cells also significantly decreased the ability of LT to enhance bacterial adherence. Overall, our data indicated that both subunits are equally responsible for LT-enhanced bacterial adherence, the LTA subunit contributes to this process mainly by increasing bacterial adhesion molecules expression, while LTB subunit mainly by mediating the initial interaction with the GM1 receptors of host cells.

Solvent-sensitive nanoparticle-enhanced PCR assay for the detection of enterotoxigenic Escherichia coli

Stimulus-responsive nanoparticles are among the most utilized nanoscale materials in biomedical applications. As these nanoparticles exhibit a manipulable response to a particular stimulus, such as pH, heat, and organic solvent, they are potential signalling units in diagnostic assays. This study aims to enhance the limit of detection and reduce the turnaround time of magnetic nanoparticle polymerase chain reaction (PCR) enzyme-linked gene assay (MELGA), an advanced PCR-based technique termed the solvent-sensitive nanoparticle (SSNP)-enhanced PCR assay. This technique was proposed to detect pathogenic enterotoxigenic Escherichia coli (ETEC) through applying stimulus-responsive nanoparticles. The SSNPs were elaborated with three main components, including mesoporous silica nanoparticles as a structural unit, organic dye (Nile red) as a payload, and the corresponding organic solvent-sensitive polymer shell as "gatekeeper" (poly(maleic anhydride-alt-methyl vinyl ether, PMAMVE). A suitable organic solvent capable of inducing polymer swelling and dye dissolution was investigated by considering a solubility parameter. Using ethanol, the encapsulated Nile red can diffuse out of the SSNPs faster than other solvents and reach a constant concentration within 15 min. For the PCR inhibition study, various SSNPs concentrations (10-30 μg/reaction) were mixed with the ETEC gene and PCR reagent. The results showed that the particles in this concentration range did not inhibit PCR. By comparing the efficacy of conventional PCR, MELGA, and SSNP-enhanced PCR assay, the proposed technique showed a better detection limit than that of PCR, whereas that of MELGA was the lowest. Moreover, compared to MELGA or conventional PCR, this technique provided remarkably faster results in the postamplification process.

Evolutionary and genomic insights into the long-term colonization of in animals

The enteroinvasive bacterium Shigella flexneri is known as a highly host-adapted human pathogen. There had been no known other reservoirs reported until recently. Here 34 isolates obtained from animals (yaks, dairy cows and beef cattle) from 2016-2017 and 268 human S. flexneri isolates from China were sequenced to determine the relationships between animal and human isolates and infer the evolutionary history of animal-associated S. flexneri. The 18 animal isolates (15 yak and 3 beef cattle isolates) in PG1 were separated into 4 lineages, and the 16 animal isolates (1 yak, 5 beef cattle and 10 dairy cow isolates) in PG3 were clustered in 8 lineages. The most recent human isolates from China belonged to PG3 whereas Chinese isolates from the 1950s-1960s belonged to PG1. PG1 S. flexneri may has been transmitted to the yaks during PG1 circulation in the human population in China and has remained in the yak population since, while PG3 S. flexneri in animals were likely recent transmissions from the human population. Increased stability of the large virulence plasmid and acquisition of abundant antimicrobial resistance determinants may have enabled PG3 to expand globally and replaced PG1 in China. Our study confirms that animals may act as a reservoir for S. flexneri. Genomic analysis revealed the evolutionary history of multiple S. flexneri lineages in animals and humans in China. However, further studies are required to determine the public health threat of S. flexneri from animals.

Phylogenetic analysis and antibiotic resistance of Shigella sonnei isolates

Shigellosis is one of the most important gastric infections caused by different species of Shigella and has been regarded as a serious threat to public health. Lineage/sublineage profile of S. sonnei is strongly associated with the antibiotic resistance and population structure of this pathogen. In this study, we determined the phylogeny and antibiotic resistance profiles of S. sonnei strains, isolated from 1246 stool and 580 food samples, using multiplex PCR-HRMA genotyping and Kirby-Bauer disk diffusion methods, respectively. A total of 64 S. sonnei strains were isolated (13 food and 51 clinical isolates). Multiplex PCR-HMR assay was able to differentiate the lineages II and III, and sublineages IIIb and IIIc strains successfully considering the definite melting curves and temperatures. Lineage I and sublineage IIIa strain were not isolated in this study. We also demonstrated that most of the S. sonnei strains isolated from both food and clinical samples clustered within the lineage III and sublineage IIIc. Resistance against trimethoprim-sulfamethoxazole, tetracycline, chloramphenicol and streptomycin antibiotics were the most prevalent phenotypes among the S. sonnei lineage III and sublineage IIIc strains.

