Autotransporter protein-encoding genes of diarrheagenic Escherichia coli are found in both typical and atypical enteropathogenic E. coli strains

Distribution and virulence of Escherichia coli harboring cyclomodulins and supplementary virulence genes isolates from clinical and environmental samples

This study aimed to determine the prevalence of cyclomodulins (cdt, cnf, pks and cif) in Escherichia coli (E. coli) isolated from clinical and environmental samples, the presence of supplementary virulence genes (SVG), antibiotic resistance, and in vitro cytotoxicity. 413 E. coli were isolated from clinical (stool from obese subjects, normal weight subjects, children with diarrhea, and children without diarrhea; and urine from pregnant and non-pregnant women with urinary tract infections) and environmental (water and different foods) samples. PCR was performed to identify E. coli pathotypes, the four cyclomodulins, and 18 SVG; virulence score, cytotoxic assay, and antibiotic resistance assay were performed. Fifteen percent of E. coli were positive for cyclomodulins and were found in all isolation sources; however, in children with diarrhea, they were more frequent. The most frequent cyclomodulin was cdt. More DEC strains harbor cyclomodulins than non-DEC, and cyclomodulins were most frequent among aEPEC pathotype. SVG ehaC was associated with cyclomodulin-positive strains. Cyclomodulin-positive E. coli had a higher virulence score but no significant cytotoxic activity. They were slightly more resistant to antibiotics. In conclusion, cyclomodulins-positive E. coli was widely distributed in humans, food, and the environment, and they were associated with SVG ehaC, suggesting that these genes may play a role in the pathogenesis of the cyclomodulins. However, more research is needed.

Plasmid-encoded toxin of Escherichia coli cleaves complement system proteins and inhibits complement-mediated lysis in vitro

Plasmid-encoded toxin (Pet) is an autotransporter protein of the serine protease autotransporters of Enterobacteriaceae (SPATE) family, important in the pathogenicity of Escherichia coli. The pet gene was initially found in the enteroaggregative E. coli (EAEC) virulence plasmid, pAA2. Although this virulence factor was initially described in EAEC, an intestinal E. coli pathotype, pet may also be present in other pathotypes, including extraintestinal pathogenic strains (ExPEC). The complement system is an important defense mechanism of the immune system that can be activated by invading pathogens. Proteases produced by pathogenic bacteria, such as SPATEs, have proteolytic activity and can cleave components of the complement system, promoting bacterial resistance to human serum. Considering these factors, the proteolytic activity of Pet and its role in evading the complement system were investigated. Proteolytic assays were performed by incubating purified components of the complement system with Pet and Pet S260I (a catalytic site mutant) proteins. Pet, but not Pet S260I, could cleave C3, C5 and C9 components, and also inhibited the natural formation of C9 polymers. Furthermore, a dose-dependent inhibition of ZnCl2-induced C9 polymerization in vitro was observed. E. coli DH5α survived incubation with human serum pre-treated with Pet. Therefore, Pet can potentially interfere with the alternative and the terminal pathways of the complement system. In addition, by cleaving C9, Pet may inhibit membrane attack complex (MAC) formation on the bacterial outer membrane. Thus, our data are suggestive of a role of Pet in resistance of E. coli to human serum.

Serine proteases autotransporter of Enterobacteriaceae: Structures, subdomains, motifs, functions, and targets

Serine protease autotransporters of Enterobacteriaceae (SPATE) constitute a superfamily of virulence factors, resembling the trypsin-like superfamily of serine proteases. SPATEs accomplish multiple functions associated to disease development of their hosts, which could be the consequence of SPATE cleavage of host cell components. SPATEs have been divided into class-1 and class-2 based on structural differences and biological effects, including similar substrate specificity, cytotoxic effects on cultured cells, and enterotoxin activity on intestinal tissues for class-1 SPATEs, whereas most class-2 SPATEs exhibit a lectin-like activity with a predilection to degrade a variety of mucins, including leukocyte surface O-glycoproteins and soluble host proteins, resulting in mucosal colonization and immune modulation. In this review, the structure of class-1 and class-2 are analyzed, making emphasis on their putative functional subdomains as well as a description of their function is provided, including prototypical mechanism of action.

Phylogenetic Groups, Pathotypes and Antimicrobial Resistance of Escherichia coli Isolated from Western Lowland Gorilla Faeces (Gorilla gorilla gorilla) of Moukalaba-Doudou National Park (MDNP)

(1) Background: Terrestrial mammals in protected areas have been identified as a potential source of antimicrobial-resistant bacteria. Studies on antimicrobial resistance in gorillas have already been conducted. Thus, this study aimed to describe the phylogroups, pathotypes and prevalence of antimicrobial resistance of Escherichia coli isolated from western lowland gorilla's faeces living in MDNP. (2) Materials and Methods: Ninety-six faecal samples were collected from western lowland gorillas (Gorilla gorilla gorilla) during daily monitoring in the MDNP. Sixty-four E. coli isolates were obtained and screened for phylogenetic and pathotype group genes by polymerase chain reaction (PCR) after DNA extraction. In addition, antimicrobial susceptibility was determined by the disk diffusion method on Mueller Hinton agar. (3) Results: Sixty-four (64%) isolates of E. coli were obtained from samples. A high level of resistance to the beta-lactam family, a moderate rate for fluoroquinolone and a low rate for aminoglycoside was obtained. All E. coli isolates were positive in phylogroup PCR with a predominance of A (69% ± 11.36%), followed by B2 (20% ± 19.89%) and B1 (10% ± 8.90%) and low prevalence for D (1% ± 3.04%). In addition, twenty E. coli isolates (31%) were positive for pathotype PCR, such as EPEC (85% ± 10.82%) and EPEC/EHEC (15% ± 5.18%) that were obtained in this study. The majority of these MDR E. coli (DECs) belonged to phylogenetic group A, followed by MDR E. coli (DECs) belonging to group B2. (4) Conclusion: This study is the first description of MDR E. coli (DECs) assigned to phylogroup A in western lowland gorillas from the MDNP in Gabon. Thus, wild gorillas in MDNP could be considered as asymptomatic carriers of potential pathogenic MDR E. coli (DECs) that may present a potential risk to human health.

