Shiga toxin 2 is specifically released from bacterial cells by two different mechanisms

Deletion of Sphingosine Kinase 2 Attenuates Acute Kidney Injury in Mice with Hemolytic-Uremic Syndrome

Typical hemolytic uremic syndrome (HUS) can occur as a severe systemic complication of infections with Shiga toxin (Stx)-producing Escherichia coli. Its pathology can be induced by Stx types, resulting in toxin-mediated damage to renal barriers, inflammation, and the development of acute kidney injury (AKI). Two sphingosine kinase (SphK) isozymes, SphK1 and SphK2, have been shown to be involved in barrier maintenance and renal inflammatory diseases. Therefore, we sought to determine their role in the pathogenesis of HUS. Experimental HUS was induced by the repeated administration of Stx2 in wild-type (WT) and SphK1 (SphK1-/-) or SphK2 (SphK2-/-) null mutant mice. Disease severity was evaluated by assessing clinical symptoms, renal injury and dysfunction, inflammatory status and sphingolipid levels on day 5 of HUS development. Renal inflammation and injury were found to be attenuated in the SphK2-/- mice, but exacerbated in the SphK1-/- mice compared to the WT mice. The divergent outcome appeared to be associated with oppositely altered sphingolipid levels. This study represents the first description of the distinct roles of SphK1-/- and SphK2-/- in the pathogenesis of HUS. The identification of sphingolipid metabolism as a potential target for HUS therapy represents a significant advance in the field of HUS research.

Transcriptomic and proteomic analysis of the virulence inducing effect of ciprofloxacin on enterohemorrhagic Escherichia coli

Enterohemorrhagic E. coli (EHEC) is considered to be the most dangerous pathotype of E. coli, as it causes severe conditions such as hemorrhagic colitis (HC) and hemolytic uremic syndrome (HUS). Antibiotic treatment of EHEC infections is generally not recommended since it may promote the production of the Shiga toxin (Stx) and lead to worsened symptoms. This study explores how exposure to the fluoroquinolone ciprofloxacin reorganizes the transcriptome and proteome of EHEC O157:H7 strain EDL933, with special emphasis on virulence-associated factors. As expected, exposure to ciprofloxacin caused an extensive upregulation of SOS-response- and Stx-phage proteins, including Stx. A range of other virulence-associated factors were also upregulated, including many genes encoded by the LEE-pathogenicity island, the enterohemolysin gene (ehxA), as well as several genes and proteins involved in LPS production. However, a large proportion of the genes and proteins (17 and 8%, respectively) whose expression was upregulated upon ciprofloxacin exposure (17 and 8%, respectively) are not functionally assigned. This indicates a knowledge gap in our understanding of mechanisms involved in EHECs response to antibiotic-induced stress. Altogether, the results contribute to better understanding of how exposure to ciprofloxacin influences the virulome of EHEC and generates a knowledge base for further studies on how EHEC responds to antibiotic-induced stress. A deeper understanding on how EHEC responds to antibiotics will facilitate development of novel and safer treatments for EHEC infections.

Induced Burkholderia prophages detected from the hemoculture: a biomarker for Burkholderia pseudomallei infection

Bacteriophages (phages), viruses that infect bacteria, are found in abundance not only in the environment but also in the human body. The use of phages for the diagnosis of melioidosis, a tropical infectious disease caused by Burkholderia pseudomallei, is emerging as a promising novel approach, but our understanding of conditions under which Burkholderia prophages can be induced remains limited. Here, we first demonstrated the isolation of Burkholderia phages from the hemocultures of melioidosis patients. The B. pseudomallei-positive hemoculture bottles were filtered to remove bacteria, and then phages were isolated and purified by spot and double agar overlay plaque assays. Forty blood samples (hemoculture-confirmed melioidosis) were tested, and phages were found in 30% of the samples. Transmission electron microscopy and genome analysis of the isolated phages, vB_HM387 and vB_HM795, showed that both phages are Myoviruses. These two phages were stable at a pH of 5-7 and temperatures of 25-37°C, suggesting their ability to survive in human blood. The genome sizes of vB_HM387 and vB_HM795 are 36.3 and 44.0 kb, respectively. A phylogenetic analysis indicated that vB_HM387 has homologs, but vB_HM795 is a novel Myovirus, suggesting the heterogeneity of Burkholderia phages in melioidosis patients. The key finding that Burkholderia phages could be isolated from the blood of melioidosis patients highlights the potential application of phage-based assays by detecting phages in blood as a pathogen-derived biomarker of infection.

A prophage encoded ribosomal RNA methyltransferase regulates the virulence of Shiga-toxin-producing Escherichia coli (STEC)

Shiga toxin (Stx) released by Shiga toxin producing Escherichia coli (STEC) causes life-threatening illness. Its production and release require induction of Stx-encoding prophage resident within the STEC genome. We identified two different STEC strains, PA2 and PA8, bearing Stx-encoding prophage whose sequences primarily differ by the position of an IS629 insertion element, yet differ in their abilities to kill eukaryotic cells and whose prophages differ in their spontaneous induction frequencies. The IS629 element in ϕPA2, disrupts an ORF predicted to encode a DNA adenine methyltransferase, whereas in ϕPA8, this element lies in an intergenic region. Introducing a plasmid expressing the methyltransferase gene product into ϕPA2 bearing-strains increases both the prophage spontaneous induction frequency and virulence to those exhibited by ϕPA8 bearing-strains. However, a plasmid bearing mutations predicted to disrupt the putative active site of the methyltransferase does not complement either of these defects. When complexed with a second protein, the methyltransferase holoenzyme preferentially uses 16S rRNA as a substrate. The second subunit is responsible for directing the preferential methylation of rRNA. Together these findings reveal a previously unrecognized role for rRNA methylation in regulating induction of Stx-encoding prophage.

Shiga Toxin (Stx) Phage-Encoded Lytic Genes Are Not Required for the Mouse Virulence of O157:H7 Escherichia coli Stx2-Producing Clinical Isolates

Shiga toxin (Stx)-producing Escherichia coli (STEC) is a major cause of foodborne diarrheal illness in the United States and globally, and serotype O157:H7 is frequently associated with STEC outbreaks and sporadic cases in the United States. Severe systemic diseases associated with STEC are mediated by Stx types, particularly subtype Stx2a, encoded on inducible bacteriophages. We previously identified two STEC O157:H7 clinical isolates, JH2010 and JH2012, that exhibit a large difference in virulence in a streptomycin (Str)-treated mouse model. In this study, we aimed to identify a genetic basis for the difference in virulence between those strains. Comparison of the stx2a phage sequences showed that JH2012 lacks the lytic genes S and R on the phage genome. We also demonstrated that compared to JH2012 cultures, cultures of JH2010 released more Stx2 into the supernatant and were more sensitive to bacterial lysis during growth with ciprofloxacin (Cip), an inducer of stx phages. We therefore generated an stx2a phage SR deletion mutant strain of JH2010 to determine if those genes were responsible for the high virulence of that strain. We found that deletion of the SR genes from the stx2a phage in JH2010, and another O157:H7 strain, JH2016, resulted in increased cellular retention of Stx2, but there was no difference in virulence compared to the wild-type strains. Our results indicate that the stx2a phage SR genes are involved in Stx2 localization and phage-mediated cell lysis in vitro but that they are not required in wild-type STEC strains for virulence in a mouse model. IMPORTANCE The release of Stx from STEC has been thought to be tied to phage-mediated lysis of the host bacterial cell. In this study, we found that the stx2a phage lytic genes are not required for the virulence of pathogenic O157:H7 clinical isolates in a murine model of STEC infection or for release of Stx2a into the supernatant of bacterial cultures. These results point to an alternate mechanism for Stx2a release from STEC strains.