The First-in-Human Synthetic Glycan-Based Conjugate Vaccine Candidate against Shigella

Shigella, the causative agent of shigellosis, is among the main causes of diarrheal diseases with still a high morbidity in low-income countries. Relying on chemical synthesis, we implemented a multidisciplinary strategy to design SF2a-TT15, an original glycoconjugate vaccine candidate targeting Shigella flexneri 2a (SF2a). Whereas the SF2a O-antigen features nonstoichiometric O-acetylation, SF2a-TT15 is made of a synthetic 15mer oligosaccharide, corresponding to three non-O-acetylated repeats, linked at its reducing end to tetanus toxoid by means of a thiol-maleimide spacer. We report on the scale-up feasibility under GMP conditions of a high yielding bioconjugation process established to ensure a reproducible and controllable glycan/protein ratio. Preclinical and clinical batches complying with specifications from ICH guidelines, WHO recommendations for polysaccharide conjugate vaccines, and (non)compendial tests were produced. The obtained SF2a-TT15 vaccine candidate passed all toxicity-related criteria, was immunogenic in rabbits, and elicited bactericidal antibodies in mice. Remarkably, the induced IgG antibodies recognized a large panel of SF2a circulating strains. These preclinical data have paved the way forward to the first-in-human study for SF2a-TT15, demonstrating safety and immunogenicity. This contribution discloses the yet unreported feasibility of the GMP synthesis of conjugate vaccines featuring a unique homogeneous synthetic glycan hapten fine-tuned to protect against an infectious disease.

Evaluation of a simple, rapid and field-adapted diagnostic assay for enterotoxigenic E. coli and Shigella

Understanding the global burden of enterotoxigenic E. coli (ETEC) and Shigella diarrhea as well as estimating the cost effectiveness of vaccines to control these two significant pathogens have been hindered by the lack of a diagnostic test that is rapid, simple, sensitive, and can be applied to the endemic countries. We previously developed a simple and rapid assay, Rapid Loop mediated isothermal amplification based Diagnostic Test (RLDT) for the detection of ETEC and Shigella spp. (Shigella). In this study, the RLDT assay was evaluated in comparison with quantitative PCR (qPCR), culture and conventional PCR for the detection of ETEC and Shigella. This validation was performed using previously collected stool samples from endemic countries, from the travelers to the endemic countries, as well as samples from a controlled human infection model study of ETEC. The performance of RLDT from dried stool spots was also validated. RLDT resulted in excellent sensitivity and specificity compared to qPCR (99% and 99.2% respectively) ranging from 92.3 to 100% for the individual toxin genes of ETEC and 100% for Shigella. Culture was less sensitive compared to RLDT. No significant differences were noted in the performance of RLDT using samples from various sources or stool samples from moderate to severe diarrhea or asymptomatic infections. RLDT performed equally well in detection of ETEC and Shigella from the dried stool samples on filter papers. This study established that RLDT is sufficiently sensitive and specific to be used as a simple and rapid diagnostic assay to detect ETEC and Shigella in endemic countries to determine disease burden of these pathogens in the national and subnational levels. This information will be important to guide public health and policy makers to prioritize resources for accelerating the development and introduction of effective preventative and/or treatment interventions against these enteric infections.

Enterotoxigenic E. coli (ETEC) and Shigella spp (Shigella) causes significant global morbidity and mortality, especially in low-and middle-income countries (LMICs). Since culture methods to detect Shigella are not sensitive, and the methods used to detect ETEC have not been feasible outside of specialized, well-equipped laboratories, the true burden of these pathogens at national and sub-national levels are mostly not available. Morbidity and mortality estimates, for these two pathogens are crucial to assess their relative public health importance in LMICs. We developed a simple and rapid diagnostic assay called the RLDT (Rapid Loop-mediated isothermal amplification based Diagnostic Test) for detection of ETEC and Shigella. In this study we evaluated RLDT compared to other currently available assays using previously collected stool samples. Our data showed that the RLDT assay exhibited high sensitivity and specificity for detection of ETEC and Shigella, with its result available within 50 minutes. The sensitivity of RLDT was higher than culture for these pathogens. We conclude that RLDT could be used as a rapid and simple diagnostic test to determine the burden of ETEC and Shigella in LMICs as well as in clinical vaccine trials of these pathogens.

A colloidal gold immunochromatographic strip assay for the rapid detection of Shigella in milk and meat products

Anti-Shigella mAb was produced using IpaC and an immunochromatographic strip was developed to detect different serotypes of Shigella in food.

GMMA as a 'plug and play' technology to tackle infectious disease to improve global health: context and perspectives for the future

INTRODUCTION

Generalized-Modules-for-Membrane-Antigens (GMMA) is a technology platform developed to design outer membrane vesicle (OMV)-based vaccines. GMMA are basically OMVs derived from a bacterial strain specifically engineered to obtain a fit-for-purpose and affordable vaccine by potentiating, or deleting, expression of specific genes. OMVs can be used as a carrier for antigens by inducing their expression on them, with the aim to improve antigen immunogenicity and design multivalent combination vaccines.

AREAS COVERED

We expanded this finding to show that the chemical conjugation of different proteic and/or polysaccharidic antigens, to GMMA, is a methodology complementary to the genetic manipulation to obtain highly effective combination vaccines. Here we discuss our findings with a specific focus on the impact that GMMA technology can have on global health, as this technology platform is particularly suited to support the development of affordable vaccines for low-income countries.