Nonfimbrial Adhesin Mutants Reveal Divergent Escherichia coli O157:H7 Adherence Mechanisms on Human and Cattle Epithelial Cells

Shiga toxin-producing, enterohemorrhagic Escherichia coli (EHEC) serotype O157:H7 is a major foodborne pathogen causing symptoms ranging from simple intestinal discomfort to bloody diarrhea and life-threatening hemolytic uremic syndrome in humans. Cattle can be asymptomatically colonized by O157:H7 predominantly at the rectoanal junction (RAJ). Colonization of the RAJ is highly associated with the shedding of O157:H7 in bovine feces. Supershedding (SS) is a phenomenon that has been reported in some cattle that shed more than 10⁴ colony-forming units of O57:H7 per gram of feces, 100–1000 times more or greater than normal shedders. The unique bovine RAJ cell adherence model revealed that O157:H7 employs a LEE-independent mechanism of attachment to one of the RAJ cell types, the squamous epithelial (RSE) cells. Nine nonfimbrial adhesins were selected to determine their role in the characteristic hyperadherent phenotype of SS O157 on bovine RSE cells, in comparison with human HEp-2 cells. A number of single nucleotide polymorphisms (SNPs) were found amongst these nonfimbrial adhesins across a number of SS isolates. In human cells, deletion of yfaL reduced the adherence of both EDL933 and SS17. However, deletion of eae resulted in a significant loss of adherence in SS17 whereas deletion of wzzB and iha in EDL933 resulted in the same loss of adherence to HEp-2 cells. On RSE cells, none of these nonfimbrial deletion mutants were able to alter the adherence phenotype of SS17. In EDL933, deletion of cah resulted in mitigated adherence. Surprisingly, four nonfimbrial adhesin gene deletions were actually able to confer the hyperadherent phenotype on RSE cells. Overall, this study reveals that the contribution of nonfimbrial adhesins to the adherence mechanisms and functions of O157:H7 is both strain and host cell type dependent as well as indicates a possible role of these nonfimbrial adhesins in the SS phenotype exhibited on RSE cells.

Molecular Characterization of Cronobacter sakazakii Strains Isolated from Powdered Milk

Cronobacter spp. are opportunistic pathogens of the Enterobacteriaceae family. The organism causes infections in all age groups, but the most serious cases occur in outbreaks related to neonates with meningitis and necrotizing enterocolitis. The objective was to determine the in silico and in vitro putative virulence factors of six Cronobacter sakazakii strains isolated from powdered milk (PM) in the Czech Republic. Strains were identified by MALDI-TOF MS and whole-genome sequencing (WGS). Virulence and resistance genes were detected with the Ridom SeqSphere+ software task template and the Comprehensive Antibiotic Resistance Database (CARD) platform. Adherence and invasion ability were performed using the mouse neuroblastoma (N1E-115 ATCCCRL-2263) cell line. The CRISPR-Cas system was searched with CRISPRCasFinder. Core genome MLST identified four different sequence types (ST1, ST145, ST245, and ST297) in six isolates. Strains 13755-1B and 1847 were able to adhere in 2.2 and 3.2 × 106 CFU/mL, while 0.00073% invasion frequency was detected only in strain 1847. Both strains 13755-1B and 1847 were positive for three (50.0%) and four virulence genes, respectively. The cpa gene was not detected. Twenty-eight genes were detected by WGS and grouped as flagellar or outer membrane proteins, chemotaxis, hemolysins, and invasion, plasminogen activator, colonization, transcriptional regulator, and survival in macrophages. The colistin-resistance-encoding mcr-9.1 and cephalothin-resis-encoding blaCSA genes and IncFII(pECLA) and IncFIB(pCTU3) plasmids were detected. All strains exhibited CRISPR matrices and four of them two type I-E and I-F matrices. Combined molecular methodologies improve Cronobacter spp. decision-making for health authorities to protect the population.

Molecular Characterization and Antimicrobial Resistance of Enteropathogenic Escherichia coli in Children from Ahvaz, Iran

Background: Enteropathogenic Escherichia coli (EPEC) is one of the most important pathogens among young children worldwide. Both eae and bfp genes have been used to identify EPEC strains and categorize them into typical and atypical strains. They may be an emerging pathogen in both developing and developed countries. Objectives: This study was primarily conducted to assess the epidemiology, drug resistance, and β-lactamase distribution of EPEC, as well as the detection of efa1/lifA in atypical strains. Methods: A total of 251 E. coli strains isolated from children with diarrhea were evaluated for their EPEC pathotype by PCR for the presence of eae, stx1, stx2, and bfp genes. Serogrouping with polyvalent antisera was performed to confirm EPEC strains. Atypical EPEC-containing samples were evaluated for the efa1/lifA gene. EPEC isolates were assessed to recognize the antibiotic resistance and screened to detect extended-spectrum β-lactamases (ESBLs). Results: Enteropathogenic E. coli strains were detected in 17 (6.78%) of E. coli isolates by PCR. The prevalence of typical and atypical strains was determined at 35.3% and 64.7%. All strains were completely susceptible to colistin, imipenem, and meropenem. The prevalence of blaCTX-M and blaTEM genes was calculated at 70.58% and 58.82%, respectively. Conclusions: Enteropathogenic E. coli isolates are completely sensitive to carbapenems, and precise therapeutic strategies are required to prevent the spread of these beta-lactamase genes among diarrheagenic E. coli.