Dynamic changes in Shiga toxin (Stx) 1 transducing phage throughout the evolution of O26:H11 Stx-producing Escherichia coli

Shiga toxin (Stx) is the key virulence factor of Stx-producing Escherichia coli (STEC). All known Stxs (Stx1 and Stx2) are encoded by bacteriophages (Stx phages). Although the genetic diversity of Stx phages has frequently been described, systematic analyses of Stx phages in a single STEC lineage are limited. In this study, focusing on the O26:H11 STEC sequence type 21 (ST21) lineage, where the stx1a gene is highly conserved, we analysed the Stx1a phages in 39 strains representative of the entire ST21 lineage and found a high level of variation in Stx1a phage genomes caused by various mechanisms, including replacement by a different Stx1a phage at the same or different locus. The evolutionary timescale of events changing Stx1a phages in ST21 was also determined. Furthermore, by using an Stx1 quantification system developed in this study, we found notable variations in the efficiency of Stx1 production upon prophage induction, which sharply contrasted with the conserved iron regulated Stx1 production. These variations were associated with the Stx1a phage alteration in some cases but not in other cases; thus, Stx1 production in this STEC lineage was determined by differences not only in Stx1 phages but also in host-encoded factors.

Transduction of stx2a mediated by phage (Φ11-3088) from Escherichia coli O104:H4 in vitro and in situ during sprouting of mung beans

Escherichia coli O104:H4 strain 11-3088 encoding Stx2a is epidemiologically related to the foodborne outbreak associated with sprouts in Germany, 2011. Sprouting provides suitable conditions for bacterial growth and may lead to transduction of non-pathogenic strains of E. coli with Stx phages. Although transduction of E. coli by Stx phages in food has been documented, data on the phages from E. coli O104:H4 is limited. This study determined the host range of the bacteriophage Φ11-3088 from E. coli O104:H4 using E. coli O104:H4 ∆stx2::gfp::amp^r and demonstrated phage transduction during sprouting. The Φ11-3088∆stx transduced 5/45 strains, including generic E. coli, pap-positive E. coli O103:H2, ETEC, and S. sonnei. The expression level of Φ11-3088∆stx differed among lysogens upon induction. Of the 3 highly induced lysogens, the lytic cycle was induced in E. coli O104:H4∆stx2::gfp::amp^r and O103:H2 but not in S. sonnei. E. coli DH5α was the only strain susceptible to lytic infection by Φ11-3088∆stx. To explore the effect of drying and rehydration during seed storage and sprouting on phage induction and transduction, mung beans inoculated with the phage donor E. coli O104:H4∆stx2::gfp::amp^r (8 log CFU/g) were dried, rehydrated, and incubated with the phage recipient E. coli DH5α (7 log CFU/g) for 96 h. Sprouted seeds harbored about 3 log CFU/g of putative lysogens that acquired ampicillin resistance. At the end of sprouting, 71 % of putative lysogens encoded gfp, confirming phage transduction. Overall, stx transfer by phages may increase the cell counts of STEC during sprouting by converting generic E. coli to STEC.

The effect of a spontaneous induction prophage, phi458, on biofilm formation and virulence in avian pathogenic Escherichia coli

Prophage sequences are present in most bacterial genomes and account for up to 20% of its host genome. Integration of temperate phages may have an impact on the expression of host genes, while some prophages could turn into the lytic cycle and affect bacterial host biological characteristics. We investigated the role of spontaneous induction prophages in avian pathogenic Escherichia coli (APEC), which is the causative agent of avian colibacillosis in poultry, and considered a potential zoonotic bacterium related to the fact it serves as an armory of extraintestinal pathogenic E. coli. We found that APEC strain DE458 had a high spontaneous induction rate in vivo and in vitro. The released phage particles, phi458, were isolated, purified, and sequenced, and the deletion mutant, DE458Δphi458, was constructed and characterized. Biofilm formation of DE458Δphi458 was strongly decreased compared to that of the wild-type strain (p < 0.01). In addition, while the addition of DNase (100 μg/ml) did not affect prophage release but could digest eDNA, it significantly reduced the biofilm production of DE458 biofilm to a level close to that of DE458Δphi458. Compared to DE458, the adhesion and invasion abilities of DE458Δphi458 increased by approximately 6-20 times (p < 0.05). The virulence of DE458Δphi458 was enhanced by approximately 10-fold in chickens based on a 50% lethal dose. Furthermore, avian infection assays showed that the bacterial loads of DE458Δphi458 in the lung and liver were increased by 16.5- and 10-fold (p < 0.05), respectively, compared with those of the WT strain. The qRT-PCR revealed that deletion of phi458 led to upregulation of type I fimbriate-related gene fimH and curli-related gene csgC by 3- and 2.8-fold, respectively (p < 0.01). Our study revealed that phi458 promoted biofilm formation by spontaneously inducing and decreasing virulence by repressing virulence genes.

Occurrence and Reduction of Shiga Toxin-Producing Escherichia coli in Wastewaters in the Kathmandu Valley, Nepal

Inadequately treated effluents discharged from wastewater treatment plants (WWTPs) severely affect the environment and the surrounding population. This study analyzed the presence of the Shiga toxin-producing Escherichia coli (STEC) genes, stx1, and stx2, and the E. coli gene, sfmD, in municipal WWTP A (n = 11) and B (n = 11) where the reductions were also evaluated; hospitals (n = 17), sewage treatment plants (STPs) (n = 4) and non-functional WWTPs (not-working WWTPs) (n = 5) in the Kathmandu Valley, Nepal. The sfmD gene was detected in 100% of the samples in WWTPs, hospitals, and not-working WWTPs and 50% of STP samples. The highest detection of stx1 and stx2 was shown in the WWTP influents, followed by WWTP effluents, not-working WWTP wastewater, hospital wastewater, and STP wastewater. Log10 reduction values of sfmD, stx1, and stx2 in WWTP A were 1.7 log10, 1.7 log10, 1.4 log10, whereas those in WWTP B were 0.5 log10, 0.6 log10, 0.5 log10, respectively, suggesting the ineffective treatment of STEC in the wastewater in the Kathmandu Valley. The high concentrations of the stx genes in the wastewaters suggest the increasing presence of aggressive STEC in the Kathmandu Valley, which should be a major public health concern.

Platelets and Escherichia coli: A Complex Interaction

Apart from their involvement in hemostasis, platelets have been recognized for their contribution to inflammation and defense against microbial agents. The interaction between platelets and bacteria has been well studied in the model of Staphylococcus and Streptococcus but little described in Gram-negative bacteria, especially Escherichia coli. Being involved in the hemolytic uremic syndrome as well as sepsis, it is important to study the mechanisms of interaction between platelets and E. coli. Results of the published studies are heterogeneous. It appears that some strains interact with platelets through the toll-like receptor-4 (TLR-4) and others through the Fc gamma glycoprotein. E. coli mainly uses lipopolysaccharide (LPS) to activate platelets and cause the release of antibacterial molecules, but this is not the case for all strains. In this review, we describe the different mechanisms developed in previous studies, focusing on this heterogeneity of responses that may depend on several factors; mainly, the strain studied, the structure of the LPS and the platelet form used in the studies. We can hypothesize that the structure of O-antigen and an eventual resistance to antibiotics might explain this difference.

Genome Sequence Analysis and Characterization of Shiga Toxin 2 Production by Escherichia coli O157:H7 Strains Associated With a Laboratory Infection

A laboratory-acquired E. coli O157:H7 infection with associated severe sequelae including hemolytic uremic syndrome occurred in an individual working in the laboratory with a mixture of nalidixic acid-resistant (Nal^R) O157:H7 mutant strains in a soil-biochar blend. The patient was hospitalized and treated with an intravenous combination of metronidazole and levofloxacin. The present study investigated the source of this severe laboratory acquired infection and further examined the influence of the antibiotics used during treatment on the expression and production of Shiga toxin. Genomes of two Stx_2a-and eae-positive O157:H7 strains isolated from the patient's stool were sequenced along with two pairs of the wt strains and their derived Nal^R mutants used in the laboratory experiments. High-resolution SNP typing determined the strains' individual genetic relatedness and unambiguously identified the two laboratory-derived Nal^R mutant strains as the source of the researcher's life-threatening disease, rather than a conceivable ingestion of unrelated O157:H7 isolates circulating at the same time. It was further confirmed that in sublethal doses, the antibiotics increased toxin expression and production. Our results support a simultaneous co-infection with clinical strains in the laboratory, which were the causative agents of previous O157:H7 outbreaks, and further that the administration of antibiotics may have impacted the outcome of the infection.