EXPERT OPINION

We believe that it is critical to elucidate the mode of action of GMMA immunogenicity and have provided a summarized description of the immunological questions to be addressed in the near future. The improved knowledge of GMMA might lead to designing more effective and safer GMMA-based vaccines to tackle the most serious vaccine-preventable diseases.

Multidrug-Resistant Lineage of Enterotoxigenic Escherichia coli ST182 With Serotype O169:H41 in Airline Waste

Enterotoxigenic Escherichia coli (ETEC) is the primary aetiologic agent of traveller’s diarrhoea and a significant cause of diarrhoeal disease and death in developing countries. ETEC O169:H41 strains are known to cause both traveller’s diarrhoea and foodborne outbreaks in developed countries and are cause for concern. Here, whole-genome sequencing (WGS) was used to assemble 46 O169:H41 (ST182) E. coli draft genomes derived from two airplane waste samples sourced from a German international airport. The ST182 genomes were compared with all 84 publicly available, geographically diverse ST182 genomes to construct a core genome-based phylogenetic tree. ST182 isolates were all phylogroup E, the majority serotype O169:H41 (n = 121, 93%) and formed five major clades. The airplane waste isolates differed by an average of 15 core SNPs (range 0–45) but their accessory genome content was diverse. While uncommon in other ST182 genomes, all airplane-derived ST182 isolates carried: (i) extended-spectrum β-lactamase gene bla_CTX–M–15 notably lacking the typical adjacent ISEcp1; (ii) qnrS1 and the S83L mutation in gyrA, both conferring resistance to fluoroquinolones; and (iii) a class 1 integron structure (IS26-intI1_Δ648-dfrA17-aadA5-qacEΔ1-sul1-ORF-srpC-padR-IS6100-mphR-mrx-mphA-IS26) identified previously in major extraintestinal pathogenic E. coli STs but not in ETEC. ST182 isolates carried ETEC-specific virulence factors STp + CS6. Adhesin/invasin tia was identified in 89% of aircraft ST182 isolates (vs 23%) and was located on a putative genomic island within a hotspot region for various insertions including PAI I₅₃₆ and plasmid-associated transposons. The most common plasmid replicons in this collection were IncFII (100%; F2:A-:B-) and IncB/O/K/Z (89%). Our data suggest that potentially through travel, E. coli ST182 are evolving a multidrug-resistant profile through the acquisition of class 1 integrons and different plasmids.

Iron influences the expression of colonization factor CS6 of enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is a major pathogen of acute watery diarrhoea. The pathogenicity of ETEC is linked to adherence to the small intestine by colonization factors (CFs) and secretion of heat-labile enterotoxin (LT) and/or heat-stable enterotoxin (ST). CS6 is one of the most common CFs in our region and worldwide. Iron availability functions as an environmental cue for enteropathogenic bacteria, signalling arrival within the human host. Therefore, iron could modify the expression of CS6 in the intestine. The objective of this study was to determine the effect of iron availability on CS6 expression in ETEC. This would help in understanding the importance of iron during ETEC pathogenesis. ETEC strain harbouring CS6 was cultured under increasing concentrations of iron salt to assess the effect on CS6 RNA expression by quantitative RT-PCR, protein expression by ELISA, promoter activity by β-galactosidase activity, and epithelial adhesion on HT-29 cells. RNA expression of CS6 was maximum in presence of 0.2 mM iron (II) salt. The expression increased by 50-fold, which also reduced under iron-chelation conditions and an increased iron concentration of 0.4 mM or more. The surface expression of CS6 also increased by 60-fold in presence of 0.2 mM iron. The upregulation of CS6 promoter activity by 25-fold under this experimental condition was in accordance with the induction of CS6 RNA and protein. This increased CS6 expression was independent of ETEC strains. Bacterial adhesion to HT-29 epithelial cells was also enhanced by five-fold in the presence of 0.2 mM iron salt. These findings suggest that CS6 expression is dependent on iron concentration. However, with further increases in iron concentration beyond 0.2 mM CS6 expression is decreased, suggesting that there might be a strong regulatory mechanism for CS6 expression under different iron concentrations.

Distribution of virulence factors and its relatedness towards the antimicrobial response of enterotoxigenic Escherichia coli strains isolated from patients in Kolkata, India

AIM

Enterotoxigenic Escherichia coli (ETEC) is one of the most widely recognized diarrhoeal pathogens in developing countries. The advancement of ETEC vaccine development depends on the antigenic determinants of the ETEC isolates from a particular geographical region. So, the aim here was to comprehend the distribution of virulence determinants of the clinical ETEC strains of this region. Additionally, an attempt was made to find any correlation with the antimicrobial response pattern.