Comparative genomic analysis provides insight into the phylogeny and virulence of atypical enteropathogenic Escherichia coli strains from Brazil

Background

Atypical enteropathogenic Escherichia coli (aEPEC) are one of the most frequent intestinal E. coli pathotypes isolated from diarrheal patients in Brazil. Isolates of aEPEC contain the locus of enterocyte effacement, but lack the genes of the bundle-forming pilus of typical EPEC, and the Shiga toxin of enterohemorrhagic E. coli (EHEC). The objective of this study was to evaluate the phylogeny and the gene content of Brazilian aEPEC genomes compared to a global aEPEC collection.

Methodology

Single nucleotide polymorphism (SNP)-based phylogenomic analysis was used to compare 106 sequenced Brazilian aEPEC with 221 aEPEC obtained from other geographic origins. Additionally, Large-Scale BLAST Score Ratio was used to determine the shared versus unique gene content of the aEPEC studied.

Principal Findings

Phylogenomic analysis demonstrated the 106 Brazilian aEPEC were present in phylogroups B1 (47.2%, 50/106), B2 (23.6%, 25/106), A (22.6%, 24/106), and E (6.6%, 7/106). Identification of EPEC and EHEC phylogenomic lineages demonstrated that 42.5% (45/106) of the Brazilian aEPEC were in four of the previously defined lineages: EPEC10 (17.9%, 19/106), EPEC9 (10.4%, 11/106), EHEC2 (7.5%, 8/106) and EPEC7 (6.6%, 7/106). Interestingly, an additional 28.3% (30/106) of the Brazilian aEPEC were identified in five novel lineages: EPEC11 (14.2%, 15/106), EPEC12 (4.7%, 5/106), EPEC13 (1.9%, 2/106), EPEC14 (5.7%, 6/106) and EPEC15 (1.9%, 2/106). We identified 246 genes that were more frequent among the aEPEC isolates from Brazil compared to the global aEPEC collection, including espG2, espT and espC (P<0.001). Moreover, the nleF gene was more frequently identified among Brazilian aEPEC isolates obtained from diarrheagenic patients when compared to healthy subjects (69.7% vs 41.2%, P<0.05).

Conclusion

The current study demonstrates significant genomic diversity among aEPEC from Brazil, with the identification of Brazilian aEPEC isolates to five novel EPEC lineages. The greater prevalence of some virulence genes among Brazilian aEPEC genomes could be important to the specific virulence strategies used by aEPEC in Brazil to cause diarrheal disease.

Atypical EPEC (aEPEC) is one of the most frequent diarrheagenic Escherichia coli pathotypes isolated from patients in Brazil and is associated with diarrheal outbreaks. This study is the first to sequence the genomes of a collection of aEPEC isolates from a South American country, Brazil, and compare their phylogenetic relationships and gene content with a global collection of aEPEC. This approach identified Brazilian aEPEC genomes in previously characterized EPEC/EHEC phylogenomic lineages and resulted in the identification of five novel EPEC phylogenomic lineages, designated EPEC11 to EPEC15. We also observed that virulence genes, such as espG2, espT and espC were more frequently identified among the Brazilian aEPEC genomes, demonstrating potential differences in the virulence repertoire of this pathogen in Brazil.

Secreted autotransporter toxin (Sat) induces cell damage during enteroaggregative Escherichia coli infection

Secreted autotransporter toxin (Sat) is a 107-kDa serine protease autotransporter of Enterobacteriaceae (SPATE) presenting cytotoxic activity in renal and bladder cells. Further studies have detected the Sat-encoding gene (sat) in enteroaggregative Escherichia coli (EAEC) and in E. coli strains isolated from neonatal septicemia and meningitis. Here, we investigated the role of Sat as a cytotoxin of EAEC. Sat was purified from a strain of E. coli harboring sat (DEC/Sat+, O126:H2) and used to raise antibodies in rabbit. The presence of Sat was detected by ELISA in the supernatant of 93.7% of EAEC strains harboring sat and in none lacking the gene. The effect of Sat during infection was investigated in polarized Caco-2 cells infected with Sat-producing EAEC (CV323/77, O125ab:H21). This strain induced intense cell detachment, which was inhibited by PMSF or Sat antiserum. Also, sat transcription and Sat production were detected during infection. Here we demonstrate that Sat is internalized in polarized cells leading to F-actin disruption which preceded cell detachment. A comparative study of the toxin action in cell lines corresponding to the infection sites in which bacteria carrying the sat gene have been isolated was performed. Cells originating from the gastrointestinal tract (Caco-2), urinary (LLC-PK1) and endothelium (HUVEC) were incubated with purified Sat. The time required for observation of cell damage differed according to the cell line. HUVEC cells were more sensitive to Sat than cells derived from urinary and intestinal tracts. The intense activity of Sat on the endothelial cells suggests that Sat could also be a virulence factor for the bacteria in the bloodstream. In addition, this is the first work demonstrating that Sat induces cytotoxic effect during EAEC infection in vitro. The cell damage observed during infection indicates that Sat may be another toxin with cytotoxic role in the EAEC pathogenesis.

Dynamics of expression, secretion and translocation of type III effectors during enteropathogenic Escherichia coli infection

Enteropathogenic Escherichia coli (EPEC) is an important cause of infant diarrhea and mortality worldwide. The locus of enterocyte effacement (LEE) pathogenicity island in the EPEC genome encodes a type 3 secretion system (T3SS). This nanomachine directly injects a sophisticated arsenal of effectors into host cells, which is critical for EPEC pathogenesis. To colonize the gut mucosa, EPEC alters its gene expression in response to host environmental signals. Regulation of the LEE has been studied extensively, revealing key mechanisms of transcriptional regulation, and more recently at the posttranscriptional and posttranslational levels. Moreover, the T3SS assembly and secretion is a highly coordinated process that ensures hierarchical delivery of effectors upon cell contact. EPEC effectors and virulence factors not only manipulate host cellular processes, but also modulate effector translocation by controlling T3SS formation. In this review, we focus on the regulation of EPEC virulence genes and modulation of effector secretion and translocation.