Enhanced production of Shiga toxin 1 in enterohaemorrhagic Escherichia coli by oxygen

Enterohaemorrhagic Escherichia coli (EHEC) produces Shiga toxin 1 (Stx1) and Shiga toxin 2 (Stx2). Although stx1 and stx2 were found within the late operons of the Stx-encoding phages (Stx-phages), stx1 could mainly be transcribed from the stx1 promoter (P _Stx1), which represents the functional operator-binding site (Fur box) for the transcriptional regulator Fur (ferric uptake regulator), upstream of stx1. In this study, we found that the production of Stx1 by EHEC was affected by oxygen concentration. Increased Stx1 production in the presence of oxygen is dependent on Fur, which is an Fe²⁺-responsive transcription factor. The intracellular Fe²⁺ pool was lower under microaerobic conditions than under anaerobic conditions, suggesting that lower Fe²⁺ availability drove the formation of less Fe²⁺-Fur, less DNA binding to the P _Stx1 region, and an increase in Stx1 production.

Primary Human Colon Epithelial Cells (pHCoEpiCs) Do Express the Shiga Toxin (Stx) Receptor Glycosphingolipids Gb3Cer and Gb4Cer and Are Largely Refractory but Not Resistant towards Stx

Shiga toxin (Stx) is released by enterohemorrhagic Escherichia coli (EHEC) into the human intestinal lumen and transferred across the colon epithelium to the circulation. Stx-mediated damage of human kidney and brain endothelial cells and renal epithelial cells is a renowned feature, while the sensitivity of the human colon epithelium towards Stx and the decoration with the Stx receptor glycosphingolipids (GSLs) globotriaosylceramide (Gb3Cer, Galα1-4Galβ1-4Glcβ1-1Cer) and globotetraosylceramide (Gb4Cer, GalNAcβ1-3Galα1-4Galβ1-4Glcβ1-1Cer) is a matter of debate. Structural analysis of the globo-series GSLs of serum-free cultivated primary human colon epithelial cells (pHCoEpiCs) revealed Gb4Cer as the major neutral GSL with Cer (d18:1, C16:0), Cer (d18:1, C22:1/C22:0) and Cer (d18:1, C24:2/C24:1) accompanied by minor Gb3Cer with Cer (d18:1, C16:0) and Cer (d18:1, C24:1) as the dominant lipoforms. Gb3Cer and Gb4Cer co-distributed with cholesterol and sphingomyelin to detergent-resistant membranes (DRMs) used as microdomain analogs. Exposure to increasing Stx concentrations indicated only a slight cell-damaging effect at the highest toxin concentration of 1 µg/mL for Stx1a and Stx2a, whereas a significant effect was detected for Stx2e. Considerable Stx refractiveness of pHCoEpiCs that correlated with the rather low cellular content of the high-affinity Stx-receptor Gb3Cer renders the human colon epithelium questionable as a major target of Stx1a and Stx2a.

Vitamin K Analogs Influence the Growth and Virulence Potential of Enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) causes serious foodborne disease worldwide. It produces the very potent Shiga toxin 2 (Stx2). The Stx2-encoding genes are located on a prophage, and production of the toxin is linked to the synthesis of Stx phages. There is, currently, no good treatment for EHEC infections, as antibiotics may trigger lytic cycle activation of the phages and increased Stx production. This study addresses how four analogs of vitamin K, phylloquinone (K1), menaquinone (K2), menadione (K3), and menadione sodium bisulfite (MSB), influence growth, Stx2-converting phage synthesis, and Stx2 production by the EHEC O157:H7 strain EDL933. Menadione and MSB conferred a concentration-dependent negative effect on bacterial growth, while phylloquinone or menaquinone had little and no effect on bacterial growth, respectively. All four vitamin K analogs affected Stx2 phage production negatively in uninduced cultures and in cultures induced with either hydrogen peroxide (H₂O₂), ciprofloxacin, or mitomycin C. Menadione and MSB reduced Stx2 production in cultures induced with either H₂O₂ or ciprofloxacin. MSB also had a negative effect on Stx2 production in two other EHEC isolates tested. Phylloquinone and menaquinone had, on the other hand, variable and concentration-dependent effects on Stx2 production. MSB, which conferred the strongest inhibitory effect on both Stx2 phage and Stx2 production, improved the growth of EHEC in the presence of H₂O₂ and ciprofloxacin, which could be explained by the reduced uptake of ciprofloxacin into the bacterial cell. Together, the data suggest that vitamin K analogs have a growth- and potential virulence-reducing effect on EHEC, which could be of therapeutic interest.IMPORTANCE Enterohemorrhagic E. coli (EHEC) can cause serious illness and deaths in humans by producing toxins that can severely damage our intestines and kidneys. There is currently no optimal treatment for EHEC infections, as antibiotics can worsen disease development. Consequently, the need for new treatment options is urgent. Environmental factors in our intestines can affect the virulence of EHEC and help our bodies fight EHEC infections. The ruminant intestine, the main reservoir for EHEC, contains high levels of vitamin K, but the levels are variable in humans. This study shows that vitamin K analogs can inhibit the growth of EHEC and/or production of its main virulence factor, the Shiga toxin. They may also inhibit the spreading of the Shiga toxin encoding bacteriophage. Our findings indicate that vitamin K analogs have the potential to suppress the development of serious disease caused by EHEC.

Overall changes in the transcriptome of Escherichia coli O26:H11 induced by a subinhibitory concentration of ciprofloxacin

AIMS

The goal was to explore the effects of subinhibitory concentration (SIC) (0·5 MIC = 20 µg l^-1 ) of ciprofloxacin on the transcriptome of enterohaemorrhagic Escherichia coli O26:H11 isolate by 60 minutes of exposure.

MATERIALS AND RESULTS

We used a combination of comparative genomic and transcriptomic (RNAseq) analyses. The whole genome of the E. coli O26:H11 #30934 strain of bovine origin was sequenced and assembled. This genome was next used as reference for the differential gene expression analysis. A whole-genome-based analysis of 36 publicly available E. coli O26:H11 genomes was performed to define the core and the accessory transcriptome of E. coli O26:H11. Using RNAseq and RT-qPCR analysis we observed overexpression of the SOS response and of T3SS effectors, together with the inhibition of specific motility-associated genes. Among the large set of transposases present, only three were activated, suggesting moderate transposition of genes with low doses of ciprofloxacin. Our results illustrated that transcriptional repressors, such as the CopG family protein, belonging to the core genome of E. coli O26:H11, are altered in response to fluoroquinolone exposure. The gene ontology enrichment analysis showed SIC of ciprofloxacin induced binding functions and catalytic activities, including mostly transferase and hydrolase proteins. The amino acid pathways involved in metabolic processes were significantly enhanced after the treatment.

CONCLUSIONS

Although the core genome of E. coli O26:H11 constituted only 54·5% of the whole genome, we demonstrated that most differentially expressed genes were associated with the core genome of E. coli O26:H11, and that effects on the mobile genetic element, phage, and plasmid-related genes were rare.

SIGNIFICANCE AND IMPACT OF THE STUDY

For the first time the effect of low dose of ciprofloxacin on the core transcriptome of E. coli O26:H11 was described. The effects on the main biological functions and protein classes including transcriptional regulators were illustrated.