METHODS AND RESULTS

Multiplex PCR was employed to identify virulence determinants followed by confirmatory singleplex PCR. For observation of antibiotic response, the Kirby-Bauer method was used. Out of 379 strains, 46% of strains harboured both the enterotoxins ST and LT, whereas 15% were LT only. Among the major colonization factors (CFs), CS6 (41%) was the most prevalent followed by CFA/I (35%) and CFA/III was the lowest (3%). Among the minor CFs, CS21 (25%) was most prevalent, while CS15 showed the lowest (3%) presence. Among the non-classical virulence factors, EatA (69%) was predominant. ETEC strains harbouring CS6 showed resistance towards the commonly used drug Ciprofloxacin (70%).

CONCLUSION

CS6 and elt+est toxin genes co-occurred covering 51% of the isolates. CS21 was found in most strains with est genes (43%). EatA was found to occur frequently when ST was present alone or with LT. CS6-harbouring strains showed an independent correlation to antimicrobial resistance.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study would aid in identifying the commonly circulating ETEC isolates of Kolkata, India, and their prevalent virulence determinants. Knowledge of antibiotic resistance patterns would also help in the appropriate use of antibiotics. Furthermore, the study would aid in identifying the multivalent antigens suitable for region-specific ETEC vaccines with maximum coverage.

Glycoconjugation of Shigella flexneri type 2a O-polysaccharide with CRM197 as a potential vaccine candidate for shigellosis

Shigellosis, a diarrheal disorder caused by an entero-invasive bacterium Shigella, is a major concern among children often leading to mortality. As most of these strains have developed universal antibiotic resistance, the development of a vaccine is crucial in combating the infection. The O-specific polysaccharide (O-PSs) from S. flexneri type 2a is considered to be the major disease-causing antigen in shigellosis. Therefore, the O-PSs conjugated with carrier proteins, can serve as a potential high molecular weight vaccine candidate. Accordingly, in the present study, O-PS extracted from S. flexneri 2a is conjugated with Cross-Reactive Material (CRM₁₉₇), a non-toxic mutant of diphtheria toxin. We derivatized CRM₁₉₇ and O-PS separately with adipic acid dihydrazide (ADH) and reacted with their counterparts to probe the conjugation efficacy. Among the two strategies, the CRM₁₉₇-ADH treated with O-PS has yielded a stable glycoconjugate of 311 kDa. The conjugation efficiency has been probed by estimating the free protein, free O-PS and O-PS:CRM₁₉₇ ratio using slot-blot, size exclusion and high-performance anion exchange chromatography techniques. The conjugate exhibited enhanced shelf-life of three months. The cytotoxicity studies with Vero/MRC-5 cells have confirmed the non-toxicity of the conjugate, which makes the glycoconjugate a potential vaccine candidate for shigellosis.

Preclinical Characterization of Immunogenicity and Efficacy against Diarrhea from MecVax, a Multivalent Enterotoxigenic E. coli Vaccine Candidate

There are no vaccines licensed for enterotoxigenic Escherichia coli (ETEC), a leading cause of diarrhea for children in developing countries and international travelers. Virulence heterogeneity among strains and difficulties identifying safe antigens for protective antibodies against STa, a potent but poorly immunogenic heat-stable toxin which plays a key role in ETEC diarrhea, are challenges in ETEC vaccine development. To overcome these challenges, we applied a toxoid fusion strategy and a novel epitope- and structure-based multiepitope fusion antigen (MEFA) vaccinology platform to construct two chimeric multivalent proteins, toxoid fusion 3xSTa_N12S-mnLT_R192G/L211A and adhesin CFA/I/II/IV MEFA, and demonstrated that the proteins induced protective antibodies against STa and heat-labile toxin (LT) produced by all ETEC strains or the seven most important ETEC adhesins (CFA/I and CS1 to CS6) expressed by the ETEC strains causing 60 to 70% of diarrheal cases and moderate to severe cases. Combining two proteins, we prepared a protein-based multivalent ETEC vaccine, MecVax. MecVax was broadly immunogenic; mice and pigs intramuscularly immunized with MecVax developed no apparent adverse effects but had robust antibody responses to the target toxins and adhesins. Importantly, MecVax-induced antibodies were broadly protective, demonstrated by significant adherence inhibition against E. coli bacteria producing any of the seven adhesins and neutralization of STa and cholera toxin (CT) enterotoxicity. Moreover, MecVax protected against watery diarrhea and provided over 70% and 90% protection against any diarrhea from an STa-positive or an LT-positive ETEC strain in a pig challenge model. These results indicated that MecVax induces broadly protective antibodies and prevents diarrhea preclinically, signifying that MecVax is potentially an effective injectable vaccine for ETEC. IMPORTANCE Enterotoxigenic Escherichia coli (ETEC) bacteria are a top cause of children's diarrhea and travelers' diarrhea and are responsible for over 220 million diarrheal cases and more than 100,000 deaths annually. A safe and effective ETEC vaccine can significantly improve public health, particularly in developing countries. Data from this preclinical study showed that MecVax induces broadly protective antiadhesin and antitoxin antibodies, becoming the first ETEC vaccine candidate to induce protective antibodies inhibiting adherence of the seven most important ETEC adhesins and neutralizing the enterotoxicity of not only LT but also STa toxin. More importantly, MecVax is shown to protect against clinical diarrhea from STa-positive or LT-positive ETEC infection in a pig challenge model, recording protection from antibodies induced by the protein-based, injectable, subunit vaccine MecVax against ETEC diarrhea and perhaps the possibility of intramuscularly administered protein vaccines for protection against intestinal mucosal infection.