Serine protease autotransporters of Enterobacteriaceae (SPATEs) are largely distributed among Escherichia coli isolated from the bloodstream

Extraintestinal pathogenic Escherichia coli (ExPEC) is the major cause of Gram-negative-related sepsis. Bacterial survival in the bloodstream is mediated by a variety of virulence traits, including those mediating immune system evasion. Serine protease autotransporters of Enterobacteriaceae (SPATE) constitute a superfamily of virulence factors that can cause tissue damage and cleavage of molecules of the complement system, which is a key feature for the establishment of infection in the bloodstream. In this study, we analyzed 278 E. coli strains isolated from human bacteremia from inpatients of both genders, different ages, and clinical conditions. These strains were screened for the presence of SPATE-encoding genes as well as for phylogenetic classification and intrinsic virulence of ExPEC. SPATE-encoding genes were detected in 61.2% of the strains and most of these strains (44.6%) presented distinct SPATE-encoding gene profiles. sat was the most frequent gene among the entire collection, found in 34.2%, followed by vat (28.4%), pic (8.3%), and tsh (4.7%). Although in low frequencies, espC (0.7%), eatA (1.1%), and espI (1.1%) were detected and are being reported for the first time in extraintestinal isolates. The presence of SPATE-encoding genes was positively associated to phylogroup B2 and intrinsic virulent strains. These findings suggest that SPATEs are highly prevalent and involved in diverse steps of the pathogenesis of bacteremia caused by E. coli.

Escherichia coli ST302: Genomic Analysis of Virulence Potential and Antimicrobial Resistance Mediated by Mobile Genetic Elements

aEPEC are associated with persistent diarrhea, and diarrheal outbreaks in both humans and animals worldwide. They are differentiated from typical EPEC by the lack of bundle-forming pili, and from EHEC by the lack of phage-mediated stx toxins. However, phylogenetic analyses often associate aEPEC with EHEC, promoting the hypothesis that aEPEC are the progenitors of EHEC, which is supported by aEPEC conversion to EHEC by stx-carrying phages. While aEPEC can cause disease outright, the potential to acquire stx, one of the most potent bacterial toxins known, merits close monitoring. Escherichia coli ST302 (O108:H9, O182:H9, O45:H9) are aEPEC that have been isolated from diarrheic human, pig and rabbit hosts, as well as in healthy pigs, however, no study to date has focused on E. coli ST302 strains. Through WGS and hybrid assembly we present the first closed chromosome, and two circularized plasmids of an ST302 strain - F2_18C, isolated from a healthy pig in Australia. A phylogenetic analysis placed E. coli ST302 strains in proximity to EHEC ST32 (O145:H28) strains. Public databases were interrogated for WGSs of E. coli ST302 strains and short-read gene screens were used to compare their virulence-associated gene (VAG) and antimicrobial resistance gene (ARG) cargo. E. coli ST302 strains carry diverse VAGs, including those that typically associated with extraintestinal pathogenic E. coli (ExPEC). Plasmid comparisons showed that pF2_18C_FIB shared homology with EHEC virulence plasmids such as pO103 while pF2_18C_HI2 is a large multidrug resistance IncHI2:ST3 plasmid. A comparison of 33 HI2:ST3 plasmids demonstrated that those of Australian origin have not acquired resistances to extended-spectrum beta-lactams, colistin, fosfomycin or rifampicin, unlike those originating from Asia. F2_18C was shown to carry two additional pathogenicity islands – ETT2, and the STEC-associated PAI_CL3, plasmid-associated heavy metal resistance genes, as well as several unoccupied stx-phage attachment sites. This study sheds light on the virulence and AMR potential of E. coli ST302 strains and informs AMR genomic surveillance.

Virulence-Related Genes and Coenteropathogens Associated with Clinical Outcomes of Enteropathogenic Escherichia coli Infections in Children from the Brazilian Semiarid Region: a Case-Control Study of Diarrhea

Enteropathogenic Escherichia coli (EPEC) is a major cause of diarrhea in children from developing countries and presents high genetic variability. We aimed to characterize the EPEC virulence-related gene (VRG) distribution and copathogens associated with diarrhea and nutrition-related outcomes in children from the low-income Brazilian semiarid region. A cross-sectional case-control study of diarrhea was conducted in 1,191 children aged 2 to 36 months from the northeast region of Brazil. Stool samples were collected and clinical, epidemiological, and anthropometric data were identified from each child. A broad molecular evaluation of enteropathogens was performed, and EPEC-positive samples were further investigated for 18 VRGs using five multiplex PCRs. EPEC was detected in 28.2% of the study population, with similar proportions among cases and controls. Typical EPEC (tEPEC) infections were more often associated with diarrhea than atypical EPEC (aEPEC) infections, while aEPEC infections presented a higher prevalence. The VRG ler, a negative regulator of the locus of enterocyte effacement, was associated with the absence of diarrhea in aEPEC-positive children; espB, a major component of the type 3 secretion system, was associated with diarrhea in tEPEC-positive children; the presence of procolonization VRGs-the combination of cesT positivity, espP negativity, and the presence of the map gene-was associated with undernutrition; and Campylobacter spp., norovirus, and enteroaggregative E. coli (EAEC) coinfections were associated with increased clinical severity in EPEC-infected children. These data identified tEPEC strains associated with diarrhea and specific VRGs of EPEC (ler, espB, cesT, and map genes) and Campylobacter spp., norovirus, and EAEC to be major contributors to diarrhea and undernutrition in children from a low-income Brazilian region.