The Virulence of Escherichia coli O157:H7 Isolates in Mice Depends on Shiga Toxin Type 2a (Stx2a)-Induction and High Levels of Stx2a in Stool

In this study we compared nine Shiga toxin (Stx)-producing Escherichia coli O157:H7 patient isolates for Stx levels, stx-phage insertion site(s), and pathogenicity in a streptomycin (Str)-treated mouse model. The strains encoded stx_2a, stx_1a and stx_2a, or stx_2a and stx_2c. All of the strains elaborated 10⁵-10⁶ cytotoxic doses 50% (CD₅₀) into the supernatant after growth in vitro as measured on Vero cells, and showed variable levels of increased toxin production after growth with sub-inhibitory levels of ciprofloxacin (Cip). The stx_2a+stx_2c+ isolates were 90–100% lethal for Str-treated BALB/c mice, though one isolate, JH2013, had a delayed time-to-death. The stx_2a+ isolate was avirulent. Both an stx_2a and a recA deletion mutant of one of the stx_2a+stx_2c+ strains, JH2010, exhibited at least a three-log decrease in cytotoxicity in vitro and both were avirulent in the mice. Stool from Str-treated mice infected with the highly virulent isolates were 10- to 100-fold more cytotoxic than feces from mice infected with the clinical isolate, JH2012, that made only Stx2a. Taken together these findings demonstrate that the stx_2a-phage from JH2010 induces to higher levels in vivo than does the phage from JH2012. The stx_1a+stx_2a+ clinical isolates were avirulent and neutralization of Stx1 in stool from mice infected with those strains indicated that the toxin produced in vivo was primarily Stx1a. Treatment of mice infected with Stx1a+Stx2a+ isolates with Cip resulted in an increase in Stx2a production in vivo and lethality in the mice. Our data suggest that high levels of Stx2a in stool are predictive of virulence in mice.

Enterohemorrhagic Escherichia coli infection inhibits colonic thiamin pyrophosphate uptake via transcriptional mechanism

Colonocytes possess a specific carrier-mediated uptake process for the microbiota-generated thiamin (vitamin B1) pyrophosphate (TPP) that involves the TPP transporter (TPPT; product of the SLC44A4 gene). Little is known about the effect of exogenous factors (including enteric pathogens) on the colonic TPP uptake process. Our aim in this study was to investigate the effect of Enterohemorrhagic Escherichia coli (EHEC) infection on colonic uptake of TPP. We used human-derived colonic epithelial NCM460 cells and mice in our investigation. The results showed that infecting NCM460 cells with live EHEC (but not with heat-killed EHEC, EHEC culture supernatant, or with non-pathogenic E. Coli) to lead to a significant inhibition in carrier-mediated TPP uptake, as well as in level of expression of the TPPT protein and mRNA. Similarly, infecting mice with EHEC led to a significant inhibition in colonic TPP uptake and in level of expression of TPPT protein and mRNA. The inhibitory effect of EHEC on TPP uptake by NCM460 was found to be associated with reduction in the rate of transcription of the SLC44A4 gene as indicated by the significant reduction in the activity of the SLC44A4 promoter transfected into EHEC infected cells. The latter was also associated with a marked reduction in the level of expression of the transcription factors CREB-1 and ELF3, which are known to drive the activity of the SLC44A4 promoter. Finally, blocking the ERK1/2 and NF-kB signaling pathways in NCM460 cells significantly reversed the level of EHEC inhibition in TPP uptake and TPPT expression. Collectively, these findings show, for the first time, that EHEC infection significantly inhibit colonic uptake of TPP, and that this effect appears to be exerted at the level of SLC44A4 transcription and involves the ERK1/2 and NF-kB signaling pathways.

Genomic Characterization of β-Glucuronidase-Positive Escherichia coli O157:H7 Producing Stx2a

Among Shiga toxin (Stx)–producing Escherichia coli (STEC) O157:H7 strains, those producing Stx2a cause more severe diseases. Atypical STEC O157:H7 strains showing a β-glucuronidase–positive phenotype (GP STEC O157:H7) have rarely been isolated from humans, mostly from persons with asymptomatic or mild infections; Stx2a-producing strains have not been reported. We isolated, from a patient with bloody diarrhea, a GP STEC O157:H7 strain (PV15-279) that produces Stx2a in addition to Stx1a and Stx2c. Genomic comparison with other STEC O157 strains revealed that PV15-279 recently emerged from the stx1a/stx2c-positive GP STEC O157:H7 clone circulating in Japan. Major virulence genes are shared between typical (β-glucuronidase–negative) and GP STEC O157:H7 strains, and the Stx2-producing ability of PV15-279 is comparable to that of typical STEC O157:H7 strains; therefore, PV15-279 presents a virulence potential similar to that of typical STEC O157:H7. This study reveals the importance of GP O157:H7 as a source of highly pathogenic STEC clones.

Bacterial 'Grounded' Prophages: Hotspots for Genetic Renovation and Innovation

Bacterial genomes are highly plastic allowing the generation of variants through mutations and acquisition of genetic information. The fittest variants are then selected by the econiche thereby allowing the bacterial adaptation and colonization of the habitat. Larger genomes, however, may impose metabolic burden and hence bacterial genomes are optimized by the loss of frivolous genetic information. The activity of temperate bacteriophages has acute consequences on the bacterial population as well as the bacterial genome through lytic and lysogenic cycles. Lysogeny is a selective advantage as the prophage provides immunity to the lysogen against secondary phage attack. Since the non-lysogens are eliminated by the lytic phages, lysogens multiply and colonize the habitat. Nevertheless, all lysogens have an imminent risk of lytic cycle activation and cell lysis. However, a mutation in the attachment sites or in the genes that encode the specific recombinase responsible for prophage excision could result in 'grounding' of the prophage. Since the lysogens with grounded prophage are immune to respective phage infection as well as dodge the induction of lytic cycle, we hypothesize that the selection of these mutant lysogens is favored relative to their normal lysogenic counterparts. These grounded prophages offer several advantages to the bacterial genome evolution through propensity for genetic variations including inversions, deletions, and insertions via horizontal gene transfer. We propose that the grounded prophages expedite bacterial genome evolution by acting as 'genetic buffer zones' thereby increasing the frequency as well as the diversity of variations on which natural selection favors the beneficial variants. The grounded prophages are also hotspots for horizontal gene transfer wherein several ecologically significant genes such as those involved in stress tolerance, antimicrobial resistance, and novel metabolic pathways, are integrated. Moreover, the high frequency of genetic changes within prophages also allows proportionate probability for the de novo genesis of genetic information. Through sequence analyses of well-characterized E. coli prophages we exemplify various roles of grounded prophages in E. coli ecology and evolution. Therefore, the temperate prophages are one of the most significant drivers of bacterial genome evolution and sites of biogenesis of genetic information.

Mass spectrometry-based Shiga toxin identification: A clinical validation

After we published our preliminary study on the use of liquid chromatography-tandem mass spectrometry (LC-MS/MS) and curated E. coli toxin databases on the identification of E. coli Shiga toxins (Stxs) in the Journal of Proteomics in year 2018, we were encouraged to further refine the method and test clinical isolates. In this study, different concentrations of mitomycin C (MMC) and ciprofloxacin (CF), two common antibiotic/chemotherapy agents capable of stimulating Stx production, were first tested and compared on three reference strains and eight clinical isolates to observe the toxin induction and subsequent identification. Notably, no differences were observed between the two agents other than the concentrations applied. Seventeen more clinical isolates were then tested using fixed MMC and CF concentrations and sample amount. This study confirms that the majority of stx2-positive E. coli strains can be stimulated to produce sufficient toxin for confident identification. This does not occur with stx1-positive E. coli isolates, however, despite the fact that both Stxs can be identified for several isolates without MMC or CF stimulation. BIOLOGICAL SIGNIFICANCE: Stxs, especially Stx2, are very important causes of severe food-borne disease, even death. This study confirms that receptor analogue-based affinity enrichment of Stxs, after MMC or CF treatment of E. coli, is useful for fast and accurate Stx2 identification through LC-MS/MS.