Contribution of Noncanonical Antigens to Virulence and Adaptive Immunity in Human Infection with Enterotoxigenic E. coli

Enterotoxigenic Escherichia coli (ETEC) contributes significantly to the substantial burden of infectious diarrhea among children living in low- and middle-income countries. In the absence of a vaccine for ETEC, children succumb to acute dehydration as well as nondiarrheal sequelae related to these infections, including malnutrition. The considerable diversity of ETEC genomes has complicated canonical vaccine development approaches defined by a subset of ETEC pathovar-specific antigens known as colonization factors (CFs). To identify additional conserved immunogens unique to this pathovar, we employed an "open-aperture" approach to capture all potential conserved ETEC surface antigens, in which we mined the genomic sequences of 89 ETEC isolates, bioinformatically selected potential surface-exposed pathovar-specific antigens conserved in more than 40% of the genomes (n = 118), and assembled the representative proteins onto microarrays, complemented with known or putative colonization factor subunit molecules (n = 52) and toxin subunits. These arrays were then used to interrogate samples from individuals with acute symptomatic ETEC infections. Surprisingly, in this approach, we found that immune responses were largely constrained to a small number of antigens, including individual colonization factor antigens and EtpA, an extracellular adhesin. In a Bangladeshi cohort of naturally infected children <2 years of age, both EtpA and a second antigen, EatA, elicited significant serologic responses that were associated with protection from symptomatic illness. In addition, children infected with ETEC isolates bearing either etpA or eatA genes were significantly more likely to develop symptomatic disease. These studies support a role for antigens not presently targeted by vaccines (noncanonical) in virulence and the development of adaptive immune responses during ETEC infections. These findings may inform vaccine design efforts to complement existing approaches.

Guanylyl cyclase C as a biomarker for immunotherapies for the treatment of gastrointestinal malignancies

Gastrointestinal cancers encompass a diverse class of tumors arising in the GI tract, including esophagus, stomach, pancreas and colorectum. Collectively, gastrointestinal cancers compose a high fraction of all cancer deaths, highlighting an unmet need for novel and effective therapies. In this context, the transmembrane receptor guanylyl cyclase C (GUCY2C) has emerged as an attractive target for the prevention, detection and treatment of many gastrointestinal tumors. GUCY2C is an intestinally-restricted protein implicated in tumorigenesis that is universally expressed by primary and metastatic colorectal tumors as well as ectopically expressed by esophageal, gastric and pancreatic cancers. This review summarizes the current state of GUCY2C-targeted modalities in the management of gastrointestinal malignancies, with special focus on colorectal cancer, the most incident gastrointestinal malignancy.

U-Omp19 from Brucella abortus increases dmLT immunogenicity and improves protection against Escherichia coli heat-labile toxin (LT) oral challenge

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Oral co-administration of dmLT with U-Omp19 increases dmLT immunogenicity.

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U-Omp19 oral co-delivery with dmLT induces anti-LT antibody responses.

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U-Omp19 co-administered with dmLT protects against oral challenge with LT.

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U-Omp19 can allow antigen dose sparing by oral route.

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U-Omp19 can be used as adjuvant in an oral vaccine formulation against ETEC.

Acute diarrhea disease caused by bacterial infections is a major global health problem. Enterotoxigenic Escherichia coli (ETEC) is one of the top causes of diarrhea-associated morbidity and mortality in young children and travelers to low-income countries. There are currently no licensed vaccines for ETEC. Induction of immunity at the site of entry of the bacteria is key to prevent infection. Current approaches to ETEC vaccines include a less toxic mutant form of E. coli heat-labile toxin (double-mutant heat-labile enterotoxin -dmLT-) with both antigenic and immunostimulatory properties. U-Omp19 is a protease inhibitor from Brucella spp. with immunostimulatory properties that has been used as oral adjuvant. In this work, we use U-Omp19 as adjuvant in an oral vaccine formulation against ETEC containing dmLT in outbred and inbred mice. To evaluate antigen dose sparing by U-Omp19 three different immunization protocols with three different doses of dmLT were evaluated. We demonstrated that U-Omp19 co-delivery increases anti-LT IgA in feces using a mid-dose of dmLT following a prime-boost protocol (after one or two boosts). Oral immunization with U-Omp19 induced protection against LT challenge when co-formulated with dmLT in CD-1 and BALB/c mice. Indeed, there was a significant increase in anti-LT IgG and IgA avidity after a single oral administration of dmLT plus U-Omp19 in comparison with dmLT delivered alone. Interestingly, sera from dmLT plus U-Omp19 vaccinated mice significantly neutralize LT effect on intestine inflammation in vivo compared with sera from the group immunized with dmLT alone. These results demonstrate the adjuvant capacity of U-Omp19 to increase dmLT immunogenicity by the oral route and support its use in an oral subunit vaccine formulation against ETEC.