Occurrence of virulence factors in Cronobacter sakazakii and Cronobacter malonaticus originated from clinical samples

BACKGROUND

Cronobacter spp. are Gram-negative, facultative-anaerobic, non-spore forming, enteric coliform bacteria, which belongs to the Enterobacteriaceae family. Cronobacter spp. are opportunistic pathogens that have brought rare but life-threatening infections such as meningitis, necrotizing enterocolitis and bloodstream infections in neonates and infants. Information on the diversity, pathogenicity and virulence of Cronobacter species obtained from various sources is still relatively scarce and fragmentary. The aim of this study was to examine and analyse different pathogenicity and virulence factors among C. sakazakii and C. malonaticus strains isolated from clinical samples.

METHODS

The thirty-six clinical Cronobacter strains have been used in this study. This bacterial collection consists of 25 strains of C. sakazakii and 11 strains of C. malonaticus, isolated from different clinical materials. Seven genes (ompA, inv, sip, aut, hly, fliC, cpa) were amplified by PCR. Moreover, the motility and the ability of these strains to adhere and invade human colorectal adenocarcinoma (HT-29) and mouse neuroblastoma (N1E-115) cell lines were investigated.

RESULTS

Our results showed that all tested strains were able to adhere to both used cell lines, HT-29 and N1E-115 cells. The invasion assay showed that 66.7% (24/36) of isolates were able to invade N1-E115 cells while 83% (30/36) of isolates were able to invade HT-29 cells. On the average, 68% of the C. sakazakii strains exhibited seven virulence factors and only 18% in C. malonaticus. All strains amplified ompA and fliC genes. The other genes were detected as follow: sip 97% (35/36), hlyA 92% (33/36), aut 94% (34/36), cpa 67% (24/36), and inv 69% (25/36).

CONCLUSIONS

C. sakazakii and C malonaticus strains demonstrate the diversity of the virulence factors present among these pathogens. It is necessary to permanently monitor the hospital environment to appropriately treat and resolve cases associated with disease. Furthermore, in-depth knowledge is needed about the source and transmission vehicles of pathogens in hospitals to adopt pertinent prevention measures.

High Prevalence of Virulence Genes in Specific Genotypes of Atypical Enteropathogenic Escherichia coli

Atypical enteropathogenic Escherichia coli (aEPEC) strains are emerging enteropathogens that have been detected worldwide. A collection of 228 aEPEC strains (121 from diarrheal patients, 27 from healthy carriers, 47 from animals and 33 from raw meats) were investigated for serotypes, virulence gene profiles and phylogenetic relationships. Sixty-six O serogroups were identified. Serogroup O51 was the most prevalent, followed by O119, O26 and O76. For the 20 virulence genes detected, statistically significant differences were observed in the overall prevalence of efa1 (lifA), nleB, nleE, set/ent, paa, and ehxA genes among strains from diarrheal patients, healthy carriers, animals and raw meats, respectively. Strains from diarrheal patients had significantly higher levels of efa1 (lifA) (29.8 vs. 0%, P = 0.0002), nleB (41.3 vs. 7.4%, P = 0.0004), nleE (43.8 vs. 7.4%, P = 0.0002) and set/ent (41.3 vs. 7.4%, P = 0.0004) genes than strains obtained from healthy carriers. The paa gene was identified more often in isolates from raw meats (63.6 vs. 14.8%, P < 0.0001), animals (42.6 vs. 14.8%, P < 0.0122), and diarrheal patients (36.4 vs. 14.8%, P < 0.0225) than in strains obtained from healthy carriers. The ehxA gene was detected more frequently in strains from raw meats than in strains from diarrheal patients (27.3 vs. 2.5%, P = 0.0000) and healthy carriers (27.3 vs. 7.4%, P = 0.0474). The phylogenetic marker, yjaA, was more frequently observed in strains among healthy carriers than in diarrheal patient strains. Among the 228 aEPEC strains, 79 sequence types (STs) were identified. The prominent STs, which comprised strains carrying the four OI-122 genes and lpfA, were ST40, ST328, and ST29. Overall, the results indicate that aEPEC strains isolated in China are highly heterogeneous. aEPEC strains that are potentially more pathogenic appear to be related to specific STs or clonal complexes and serotypes. The high prevalence of diarrhea-associated genes in animal or raw meat strains suggests a zoonotic transmission pathway for potentially human pathogenic aEPEC.

A Novel Mechanism for Protein Delivery by the Type 3 Secretion System for Extracellularly Secreted Proteins

The type 3 secretion system (T3SS) is essential for bacterial virulence through delivering effector proteins directly into the host cytosol. Here, we identified an alternative delivery mechanism of virulence factors mediated by the T3SS, which consists of the association of extracellularly secreted proteins from bacteria with the T3SS to gain access to the host cytosol. Both EspC, a protein secreted as an enteropathogenic Escherichia coli (EPEC) autotransporter, and YopH, a protein detected on the surface of Yersinia, require a functional T3SS for host cell internalization; here we provide biophysical and molecular evidence to support the concept of the EspC translocation mechanism, which requires (i) an interaction between EspA and an EspC middle segment, (ii) an EspC translocation motif (21 residues that are shared with the YopH translocation motif), (iii) increases in the association and dissociation rates of EspC mediated by EspA interacting with EspD, and (iv) an interaction of EspC with the EspD/EspB translocon pore. Interestingly, this novel mechanism does not exclude the injection model (i.e., EspF) operating through the T3SS conduit; therefore, T3SS can be functioning as an internal conduit or as an external railway, which can be used to reach the translocator pore, and this mechanism appears to be conserved among different T3SS-dependent pathogens.