Influences of epigallocatechin gallate and citric acid on Escherichia coli O157:H7 toxin gene expression and virulence-associated stress response

Citric acid and EGCG at their minimum inhibitory concentrations were tested in this study. Logarithmic phase cells of Escherichia coli O157:H7 (ATCC 43895) were exposed to EGCG and citric acid respectively. The results of RT-real time PCR showed that both EGCG and citric acid increased stx2 and oxyR expression and decreased stx1, recA and Q expression. The result of Western blotting for RecA protein further indicated that both EGCG and citric acid decreased RecA production. Both EGCG and citric acid increased the level of intracellular reactive oxygen species and H₂ O₂ production and decreased superoxide dismutase activity. Therefore, EGCG and citric acid might induce stx2 production by increasing oxidative stress response and inhibit stx1 production by suppressing SOS response. In our study, the differential effects of the two antimicrobials were observed. EGCG reduced ompC and rpoS expression. However, citric acid caused an increase in ompC and rpoS expression. Membrane permeability is associated with toxin release. Citric acid increased the outer membrane permeability of E. coli O157:H7. However, the outer membrane of E. coli O157:H7 remained unaffected by EGCG.

SIGNIFICANCE AND IMPACT OF THE STUDY

Shiga toxins are the major virulence factors of Escherichia coli O157:H7. The use of antimicrobials triggering Shiga toxin production is controversial. (-)-epigallocatechin-3-gallate (EGCG) citric acid are often used singly or in combination to prevent micro-organisms in some food products. This study evaluated toxin induction in E. coli O157:H7 in response to EGCG and citric acid and investigated the potential mechanism of action. The findings may contribute to the proper use of EGCG and citric acid as antimicrobials.

After the Fact(or): Posttranscriptional Gene Regulation in Enterohemorrhagic Escherichia coli O157:H7

To adapt to ever-changing environments, pathogens quickly alter gene expression. This can occur through transcriptional, posttranscriptional, or posttranslational regulation. Historically, transcriptional regulation has been thoroughly studied to understand pathogen niche adaptation, whereas posttranscriptional and posttranslational gene regulation has only relatively recently been appreciated to play a central role in bacterial pathogenesis. Posttranscriptional regulation may involve chaperones, nucleases, and/or noncoding small RNAs (sRNAs) and typically controls gene expression by altering the stability and/or translation of the target mRNA. In this review, we highlight the global importance of posttranscriptional regulation to enterohemorrhagic Escherichia coli (EHEC) gene expression and discuss specific mechanisms of how EHEC regulates expression of virulence factors critical to host colonization and disease progression. The low infectious dose of this intestinal pathogen suggests that EHEC is particularly well adapted to respond to the host environment.

Non-pathogenic Escherichia coli Enhance Stx2a Production of E. coli O157:H7 Through Both bamA-Dependent and Independent Mechanisms

Intestinal colonization by the foodborne pathogen Escherichia coli O157:H7 leads to serious disease symptoms, including hemolytic uremic syndrome (HUS) and hemorrhagic colitis (HC). Synthesis of one or more Shiga toxins (Stx) is essential for HUS and HC development. The genes encoding Stx, including Stx2a, are found within a lambdoid prophage integrated in the E. coli O157:H7 chromosome. Enhanced Stx2a expression was reported when specific non-pathogenic E. coli strains were co-cultured with E. coli O157:H7, and it was hypothesized that this phenotype required the non-pathogenic E. coli to be sensitive to stx-converting phage infection. We tested this hypothesis by generating phage resistant non-pathogenic E. coli strains where bamA (an essential gene and Stx phage receptor) was replaced with an ortholog from other species. Such heterologous gene replacement abolished the ability of the laboratory strain E. coli C600 to enhance toxin production when co-cultured with E. coli O157:H7 strain PA2, which belongs to the hypervirulent clade 8. The extracellular loops of BamA (loop 4, 6, 7) were further shown to be important for infection by stx2a-converting phages. However, similar gene replacement in another commensal E. coli, designated 1.1954, revealed a bamA-independent mechanism for toxin amplification. Toxin enhancement by 1.1954 was not the result of phage infection through an alternative receptor (LamB or FadL), lysogen formation by stx2a-converting phages, or the production of a secreted molecule. Collectively, these data suggest that non-pathogenic E. coli can enhance toxin production through at least two mechanisms.

Molecular Mechanisms Governing "Hair-Trigger" Induction of Shiga Toxin-Encoding Prophages

Shiga toxin (Stx)-encoding E. coli (STEC) strains are responsible for sporadic outbreaks of food poisoning dating to 1982, when the first STEC strain, E. coli O157:H7, was isolated. Regardless of STEC serotype, the primary symptoms of STEC infections are caused by Stx that is synthesized from genes resident on lambdoid prophage present in STEC. Despite similar etiology, the severity of STEC-mediated disease varies by outbreak. However, it is unclear what modulates the severity of STEC-mediated disease. Stx production and release is controlled by lytic growth of the Stx-encoding bacteriophage, which in turn, is controlled by the phage repressor. Here, we confirm our earlier suggestion that the higher spontaneous induction frequency of Stx-encoding prophage is a consequence, in part, of lower intracellular repressor levels in STEC strains versus non-STEC strains. We also show that this lowered intracellular repressor concentration is a consequence of the utilization of alternative binding/regulatory strategies by the phage repressor. We suggest that a higher spontaneous induction frequency would lead to increased virulence.

Induction of Shiga Toxin-Encoding Prophage by Abiotic Environmental Stress in Food

The prophage-encoded Shiga toxin is a major virulence factor in Stx-producing Escherichia coli (STEC). Toxin production and phage production are linked and occur after induction of the RecA-dependent SOS response. However, food-related stress and Stx-prophage induction have not been studied at the single-cell level. This study investigated the effects of abiotic environmental stress on stx expression by single-cell quantification of gene expression in STEC O104:H4 Δstx2::gfp::amp^r In addition, the effect of stress on production of phage particles was determined. The lethality of stressors, including heat, HCl, lactic acid, hydrogen peroxide, and high hydrostatic pressure, was selected to reduce cell counts by 1 to 2 log CFU/ml. The integrity of the bacterial membrane after exposure to stress was measured by propidium iodide (PI). The fluorescent signals of green fluorescent protein (GFP) and PI were quantified by flow cytometry. The mechanism of prophage induction by stress was evaluated by relative gene expression of recA and cell morphology. Acid (pH < 3.5) and H₂O₂ (2.5 mM) induced the expression of stx₂ in about 18% and 3% of the population, respectively. The mechanism of prophage induction by acid differs from that of induction by H₂O₂ H₂O₂ induction but not acid induction corresponded to production of infectious phage particles, upregulation of recA, and cell filamentation. Pressure (200 MPa) or heat did not induce the Stx2-encoding prophage (Stx2-prophage). Overall, the quantification method developed in this study allowed investigation of prophage induction and physiological properties at the single-cell level. H₂O₂ and acids mediate different pathways to induce Stx2-prophage.IMPORTANCE Induction of the Stx-prophage in STEC results in production of phage particles and Stx and thus relates to virulence as well as the transduction of virulence genes. This study developed a method for a detection of the induction of Stx-prophages at the single-cell level; membrane permeability and an indication of SOS response to environmental stress were additionally assessed. H₂O₂ and mitomycin C induced expression of the prophage and activated a SOS response. In contrast, HCl and lactic acid induced the Stx-prophage but not the SOS response. The lifestyle of STEC exposes the organism to intestinal and extraintestinal environments that impose oxidative and acid stress. A more thorough understanding of the influence of food processing-related stressors on Stx-prophage expression thus facilitates control of STEC in food systems by minimizing prophage induction during food production and storage.