A bivalent vaccine confers immunogenicity and protection against Shigella flexneri and enterotoxigenic Escherichia coli infections in mice

Vaccine studies for Shigella flexneri and enterotoxigenic Escherichia coli have been impaired by the lack of optimal animal models. We used two murine models to show that a S. flexneri 2a bivalent vaccine (CVD 1208S-122) expressing enterotoxigenic Escherichia coli colonization factor antigen-I (CFA/I) and the binding subunits A2 and B of heat labile-enterotoxin (LTb) is immunogenic and protects against weight loss and diarrhea. These findings document the immunogenicity and pre-clinical efficacy effects of CVD 1208S-122 vaccine and suggest that further work can help elucidate relevant immune responses and ultimately its clinical efficacy in humans.

Shigella flexneri: an emerging pathogen

Shigella flexneri is a leading etiologic agent of diarrhea in low socioeconomic countries. Notably, various serotypes in S. flexneri are reported from different regions of the world. The precise approximations of illness and death owing to shigellosis are missing in low socioeconomic countries, although it is widespread in different regions. The inadequate statistics available reveal S. flexneri to be a significant food and waterborne pathogen. All over the world, different antibiotic-resistant strains of S. flexneri serotypes have been emerged especially multidrug-resistant strains. Recently, increased resistance was observed in cephalosporins (3rd generation), azithromycin, and fluoroquinolones. There is a need for a continuous surveillance study on antibiotic resistance that will be helpful in the update of the antibiogram. The shigellosis burden can be reduced by adopting preventive measures like delivery of safe drinking water, suitable sanitation, and development of an effective and inexpensive multivalent vaccine. This review attempts to provide the recent findings of S. flexneri related to epidemiology and the emergence of multidrug resistance.

Designing and evaluation of an antibody-targeted chimeric recombinant vaccine encoding Shigella flexneri outer membrane antigens

Shigellosis is one of the most common diseases found in the developing countries, especially those countries that are prone flood. The causative agent for this disease is the Shigella species. This organism is one of the third most common enteropathogens responsible for childhood diarrhea. Since Shigella can survive gastric acidity and is an intracellular pathogen, it becomes difficult to treat. Also, uncontrolled use of antibiotics has led to development of resistant strains which poses a threat to public health. Therefore, there is a need for long term control of Shigella infection which can be achieved by designing a proper and effective vaccine. In this study, emphasis was made on designing a candidate that could elicit both B-cell and T-cell immune response. Hence B- and T-cell epitopes of outer membrane channel protein (OM) and putative lipoprotein (PL) from S. flexneri 2a were computationally predicted using immunoinformatics approach and a chimeric construct (chimeric-OP) containing the immunogenic epitopes selected from OM and PL was designed, cloned and expressed in E. coli system. The immunogenicity of the recombinant chimeric-OP was assessed using Shigella antigen infected rabbit antibody. The result showed that the chimeric-OP was a synthetic peptide candidate suitable for the development of vaccine and immunodiagnostics against Shigella infection.

Conservation and global distribution of non-canonical antigens in Enterotoxigenic Escherichia coli

BACKGROUND

Enterotoxigenic Escherichia coli (ETEC) cause significant diarrheal morbidity and mortality in children of resource-limited regions, warranting development of effective vaccine strategies. Genetic diversity of the ETEC pathovar has impeded development of broadly protective vaccines centered on the classical canonical antigens, the colonization factors and heat-labile toxin. Two non-canonical ETEC antigens, the EtpA adhesin, and the EatA mucinase are immunogenic in humans and protective in animal models. To foster rational vaccine design that complements existing strategies, we examined the distribution and molecular conservation of these antigens in a diverse population of ETEC isolates.

METHODS

Geographically diverse ETEC isolates (n = 1159) were interrogated by PCR, immunoblotting, and/or whole genome sequencing (n = 46) to examine antigen conservation. The most divergent proteins were purified and their core functions assessed in vitro.

RESULTS

EatA and EtpA or their coding sequences were present in 57.0% and 51.5% of the ETEC isolates overall, respectively; and were globally dispersed without significant regional differences in antigen distribution. These antigens also exhibited >93% amino acid sequence identity with even the most divergent proteins retaining the core adhesin and mucinase activity assigned to the prototype molecules.

CONCLUSIONS

EtpA and EatA are well-conserved molecules in the ETEC pathovar, suggesting that they serve important roles in virulence and that they could be exploited for rational vaccine design.