The type 3 secretion system is essential for injection of virulence factors, which are delivered directly into the cytosol of the host cells for usurping and subverting host processes. Recent studies have shown that these effectors proteins indeed travel inside an “injectisome” conduit through a single step of translocation by connecting the bacterium and host cell cytoplasms. However, all findings are not compatible with this model. For example, both YopH, a protein detected on the surface of Yersinia, and EspC, an autotransporter protein secreted by enteropathogenic E. coli, require a functional T3SS for host cell translocation. Both proteins have an intermediate extracellular step before their T3SS-dependent translocation. Here, we show an alternative delivery mechanism for these extracellularly secreted virulence factors that are then incorporated into the T3SS to enter the cells; this novel mechanism coexists with but diverges from the canonical injection model that involves the passage of the protein inside the injectisome.

Overexpressed Proteins in Hypervirulent Clade 8 and Clade 6 Strains of Escherichia coli O157:H7 Compared to E. coli O157:H7 EDL933 Clade 3 Strain

Escherichia coli O157:H7 is responsible for severe diarrhea and hemolytic uremic syndrome (HUS), and predominantly affects children under 5 years. The major virulence traits are Shiga toxins, necessary to develop HUS and the Type III Secretion System (T3SS) through which bacteria translocate effector proteins directly into the host cell. By SNPs typing, E. coli O157:H7 was separated into nine different clades. Clade 8 and clade 6 strains were more frequently associated with severe disease and HUS. In this study, we aimed to identify differentially expressed proteins in two strains of E. coli O157:H7 (clade 8 and clade 6), obtained from cattle and compared them with the well characterized reference EDL933 strain (clade 3). Clade 8 and clade 6 strains show enhanced pathogenicity in a mouse model and virulence-related properties. Proteins were extracted and analyzed using the TMT-6plex labeling strategy associated with two dimensional liquid chromatography and mass spectrometry in tandem. We detected 2241 proteins in the cell extract and 1787 proteins in the culture supernatants. Attention was focused on the proteins related to virulence, overexpressed in clade 6 and 8 strains compared to EDL933 strain. The proteins relevant overexpressed in clade 8 strain were the curli protein CsgC, a transcriptional activator (PchE), phage proteins, Stx2, FlgM and FlgD, a dienelactone hydrolase, CheW and CheY, and the SPATE protease EspP. For clade 6 strain, a high overexpression of phage proteins was detected, mostly from Stx2 encoding phage, including Stx2, flagellin and the protease TagA, EDL933_p0016, dienelactone hydrolase, and Haemolysin A, amongst others with unknown function. Some of these proteins were analyzed by RT-qPCR to corroborate the proteomic data. Clade 6 and clade 8 strains showed enhanced transcription of 10 out of 12 genes compared to EDL933. These results may provide new insights in E. coli O157:H7 mechanisms of pathogenesis.

Diarrheagenic Escherichia coli

Most Escherichia coli strains live harmlessly in the intestines and rarely cause disease in healthy individuals. Nonetheless, a number of pathogenic strains can cause diarrhea or extraintestinal diseases both in healthy and immunocompromised individuals. Diarrheal illnesses are a severe public health problem and a major cause of morbidity and mortality in infants and young children, especially in developing countries. E. coli strains that cause diarrhea have evolved by acquiring, through horizontal gene transfer, a particular set of characteristics that have successfully persisted in the host. According to the group of virulence determinants acquired, specific combinations were formed determining the currently known E. coli pathotypes, which are collectively known as diarrheagenic E. coli. In this review, we have gathered information on current definitions, serotypes, lineages, virulence mechanisms, epidemiology, and diagnosis of the major diarrheagenic E. coli pathotypes.

EspC, an Autotransporter Protein Secreted by Enteropathogenic Escherichia coli, Causes Apoptosis and Necrosis through Caspase and Calpain Activation, Including Direct Procaspase-3 Cleavage

Enteropathogenic Escherichia coli (EPEC) has the ability to antagonize host apoptosis during infection through promotion and inhibition of effectors injected by the type III secretion system (T3SS), but the total number of these effectors and the overall functional relationships between these effectors during infection are poorly understood. EspC produced by EPEC cleaves fodrin, paxillin, and focal adhesion kinase (FAK), which are also cleaved by caspases and calpains during apoptosis. Here we show the role of EspC in cell death induced by EPEC. EspC is involved in EPEC-mediated cell death and induces both apoptosis and necrosis in epithelial cells. EspC induces apoptosis through the mitochondrial apoptotic pathway by provoking (i) a decrease in the expression levels of antiapoptotic protein Bcl-2, (ii) translocation of the proapoptotic protein Bax from cytosol to mitochondria, (iii) cytochrome c release from mitochondria to the cytoplasm, (iv) loss of mitochondrial membrane potential, (v) caspase-9 activation, (vi) cleavage of procaspase-3 and (vii) an increase in caspase-3 activity, (viii) PARP proteolysis, and (ix) nuclear fragmentation and an increase in the sub-G1 population. Interestingly, EspC-induced apoptosis was triggered through a dual mechanism involving both independent and dependent functions of its EspC serine protease motif, the direct cleavage of procaspase-3 being dependent on this motif. This is the first report showing a shortcut for induction of apoptosis by the catalytic activity of an EPEC protein. Furthermore, this atypical intrinsic apoptosis appeared to induce necrosis through the activation of calpain and through the increase of intracellular calcium induced by EspC. Our data indicate that EspC plays a relevant role in cell death induced by EPEC.

IMPORTANCE

EspC, an autotransporter protein with serine protease activity, has cytotoxic effects on epithelial cells during EPEC infection. EspC causes cytotoxicity by cleaving fodrin, a cytoskeletal actin-associated protein, and focal adhesion proteins (i.e., FAK); interestingly, these proteins are also cleaved during apoptosis and necrosis. Here we show that EspC is able to cause cell death, which is characterized by apoptosis: by dissecting the apoptotic pathway and considering that EspC is translocated by an injectisome, we found that EspC induces the mitochondrial apoptotic pathway. Remarkably, EspC activates this pathway by two distinct mechanisms-either by using or not using its serine protease motif. Thus, we show for the first time that this serine protease motif is able to cleave procaspase-3, thereby reaching the terminal stages of caspase cascade activation leading to apoptosis. Furthermore, this overlapped apoptosis appears to potentiate cell death through necrosis, where EspC induces calpain activation and increases intracellular calcium.