Shiga Toxin Subtypes of Non-O157 Escherichia coli Serogroups Isolated from Cattle Feces

Shiga toxin producing Escherichia coli (STEC) are important foodborne pathogens responsible for human illnesses. Cattle are a major reservoir that harbor the organism in the hindgut and shed in the feces. Shiga toxins (Stx) are the primary virulence factors associated with STEC illnesses. The two antigenically distinct Stx types, Stx1 and Stx2, encoded by stx1 and stx2 genes, share approximately 56% amino acid sequence identity. Genetic variants exist within Stx1 and Stx2 based on differences in amino acid composition and in cytotoxicity. The objective of our study was to identify the stx subtypes in strains of STEC serogroups, other than O157, isolated from cattle feces. Shiga toxin gene carrying E. coli strains (n = 192), spanning 27 serogroups originating from cattle (n = 170) and human (n = 22) sources, were utilized in the study. Shiga toxin genes were amplified by PCR, sequenced, and nucleotide sequences were translated into amino acid sequences using CLC main workbench software. Shiga toxin subtypes were identified based on the amino acid motifs that define each subtype. Shiga toxin genotypes were also identified at the nucleotide level by in silico restriction fragment length polymorphism (RFLP). Of the total 192 STEC strains, 93 (48.4%) were positive for stx1 only, 43 (22.4%) for stx2 only, and 56 (29.2%) for both stx1 and stx2. Among the 149 strains positive for stx1, 132 (88.6%) were stx1a and 17 (11.4%) were stx1c. Shiga toxin 1a was the most common subtype of stx1 among cattle (87.9%; 123/140) and human strains (100%; 9/9) of non-O157 serogroups. Of the total 99 strains positive for stx2, 79 were stx2a (79.8%), 11 (11.1%) were stx2c, 12 (12.1%) were stx2d. Of the 170 strains originating from cattle feces, 58 (34.1%) were stx2a subtype, 11 (6.5%) were stx2c subtype, and 11 were of subtype stx2d (6.5%). All but one of the human strains were positive for stx2a. Three strains of cattle origin were positive for both stx2a and stx2d. In conclusion, a number of non-O157 STEC serogroups harbored by cattle possess a wide variety of Shiga toxin subtypes, with stx1a and stx2a being the most predominant stx subtypes occurring individually or in combination. Cattle are a reservoir of a number of non-O157 STEC serogroups and information on the Shiga toxin subtypes is useful in assessing the potential risk as human pathogens.

Nitric oxide-enhanced Shiga toxin production was regulated by Fur and RecA in enterohemorrhagic Escherichia coli O157

Enterohemorrhagic Escherichia coli (EHEC) produces Shiga toxin 1 (Stx1) and Shiga toxin 2 (Stx2). Nitric oxide (NO), which acts as an antimicrobial defense molecule, was found to enhance the production of Stx1 and Stx2 in EHEC under anaerobic conditions. Although EHEC O157 has two types of anaerobic NO reductase genes, an intact norV and a deleted norV, in the deleted norV‐type EHEC, a high concentration of NO (12–29 μmol/L, maximum steady‐state concentration) is required for enhanced Stx1 production and a low concentration of NO (~12 μmol/L, maximum steady‐state concentration) is sufficient for enhanced Stx2 production under anaerobic conditions. These results suggested that different concentration thresholds of NO elicit a discrete set of Stx1 and Stx2 production pathways. Moreover, the enhancement of Shiga toxin production in the intact norV‐type EHEC required treatment with a higher concentration of NO than was required for enhancement of Shiga toxin production in the deleted norV‐type EHEC, suggesting that the specific NorV type plays an important role in the level of enhancement of Shiga toxin production in response to NO. Finally, Fur derepression and RecA activation in EHEC were shown to participate in the NO‐enhanced Stx1 and Stx2 production, respectively.

Characterization of the Prophage Repertoire of African Salmonella Typhimurium ST313 Reveals High Levels of Spontaneous Induction of Novel Phage BTP1

In the past 30 years, Salmonella bloodstream infections have become a significant health problem in sub-Saharan Africa and are responsible for the deaths of an estimated 390,000 people each year. The disease is predominantly caused by a recently described sequence type of Salmonella Typhimurium: ST313, which has a distinctive set of prophage sequences. We have thoroughly characterized the ST313-associated prophages both genetically and experimentally. ST313 representative strain D23580 contains five full-length prophages: BTP1, Gifsy-2^D23580, ST64B^D23580, Gifsy-1^D23580, and BTP5. We show that common S. Typhimurium prophages Gifsy-2, Gifsy-1, and ST64B are inactivated in ST313 by mutations. Prophage BTP1 was found to be a functional novel phage, and the first isolate of the proposed new species "Salmonella virus BTP1", belonging to the P22virus genus. Surprisingly, ∼10⁹ BTP1 virus particles per ml were detected in the supernatant of non-induced, stationary-phase cultures of strain D23580, representing the highest spontaneously induced phage titer so far reported for a bacterial prophage. High spontaneous induction is shown to be an intrinsic property of prophage BTP1, and indicates the phage-mediated lysis of around 0.2% of the lysogenic population. The fact that BTP1 is highly conserved in ST313 poses interesting questions about the potential fitness costs and benefits of novel prophages in epidemic S. Typhimurium ST313.

Postinfectious Hemolytic Uremic Syndrome

Post-infectious hemolytic uremic syndrome (HUS) is caused by specific pathogens in patients with no identifiable HUS-associated genetic mutation or autoantibody. The majority of episodes is due to infections by Shiga toxin (Stx) producing Escherichia coli (STEC). This chapter reviews the epidemiology and pathogenesis of STEC-HUS, including bacterial-derived factors and host responses. STEC disease is characterized by hematological (microangiopathic hemolytic anemia), renal (acute kidney injury) and extrarenal organ involvement. Clinicians should always strive for an etiological diagnosis through the microbiological or molecular identification of Stx-producing bacteria and Stx or, if negative, serological assays. Treatment of STEC-HUS is supportive; more investigations are needed to evaluate the efficacy of putative preventive and therapeutic measures, such as non-phage-inducing antibiotics, volume expansion and anti-complement agents. The outcome of STEC-HUS is generally favorable, but chronic kidney disease, permanent extrarenal, mainly cerebral complication and death (in less than 5 %) occur and long-term follow-up is recommended. The remainder of this chapter highlights rarer forms of (post-infectious) HUS due to S. dysenteriae, S. pneumoniae, influenza A and HIV and discusses potential interactions between these pathogens and the complement system.

Mechanisms that Determine the Differential Stability of Stx⁺ and Stx(-) Lysogens

Phages 933W, BAA2326, 434, and λ are evolutionarily-related temperate lambdoid phages that infect Escherichia coli. Although these are highly-similar phages, BAA2326 and 933W naturally encode Shiga toxin 2 (Stx⁺), but phage 434 and λ do not (Stx⁻). Previous reports suggest that the 933W Stx⁺ prophage forms less stable lysogens in E. coli than does the Stx⁻ prophages λ, P22, and 434. The higher spontaneous induction frequency of the Stx⁺ prophage may be correlated with both virulence and dispersion of the Stx2-encoding phage. Here, we examined the hypothesis that lysogen instability is a common feature of Stx⁺ prophages. We found in both the absence and presence of prophage inducers (DNA damaging agents, salts), the Stx⁺ prophages induce at higher frequencies than do Stx⁻ prophages. The observed instability of Stx⁺ prophages does not appear to be the result of any differences in phage development properties between Stx⁺ and Stx⁻ phages. Our results indicate that differential stability of Stx⁺ and Stx⁻ prophages results from both RecA-dependent and RecA-independent effects on the intracellular concentration of the respective cI repressors.