How genomics can be used to understand host susceptibility to enteric infection, aiding in the development of vaccines and immunotherapeutic interventions

Thanks to the modern sequencing era, the extent to which infectious disease imposes selective pressures on the worldwide human population is being revealed. This is aiding our understanding of the underlying immunological and host mechanistic defenses against these pathogens, as well as potentially assisting in the development of vaccines and therapeutics to control them. As a consequence, the workshop "How genomics can be used to understand host susceptibility to enteric infection, aiding in the development of vaccines and immunotherapeutic interventions" at the VASE 2018 meeting, aimed to discuss how genomics and related tools could be used to assist Shigella and ETEC vaccine development. The workshop featured four short presentations which highlighted how genomic applications can be used to assist in the identification of genetic patterns related to the virulence of disease, or host genetic factors that could contribute to immunity or successful vaccine responses. Following the presentations, there was an open debate with workshop attendees to discuss the best ways to utilise such genomic studies, to improve or accelerate the process of both Shigella and ETEC vaccine development. The workshop concluded by making specific recommendations on how genomic research methods could be strengthened and harmonised within the ETEC and Shigella research communities.

Booster Vaccination With GVGH Shigella sonnei 1790GAHB GMMA Vaccine Compared to Single Vaccination in Unvaccinated Healthy European Adults: Results From a Phase 1 Clinical Trial

The investigational Shigella sonnei vaccine (1790GAHB) based on GMMA (generalized modules for membrane antigens) is immunogenic, with an acceptable safety profile in adults. However, pre-vaccination anti-S. sonnei lipopolysaccharide (LPS) antibody levels seemed to impact vaccine-related immune responses. This phase 1, open-label, non-randomized extension study (ClinicalTrials.gov: NCT03089879) evaluated immunogenicity of a 1790GAHB booster dose in seven adults with undetectable antibodies prior to priming with three 1790GAHB vaccinations 2–3 years earlier (boosted group), compared to one dose in 28 vaccine-naïve individuals (vaccine-naïve group). Anti-S. sonnei LPS serum IgG geometric mean concentrations and seroresponse (increase of ≥25 EU or ≥50% from baseline antibody ≤ 50 EU and ≥50 EU, respectively) rates were calculated at vaccination (day [D]1), D8, D15, D29, D85. Safety was assessed. Geometric mean concentrations at D8 were 168 EU (boosted group) and 32 EU (vaccine-naïve group). Response peaked at D15 (883 EU) and D29 (100 EU) for the boosted and vaccine-naïve groups. Seroresponse rates at D8 were 86% (boosted group) and 24% (vaccine-naïve group) and increased at subsequent time points. Across both groups, pain (local) and fatigue (systemic) were the most frequent solicited adverse events (AEs). Unsolicited AEs were reported by 57% of boosted and 25% of vaccine-naïve participants. No deaths, serious AEs, or AEs of special interest (except one mild neutropenia case, possibly vaccination-related) were reported. One 1790GAHB dose induced a significant booster response in previously-primed adults, regardless of priming dose, and strong immune response in vaccine-naïve individuals. Vaccination was well tolerated.

Enterotoxigenic Escherichia coli Infections

PURPOSE OF REVIEW

Review recent developments pertaining to the epidemiology, molecular pathogenesis, and sequelae of enterotoxigenic Escherichia coli (ETEC) infections in addition to discussion of challenges for vaccinology.

RECENT FINDINGS

ETEC are a major cause of diarrheal illness in resource poor areas of the world where they contribute to unacceptable morbidity and continued mortality particularly among young children; yet, precise epidemiologic estimates of their contribution to death and chronic disease have been difficult to obtain. Although most pathogenesis studies, and consequently vaccine development have focused intensively on canonical antigens, more recently identified molecules unique to the ETEC pathovar may inform our understanding of ETEC virulence, and the approach to broadly protective vaccines. ETEC undeniably continue to have a substantial impact on global health; however, further studies are needed to clarify the true impact of these infections, particularly in regions where access to care may be limited. Likewise, our present understanding of the relationship of ETEC infection to non-diarrheal sequelae is presently limited, and additional effort will be required to achieve a mechanistic understanding of these diseases and to fulfill Koch's postulates on a molecular level. Precise elucidation of the role played by novel virulence factors, the global burden of acute illness, and the contribution of these pathogens and/or their toxins to non-diarrheal morbidity remain important imperatives.

Cross-Protective Shigella Whole-Cell Vaccine With a Truncated O-Polysaccharide Chain

Shigella is a highly prevalent bacterium causing acute diarrhea and dysentery in developing countries. Shigella infections are treated with antibiotics but Shigellae are increasingly resistant to these drugs. Vaccination can be a countermeasure against emerging antibiotic-resistant shigellosis. Because of the structural variability in Shigellae O-antigen polysaccharides (Oag), cross-protective Shigella vaccines cannot be derived from single serotype-specific Oag. We created an attenuated Shigella flexneri 2a strain with one rather than multiple Oag units by disrupting the Oag polymerase gene (Δwzy), which broadened protective immunogenicity by exposing conserved surface proteins. Inactivated Δwzy mutant cells combined with Escherichia coli double mutant LT(R192G/L211A) as adjuvant, induced potent antibody responses to outer membrane protein PSSP-1, and type III secretion system proteins IpaB and IpaC. Intranasal immunization with the vaccine preparation elicited cross-protective immunity against S. flexneri 2a, S. flexneri 3a, S. flexneri 6, and Shigella sonnei in a mouse pneumonia model. Thus, S. flexneri 2a Δwzy represents a promising candidate strain for a universal Shigella vaccine.