The serine protease Pic as a virulence factor of atypical enteropathogenic Escherichia coli

Autotransporter proteins (AT) are associated with bacterial virulence attributes. Originally identified in enteroaggregative Escherichia coli (EAEC), Shigella flexneri 2a and uropathogenic E. coli, the serine protease Pic is one of these AT. We have previously detected one atypical enteropathogenic E. coli strain (BA589) carrying the pic gene. In the present study, we characterized the biological activities of Pic produced by BA589 both in vitro and in vivo. Contrarily to other Pic-producers bacteria, pic in BA589 is located on a high molecular weight plasmid. PicBA589 was able to agglutinate rabbit erythrocytes, cleave mucin and degrade complement system molecules. BA589 was able to colonize mice intestines, and an intense mucus production was observed. The BA589Δpic mutant lost the capacity to colonize as well as the above-mentioned in vitro activities. Thus, Pic represents an additional virulence factor in aEPEC strain BA589, associated with adherence, colonization and evasion from the innate immune system.

Flagellin and GroEL mediates in vitro binding of an atypical enteropathogenic Escherichia coli to cellular fibronectin

Background

Enteropathogenic Escherichia coli (EPEC) is distinguished mainly by the presence of EPEC adherence factor plasmid (pEAF) in typical EPEC (tEPEC) and its absence in atypical EPEC (aEPEC). The initial adherence to the intestinal mucosa is complex and mediated by adhesins other than bundle-forming pilus, which is not produced by aEPEC. Extracellular matrix (ECM) proteins of eukaryotic cells are commonly recognized by bacterial adhesins. Therefore, binding to ECM proteins may facilitate colonization, invasion and/or signaling by intestinal pathogens. Previous studies from our group demonstrated that aEPEC O26:H11 (strain BA2103) showed high binding activity to fibronectin, not shared by its counterpart, aEPEC O26:HNM.

Results

In the present study, using mass spectrometry after fibronectin-associated immunoprecipitation, two proteins, flagellin (50 kDa) and GroEL (52 kDa), were identified and BA2103 binding ability to fibronectin was inhibited in the presence of anti-H11 and anti-GroEL sera, but not by either naïve rabbit or other unrelated sera. It was also observed that the presence of purified flagellin inhibits adhesion of BA2103 to cellular fibronectin in a dose-dependent manner. Additionally, BA2103 GroEL is similar to the same protein of uropathogenic E. coli.

Conclusions

Our results suggest that flagellin may play a role in the in vitro interaction of BA2103 with cellular fibronectin, and GroEL can be an accessory protein in this process.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0612-4) contains supplementary material, which is available to authorized users.

The Serine Protease EspC from Enteropathogenic Escherichia coli Regulates Pore Formation and Cytotoxicity Mediated by the Type III Secretion System

Type III secretion systems (T3SSs) are specialized macromolecular machines critical for bacterial virulence, and allowing the injection of bacterial effectors into host cells. The T3SS-dependent injection process requires the prior insertion of a protein complex, the translocon, into host cell membranes consisting of two-T3SS hydrophobic proteins, associated with pore-forming activity. In all described T3SS to date, a hydrophilic protein connects one hydrophobic component to the T3SS needle, presumably insuring the continuum between the hollow needle and the translocon. In the case of Enteropathogenic Escherichia coli (EPEC), the hydrophilic component EspA polymerizes into a filament connecting the T3SS needle to the translocon composed of the EspB and EspD hydrophobic proteins. Here, we identify EspA and EspD as targets of EspC, a serine protease autotransporter of Enterobacteriaceae (SPATE). We found that in vitro, EspC preferentially targets EspA associated with EspD, but was less efficient at proteolyzing EspA alone. Consistently, we found that EspC did not regulate EspA filaments at the surface of primed bacteria that was devoid of EspD, but controlled the levels of EspD and EspA secreted in vitro or upon cell contact. While still proficient for T3SS-mediated injection of bacterial effectors and cytoskeletal reorganization, an espC mutant showed increased levels of cell-associated EspA and EspD, as well as increased pore formation activity associated with cytotoxicity. EspP from enterohaemorrhagic E. coli (EHEC) also targeted translocator components and its activity was interchangeable with that of EspC, suggesting a common and important function of these SPATEs. These findings reveal a novel regulatory mechanism of T3SS-mediated pore formation and cytotoxicity control during EPEC/EHEC infection.

Pathogenesis of human diffusely adhering Escherichia coli expressing Afa/Dr adhesins (Afa/Dr DAEC): current insights and future challenges

The pathogenicity and clinical pertinence of diffusely adhering Escherichia coli expressing the Afa/Dr adhesins (Afa/Dr DAEC) in urinary tract infections (UTIs) and pregnancy complications are well established. In contrast, the implication of intestinal Afa/Dr DAEC in diarrhea is still under debate. These strains are age dependently involved in diarrhea in children, are apparently not involved in diarrhea in adults, and can also be asymptomatic intestinal microbiota strains in children and adult. This comprehensive review analyzes the epidemiology and diagnosis and highlights recent progress which has improved the understanding of Afa/Dr DAEC pathogenesis. Here, I summarize the roles of Afa/Dr DAEC virulence factors, including Afa/Dr adhesins, flagella, Sat toxin, and pks island products, in the development of specific mechanisms of pathogenicity. In intestinal epithelial polarized cells, the Afa/Dr adhesins trigger cell membrane receptor clustering and activation of the linked cell signaling pathways, promote structural and functional cell lesions and injuries in intestinal barrier, induce proinflammatory responses, create angiogenesis, instigate epithelial-mesenchymal transition-like events, and lead to pks-dependent DNA damage. UTI-associated Afa/Dr DAEC strains, following adhesin-membrane receptor cell interactions and activation of associated lipid raft-dependent cell signaling pathways, internalize in a microtubule-dependent manner within urinary tract epithelial cells, develop a particular intracellular lifestyle, and trigger a toxin-dependent cell detachment. In response to Afa/Dr DAEC infection, the host epithelial cells generate antibacterial defense responses. Finally, I discuss a hypothetical role of intestinal Afa/Dr DAEC strains that can act as "silent pathogens" with the capacity to emerge as "pathobionts" for the development of inflammatory bowel disease and intestinal carcinogenesis.