Detection of Shiga Toxin-Producing Escherichia coli from Nonhuman Sources and Strain Typing

Shiga toxin-producing Escherichia coli (STEC) strains are commonly found in the intestine of ruminant species of wild and domestic animals. Excretion of STEC with animal feces results in a broad contamination of food and the environment. Humans get infected with STEC through ingestion of contaminated food, by contact with the environment, and from STEC-excreting animals and humans. STEC strains can behave as human pathogens, and some of them, called enterohemorrhagic E. coli (EHEC), may cause hemorrhagic colitis (HC) and hemolytic-uremic syndrome (HUS). Because of the diversity of STEC types, detection strategies for STEC and EHEC are based on the identification of Shiga toxins or the underlying genes. Cultural enrichment of STEC from test samples is needed for identification, and different protocols were developed for this purpose. Multiplex real-time PCR protocols (ISO/CEN TS13136 and USDA/FSIS MLG5B.01) have been developed to specifically identify EHEC by targeting the LEE (locus of enterocyte effacement)-encoded eae gene and genes for EHEC-associated O groups. The employment of more genetic markers (nle and CRISPR) is a future challenge for better identification of EHEC from any kinds of samples. The isolation of STEC or EHEC from a sample is required for confirmation, and different cultivation protocols and media for this purpose have been developed. Most STEC strains present in food, animals, and the environment are eae negative, but some of these strains can cause HC and HUS in humans as well. Phenotypic assays and molecular tools for typing EHEC and STEC strains are used to detect and characterize human pathogenic strains among members of the STEC group.

The Shiga toxin 2 production level in enterohemorrhagic Escherichia coli O157:H7 is correlated with the subtypes of toxin-encoding phage

Enterohemorrhagic E. coli (EHEC) causes diarrhea and hemorrhagic colitis with life-threatening complications, such as hemolytic uremic syndrome. Their major virulence factor is Shiga toxin (Stx), which is encoded by bacteriophages. Of the two types of Stx, the production of Stx2, particularly that of Stx2a (a subtype of Stx2), is a major risk factor for severe EHEC infections, but the Stx2 production level is highly variable between strains. Here, we define four major and two minor subtypes of Stx2a-encoding phages according to their replication proteins. The subtypes are correlated with Stx2a titers produced by the host O157 strains, suggesting a critical role of the phage subtype in determining the Stx2a production level. We further show that one of the two subclades in the clade 8, a proposed hyper-virulent lineage of O157, carries the Stx2 phage subtype that confers the highest Stx2 production to the host strain. The presence of this subclade may explain the proposed high virulence potential of clade 8. These results provide novel insights into the variation in virulence among O157 strains and highlight the role of phage variation in determining the production level of the virulence factors that phages encode.

An in vitro combined antibiotic-antibody treatment eliminates toxicity from Shiga toxin-producing Escherichia coli

Treating Shiga toxin-producing Escherichia coli (STEC) gastrointestinal infections is difficult. The utility of antibiotics for STEC treatment is controversial, since antibiotic resistance among STEC isolates is widespread and certain antibiotics dramatically increase the expression of Shiga toxins (Stxs), which are some of the most important virulence factors in STEC. Stxs contribute to life-threatening hemolytic uremic syndrome (HUS), which develops in considerable proportions of patients with STEC infections. Understanding the antibiotic resistance profiles of STEC isolates and the Stx induction potential of promising antibiotics is essential for evaluating any antibiotic treatment of STEC. In this study, 42 O157:H7 or non-O157 STEC isolates (including the "big six" serotypes) were evaluated for their resistance against 22 antibiotics by using an antibiotic array. Tigecycline inhibited the growth of all of the tested STEC isolates and also inhibited the production of Stxs (Stx2 in particular). In combination with neutralizing antibodies to Stx1 and Stx2, the tigecycline-antibody treatment fully protected Vero cells from Stx toxicity, even when the STEC bacteria and the Vero cells were cultured together. The combination of an antibiotic such as tigecycline with neutralizing antibodies presents a promising strategy for future STEC treatments.

An evolutionary analysis of nitric oxide reductase gene norV in enterohemorrhagic Escherichia coli O157

A novel virulence gene, norV, that encodes nitric oxide (NO) reductase, was examined to investigate the emergence of enterohemorrhagic Escherichia coli (EHEC) O157 subgroup C clusters 2 and 3 from subgroup C cluster 1. Deletion of norV occurred at a point between cluster 1 and cluster 2 just after or at the same time that an stx2 bacteriophage, which retains Shiga toxin 2 gene, was inserted into wrbA, which encodes a novel multimeric flavodoxin-like protein, in EHEC O157. Sensitivity of NO to anaerobic growth was correlated with the deletion of norV in all EHEC O157 individuals tested. The C467A mutation of fimH, which encodes minor component of type 1 fimbriae, occurred within cluster 1, not as a transition from cluster 1 to cluster 2, indicating that there is a cluster 1 minority branch that leads to cluster 2. These data refine the evolutionary history of an emerging EHEC O157.

Shiga Toxin Producing Escherichia coli

Shiga toxin-producing Escherichia coli (STEC) is among the common causes of foodborne gastroenteritis. STEC is defined by the production of specific toxins, but within this pathotype there is a diverse group of organisms. This diversity has important consequences for understanding the pathogenesis of the organism, as well as for selecting the optimum strategy for diagnostic testing in the clinical laboratory. This review includes discussions of the mechanisms of pathogenesis, the range of manifestations of infection, and the several different methods of laboratory detection of Shiga toxin-producing E coli.

Approaches to treatment of emerging Shiga toxin-producing Escherichia coli infections highlighting the O104:H4 serotype

Shiga toxin-producing Escherichia coli (STEC) are a group of diarrheagenic bacteria associated with foodborne outbreaks. Infection with these agents may result in grave sequelae that include fatality. A large number of STEC serotypes has been identified to date. E. coli serotype O104:H4 is an emerging pathogen responsible for a 2011 outbreak in Europe that resulted in over 4000 infections and 50 deaths. STEC pathogenicity is highly reliant on the production of one or more Shiga toxins that can inhibit protein synthesis in host cells resulting in a cytotoxicity that may affect various organ systems. Antimicrobials are usually avoided in the treatment of STEC infections since they are believed to induce bacterial cell lysis and the release of stored toxins. Some antimicrobials have also been reported to enhance toxin synthesis and production from these organisms. Various groups have attempted alternative treatment approaches including the administration of toxin-directed antibodies, toxin-adsorbing polymers, probiotic agents and natural remedies. The utility of antibiotics in treating STEC infections has also been reconsidered in recent years with certain modalities showing promise.

Commensal E. coli Stx2 lysogens produce high levels of phages after spontaneous prophage induction

Enterohemorrhagic E. coli (EHEC) is a food-borne pathogen that causes disease ranging from uncomplicated diarrhea to life-threatening hemolytic uremic syndrome (HUS) and nervous system complications. Shiga toxin 2 (Stx2) is the major virulence factor of EHEC and is critical for development of HUS. The genes encoding Stx2 are carried by lambdoid bacteriophages and the toxin production is tightly linked to the production of phages during lytic cycle. It has previously been suggested that commensal E. coli could amplify the production of Stx2-phages and contribute to the severity of disease. In this study we examined the susceptibility of commensal E. coli strains to the Stx2-converting phage ϕ734, isolated from a highly virulent EHEC O103:H25 (NIPH-11060424). Among 38 commensal E. coli strains from healthy children below 5 years, 15 were lysogenized by the ϕ734 phage, whereas lytic infection was not observed. Three of the commensal E. coli ϕ734 lysogens were tested for stability, and appeared stable and retained the phage for at least 10 cultural passages. When induced to enter lytic cycle by H2O2 treatment, 8 out of 13 commensal lysogens produced more ϕ734 phages than NIPH-11060424. Strikingly, five of them even spontaneously (non-induced) produced higher levels of phage than the H2O2 induced NIPH-11060424. An especially high frequency of HUS (60%) was seen among children infected by NIPH-11060424 during the outbreak in 2006. Based on our findings, a high Stx2 production by commensal E. coli lysogens cannot be ruled out as a contributor to the high frequency of HUS during this outbreak.