Coli Surface Antigen 26 Acts as an Adherence Determinant of Enterotoxigenic Escherichia coli and Is Cross-Recognized by Anti-CS20 Antibodies

The coli surface antigen 26 (CS26) of enterotoxigenic Escherichia coli (ETEC) had been described as a putative adhesive pilus based on the partial sequence of the crsH gene, detected in isolates from children with diarrhea in Egypt. However, its production and activity as adherence determinant has not been experimentally addressed. The crsH was identified as a homolog of genes encoding structural subunits of ETEC colonization factors (CFs) CS12, CS18, and CS20. These CFs, along with the recently discovered CS30, belong to the γ₂ family of pili assembled by the chaperone-usher pathway (CU pili). Further, the complete CS26 locus, crsHBCDEFG, was described in an O141 ETEC strain (ETEC 100664) obtained from a diarrhea case in The Gambia, during the Global Enterics Multicenter Study. Here, we report that CS26 is a pilus of ∼10 nm in diameter, with the capacity to increase the cell adherence of the non-pathogenic strain E. coli DH10B. As for other related pili, production of CS26 seems to be regulated by phase variation. Deletion of crsHBCDEFG in ETEC 100664 significantly decreased its adherence capacity, which was recovered by in trans complementation. Furthermore, CrsH was cross-recognized by polyclonal antibodies directed against the major structural subunit of CS20, CsnA, as determined by Western blotting and immunogold labeling. ETEC CS26+ strains were found to harbor the heat-labile enterotoxin only, within three different sequence types of phylogroups A and B1, the latter suggesting acquisition through independent events of horizontal transfer. Overall, our results demonstrate that CS26 is an adhesive pilus of human ETEC. In addition, cross-reactivity with anti-CsnA antibodies indicate presence of common epitopes in γ₂-CFs.

Significance of Enterotoxigenic Escherichia coli (ETEC) Heat-Labile Toxin (LT) Enzymatic Subunit Epitopes in LT Enterotoxicity and Immunogenicity

Enterotoxigenic Escherichia coli (ETEC) strains producing heat-labile toxin (LT) and/or heat-stable toxin (STa) are a top cause of children's diarrhea and travelers' diarrhea. Holotoxin-structured GM1-binding LT is a strong immunogen and an effective adjuvant, and can serve a carrier or a platform for multivalent vaccine development. However, the significance of peptide domains or epitopes of LT particularly enzymatic LTA subunit in association with LT enterotoxicity and immunogenicity has not been characterized. In this study, we identified B-cell epitopes in silico from LTA subunit and examined epitopes for immunogenicity and association with LT enterotoxicity. Epitopes identified from LTA subunit were individually fused to a modified chicken ovalbumin carrier protein, and each epitope-ovalbumin fusion was used to immunize mice. Data showed all 11 LTA epitopes were immunogenic; epitope 7 (105SPHPYEQEVSA115) induced greater titers of anti-LT antibodies which neutralized LT enterotoxicity more effectively. To examine these epitopes for the significance in LT enterotoxicity, we constructed LT mutants by substituting each of 10 epitopes at the toxic A1 domain of LTA subunit with a foreign epitope and examined LT mutants for enterotoxicity and GM1-binding activity. Data showed that LT mutants exhibited no enterotoxicity but retained GM1-binding activity. The results from this study indicated that while not all immunodominant LTA epitopes were neutralizing, LT mutants with an individual epitope substituted lost enterotoxicity but retained GM1-binding activity. These results provided additional information to understand LT immunogenicity and enterotoxicity and suggested the potential application of LT platform for multivalent vaccines against ETEC diarrhea and other diseases.IMPORTANCE No vaccine is licensed for enterotoxigenic Escherichia coli (ETEC) strains, which remain a leading cause of diarrhea in children from developing countries and international travelers. GM1-binding heat-labile toxin (LT) which is a key virulence factor of ETEC diarrhea is a strong vaccine antigen and a self-adjuvant. LT can also serve a backbone or platform for MEFA (multiepitope fusion antigen), a newly developed structural vaccinology technology, to present heterogeneous epitopes (by replacing LT epitopes) and to mimic epitope antigenicity for development of broadly protective vaccines. Data from this study identified neutralizing LT epitopes and demonstrated that substitution of LT epitopes eliminated LT enterotoxicity without altering GM1-binding activity, suggesting LT is potentially a versatile MEFA platform to present heterogeneous epitopes for multivalent vaccines against ETEC and other pathogens.