Longitudinal Characterization of Escherichia coli in Healthy Captive Non-Human Primates

The gastrointestinal (GI) tracts of non-human primates (NHPs) are well known to harbor Escherichia coli, a known commensal of human beings and animals. While E. coli is a normal inhabitant of the mammalian gut, it also exists in a number of pathogenic forms or pathotypes, including those with predisposition for the GI tract as well as the urogenital tract. Diarrhea in captive NHPs has long been a problem in both zoo settings and research colonies, including the Como Zoo. It is an animal welfare concern, as well as a public health concern. E. coli has not been extensively studied; therefore, a study was performed during the summer of 2009 in collaboration with a zoo in Saint Paul, MN, which was previously experiencing an increased incidence and severity of diarrhea among their NHP collection. Fresh fecal samples were collected weekly from each member of the primate collection, between June and August of 2009, and E. coli were isolated. A total of 33 individuals were included in the study, representing eight species. E. coli isolates were examined for their genetic relatedness, phylogenetic relationships, plasmid replicon types, virulence gene profiles, and antimicrobial susceptibility profiles. A number of isolates were identified containing virulence genes commonly found in several different E. coli pathotypes, and there was evidence of clonal transmission of isolates between animals and over time. Overall, the manifestation of chronic diarrhea in the Como Zoo primate collection is a complex problem whose solution will require regular screening for microbial agents and consideration of environmental causes. This study provides some insight toward the sharing of enteric bacteria between such animals.

Biochemical characterization of the SPATE members EspPα and EspI

The activity of serine proteases is influenced by their substrate specificity as well as by the physicochemical conditions. Here, we present the characterization of key biochemical features of the two SPATE members EspPα and EspI from Shiga-toxin producing Escherichia coli (STEC) and enterohemorrhagic E. coli (EHEC). Both proteases show high activity at conditions mimicking the human blood stream. Optimal activities were observed at slightly alkaline pH and low millimolar concentrations of the divalent cations Ca²⁺ and Mg²⁺ at physiological temperatures indicating a function in the human host. Furthermore, we provide the first cleavage profile for EspI demonstrating pronounced specificity of this protease.

EspC promotes epithelial cell detachment by enteropathogenic Escherichia coli via sequential cleavages of a cytoskeletal protein and then focal adhesion proteins

EspC is a non-locus of enterocyte effacement (LEE)-encoded autotransporter produced by enteropathogenic Escherichia coli (EPEC) that is secreted to the extracellular milieu by a type V secretion system and then translocated into epithelial cells by the type III secretion system. Here, we show that this efficient EspC delivery into the cell leads to a cytopathic effect characterized by cell rounding and cell detachment. Thus, EspC is the main protein involved in epithelial cell cytotoxicity detected during EPEC adhesion and pedestal formation assays. The cell detachment phenotype is triggered by cytoskeletal and focal adhesion disruption. EspC-producing EPEC is able to cleave fodrin, paxillin, and focal adhesion kinase (FAK), but these effects are not observed when cells are infected with an espC isogenic mutant. Recovery of these phenotypes by complementing the mutant with the espC gene but not with the espC gene mutated in the serine protease motif highlights the role of the protease activity of EspC in the cell detachment phenotype. In vitro assays using purified proteins showed that EspC, but not EspC with an S256I substitution [EspCS256I], directly cleaves these cytoskeletal and focal adhesion proteins. Kinetics of protein degradation indicated that EspC-producing EPEC first cleaves fodrin (within the 11th and 9th repetitive units at the Q1219 and D938 residues, respectively), and this event sequentially triggers paxillin degradation, FAK dephosphorylation, and FAK degradation. Thus, cytoskeletal and focal adhesion protein cleavage leads to the cell rounding and cell detachment promoted by EspC-producing EPEC.

Atypical enteropathogenic Escherichia coli secretes plasmid encoded toxin

Plasmid encoded toxin (Pet) is a serine protease originally described in enteroaggregative Escherichia coli (EAEC) prototype strain 042 whose entire characterization was essentially obtained from studies performed with the purified toxin. Here we show that Pet is not exclusive to EAEC. Atypical enteropathogenic Escherichia coli (aEPEC) strains, isolated from diarrhea cases, express Pet and its detection in supernatants of infected HEp-2 cells coincides with the appearance of cell damage, which, in turn, were similar to those described with purified Pet. Pet secretion and the cytotoxic effects are time and culture medium dependent. In presence of DMEM supplemented with tryptone cell rounding and detachment were observed after just 5 h of incubation with the bacteria. In the absence of tryptone, the cytotoxic effects were detected only after 24 h of infection. We also show that, in addition to the prototype EAEC, other pet+ EAEC strains, also isolated from diarrhea cases, induce cellular damage in the same degree as the aEPEC. The cytotoxic effects of EAEC and aEPEC strains were significantly reduced in the presence of a serine protease inhibitor or anti-Pet IgG serum. Our results show a common aspect between the aEPEC and EAEC and provide the first evidence pointing to a role of Pet in aEPEC pathogenesis.