Impact of spontaneous prophage induction on the fitness of bacterial populations and host-microbe interactions

Bacteriophages and genetic elements, such as prophage-like elements, pathogenicity islands, and phage morons, make up a considerable amount of bacterial genomes. Their transfer and subsequent activity within the host's genetic circuitry have had a significant impact on bacterial evolution. In this review, we consider what underlying mechanisms might cause the spontaneous activity of lysogenic phages in single bacterial cells and how the spontaneous induction of prophages can lead to competitive advantages for and influence the lifestyle of bacterial populations or the virulence of pathogenic strains.

Effect of rifampicin and gentamicin on Shiga toxin 2 expression level and the SOS response in Escherichia coli O104:H4

BACKGROUND

A novel pathotype, Shiga toxin-producing Escherichia coli O104:H4, was the cause of a severe outbreak that affected European countries, mainly Germany, in 2011. The effect of different regimens of rifampicin and gentamicin were evaluated to determine possible treatment modes for the novel strain, and to evaluate the SOS response and its effect on toxin release.

MATERIALS AND METHODS

Pulsed-field gel electrophoresis (PFGE) was performed on the novel E. coli O104:H4 pathotype and two pre-outbreak E. coli O104:H4 CDC strains. Transcript levels of the stx2 and recA gene (SOS response inducer) were evaluated using quantitative real-time reverse transcriptase polymerase chain reaction (qRT-PCR) in the novel E. coli O104:H4 samples subjected to different regimens of rifampicin and gentamicin. Consequently, reverse passive latex agglutination (RPLA) was used to determine the Stx2 titers in these samples. Western blot was performed to determine the LexA levels (SOS response repressor) in E. coli O104:H4. The efficacy of treatment with antimicrobial agents was assessed in BALB/c mice.

RESULTS

The outbreak and pre-outbreak strains are closely related as shown by PFGE, which demonstrated slight genomic differences between the three strains. The transcription level of the stx2 gene in the new pathotype was 1.41- and 1.75-fold that of the 2009 EL-2050 and 2009 EL-2071 pre-outbreak strains, respectively. Moreover, the transcription level of the stx2 gene in the new pathotype was substantially decreased as a result of treatment with the different concentrations of the antimicrobial agents, but was enhanced when the antibiotics were administered at two subinhibitory levels. RPLA data were in accordance with the qRT-PCR results. E. coli O104:H4 exposed to gentamicin at both sub-minimum inhibitory concentration (MIC) levels led to high transcription levels of the recA gene and lack of expression of the LexA protein, implying that the SOS response was activated. Rifampicin at both sub-MIC levels resulted in low transcript levels of the recA gene, indicating that the SOS response was not induced. In vivo, the highest survival rate in BALB/c mice was observed in the group that was treated with the minimum bactericidal concentration (MBC) of gentamicin.

CONCLUSION

The use of antimicrobial agents in E. coli O104:H4 infection seems to be effective at the MIC and MBC levels. This provides a promising ground for treatment of E. coli O104:H4.

Probiotic Escherichia coli Nissle 1917 reduces growth, Shiga toxin expression, release and thus cytotoxicity of enterohemorrhagic Escherichia coli

Due to increased release or production of Shiga toxin by Enterohemorrhagic Escherichia coli (EHEC) after exposure to antimicrobial agents, the role of antimicrobial agents in EHEC mediated infections remains controversial. Probiotics are therefore rapidly gaining interest as an alternate therapeutic option. The well-known probiotic strain Escherichia coli Nissle 1917 (EcN) was tested in vitro to determine its probiotic effects on growth, Shiga toxin (Stx) gene expression, Stx amount and associated cytotoxicity on the most important EHEC strains of serotype O104:H4 and O157:H7. Following co-culture of EcN:EHEC in broth for 4 and 24 h, the probiotic effects on EHEC growth, toxin gene expression, Stx amount and cytotoxicity were determined using quantitative real time-PCR, Stx-ELISA and Vero cytotoxicity assays. Probiotic EcN strongly reduced EHEC numbers (cfu) of O104:H4 up to (68%) and O157:H7 to (72.2%) (p<0.05) in LB broth medium whereas the non-probiotic E. coli strain MG1655 had no effect on EHEC growth. The level of stx expression was significantly down-regulated, particularly for the stx2a gene. The stx down-regulation in EcN co-culture was not due to reduced numbers of EHEC. A significant inhibition in Stx amounts and cytotoxicity were also observed in sterile supernatants of EcN:EHEC co-cultures. These findings indicate that probiotic EcN displays strong inhibitory effects on growth, Shiga toxin gene expression, amount and cytotoxicity of EHEC strains. Thus, EcN may be considered as a putative therapeutic candidate, in particular against EHEC O104:H4 and O157:H7.

A high-throughput, precipitating colorimetric sandwich ELISA microarray for Shiga toxins

Shiga toxins 1 and 2 (Stx1 and Stx2) from Shiga toxin-producing E. coli (STEC) bacteria were simultaneously detected with a newly developed, high-throughput antibody microarray platform. The proteinaceous toxins were immobilized and sandwiched between biorecognition elements (monoclonal antibodies) and pooled horseradish peroxidase (HRP)-conjugated monoclonal antibodies. Following the reaction of HRP with the precipitating chromogenic substrate (metal enhanced 3,3-diaminobenzidine tetrahydrochloride or DAB), the formation of a colored product was quantitatively measured with an inexpensive flatbed page scanner. The colorimetric ELISA microarray was demonstrated to detect Stx1 and Stx2 at levels as low as ~4.5 ng/mL within ~2 h of total assay time with a narrow linear dynamic range of ~1-2 orders of magnitude and saturation levels well above background. Stx1 and/or Stx2 produced by various strains of STEC were also detected following the treatment of cultured cells with mitomycin C (a toxin-inducing antibiotic) and/or B-PER (a cell-disrupting, protein extraction reagent). Semi-quantitative detection of Shiga toxins was demonstrated to be sporadic among various STEC strains following incubation with mitomycin C; however, further reaction with B-PER generally resulted in the detection of or increased detection of Stx1, relative to Stx2, produced by STECs inoculated into either axenic broth culture or culture broth containing ground beef.

Shiga toxin production and translocation during microaerobic human colonic infection with Shiga toxin-producing E. coli O157:H7 and O104:H4

Haemolytic uraemic syndrome caused by Shiga toxin-producing E. coli (STEC) is dependent on release of Shiga toxins (Stxs) during intestinal infection and subsequent absorption into the bloodstream. An understanding of Stx-related events in the human gut is limited due to lack of suitable experimental models. In this study, we have used a vertical diffusion chamber system with polarized human colon carcinoma cells to simulate the microaerobic (MA) environment in the human intestine and investigate its influence on Stx release and translocation during STEC O157:H7 and O104:H4 infection. Stx2 was the major toxin type released during infection. Whereas microaerobiosis significantly reduced bacterial growth as well as Stx production and release into the medium, Stx translocation across the epithelial monolayer was enhanced under MA versus aerobic conditions. Increased Stx transport was dependent on STEC infection and occurred via a transcellular pathway other than macropinocytosis. While MA conditions had a similar general effect on Stx release and absorption during infection with STEC O157:H7 and O104:H4, both serotypes showed considerable differences in colonization, Stx production, and Stx translocation which suggest alternative virulence strategies. Taken together, our study suggests that the MA environment in the human colon may modulate Stx-related events and enhance Stx absorption during STEC infection.

[Studies on the mode of action of bacterial AB5 toxins]

Bacterial AB5 toxins are proteins, produced by pathogenic bacteria including of Vibrio cholerae, Shigella dysenteriae, and enterohaemorrhagic Escherichia coli, which are usually released into the extracellular medium and cause disease by killing or altering the metabolism of target eukaryotic cells. The toxins are usually composed of one A subunit (a toxic domain) and five B subunits (a receptor-binding domain). This article overviews the characteristics and mode of actions of AB5 toxins including cholera toxin, Shiga-like toxin, and subtilase cytotoxin, and highlights current topics related to the roles of the effectors in promoting bacterial infection.