The new allelic variant of the subtilase cytotoxin (subAB2) is common among Shiga toxin-producing Escherichia coli strains from large game animals and their meat and meat products

Pathogenicity of Shiga toxin-producing Escherichia coli (STEC) from wildlife: Should we care?

Shiga toxin-producing Escherichia coli (STEC) is one of the most frequent bacterial agents associated with food-borne outbreaks in Europe. In humans, the infection can lead to life-threatening diseases. Domestic and wild animals can harbor STEC, and ruminants are the main STEC reservoirs, although asymptomatic. In the present study we have characterized STEC from wildlife (wild boar (n = 56), red deer (n = 101), red fox (n = 37) and otter (n = 92)). Cultivable STEC (n = 52) were isolated from 17% (n = 49) of the faecal samples. All the isolates were non-O157 STEC encoding stx1 (n = 2; 4%) and/or stx2 genes (n = 51; 98%). Only one strain (2%) isolated from red fox had an antibiotic resistant phenotype. However, when the normalized resistance interpretation of epidemiological cutoffs (NRI ECOFFs) were used, 23% (n = 12) of the strains were non-wildtype to at least one of the antibiotics tested. After analysis by pulsed-field gel electrophoresis (PFGE), 20 strains were selected for whole genome sequencing and belonged to the following serotypes: O27:H30 (n = 15), O146:H28 (n = 2), O146:H21 (n = 1), O178:H19 (n = 1), and O103:H2 (n = 1). In addition to stx, all strains encode several virulence factors such as toxins, adhesins, fimbriae and secretion systems, among others. All sequenced genomes carried several mobile genetic elements (MGEs), such as prophages and/or plasmids. The core genome and the phylogenetic analysis showed close evolutionary relationships between some of the STEC recovered from wildlife and strains of clinical origin, highlighting their pathogenic potential. Overall, our results show the zoonotic potential of STEC strains originating from wildlife, highlighting the importance of monitoring their genomic characteristics following a One Health perspective, in which the health of humans is related to the health of animals, and the environment.

The enzyme subunit SubA of Shiga toxin-producing E. coli strains demonstrates comparable intracellular transport and cytotoxic activity as the holotoxin SubAB in HeLa and HCT116 cells in vitro

The subtilase cytotoxin (SubAB) is secreted by certain Shiga toxin-producing Escherichia coli (STEC) strains and is composed of the enzymatically active subunit SubA and the pentameric binding/transport subunit SubB. We previously demonstrated that SubA (10 µg/ml), in the absence of SubB, binds and intoxicates the human cervix cancer-derived epithelial cell line HeLa. However, the cellular and molecular mechanisms underlying the cytotoxic activity of SubA in the absence of SubB remained unclear. In the present study, the cytotoxic effects mediated by SubA alone were investigated in more detail in HeLa cells and the human colon cancer cell line HCT116. We found that in the absence of SubB, SubA (10 µg/ml) is internalized into the endoplasmic reticulum (ER), where it cleaves the chaperone GRP78, an already known substrate for SubA after its canonical uptake into cells via SubB. The autonomous cellular uptake of SubA and subsequent cleavage of GRP78 in cells is prevented by treatment of cells with 10 µM brefeldin A, which inhibits the transport of protein toxins into the ER. In addition, by analyzing the SubA mutant SubA_ΔC344, we identified the C-terminal SEEL motif as an ER-targeting signal. Conclusively, our results strongly suggest that SubA alone shares the same intracellular transport route and cytotoxic activity as the SubAB holotoxin.

Occurrence and characterization of stx and/or eae-positive Escherichia coli isolated from wildlife, including a typical EPEC strain from a wild boar

Shiga toxin-producing E. coli (STEC) and enteropathogenic E. coli (EPEC) strains are food-borne pathogens associated with acute diarrhea. Haemolytic-uremic syndrome (HUS) is often a complication of STEC infection. In order to examine the occurrence, serotypes, virulence and antimicrobial-resistance profiles of STEC and EPEC in wildlife, 326 faecal E. coli strains from 304 clinically healthy animals were analyzed. For this approach stx₁, stx₂ and eae genes, as well as accessory virulence determinants (ehx, hlyA, saa, tia, bfp, subAB) were PCR-screened and sequenced. Serotyping was performed employing all available O (O1-O185) and H (H1-H56) antisera. Genetic diversity was analyzed by XbaI-PFGE and phylotyping. Thirteen STEC (4.3%) and 10 EPEC (3.3%) were identified among 12 deer, 3 mouflon, 6 wild boars and 2 birds. Nine STEC showed seropathotypes B (O145:[H28]) and C (O22:H8, O128:[H2]) associated with HUS, and D (O110:H28, O146:H21, O146:[H28], ONT:H8) associated with human diarrhea. Although most isolates harbored stx_2b and stx_1c variants, stx_2a and stx_1a (related with severe disease) were also detected. Additionally, the eae gene was present in one stx_2a-positive O145:[H28] STEC from a deer and 11 STEC harbored subAB genes (mainly the subAB₂ variant). EPEC isolates showed 7 different intimin variants (β1, β2, γ1, ε1, ζ1, ι1-A, κ). Interestingly, the O49:[H10] eae-κ EPEC isolated from a wild boar was bfpA-positive showing a combination of serotype/virulence profile previously detected among human clinical tEPEC. Based on present results, wild ruminants, wild boars and to a lesser extent birds would be carriers of potentially pathogenic STEC and EPEC strains.

Genotypic study of verocytotoxic Escherichia coli isolates from deer by multiplex polymerase chain reaction

AIM

This study was planned to study the genotypes of verocytotoxigenic Escherichia coli (VTEC) in fecal samples of deer due to its public health significance.

MATERIALS AND METHODS

A total of 160 fecal samples of deer were taken from Mathura district and Kanpur Zoo and screened for VTEC genes by polymerase chain reaction (PCR).

RESULTS

All fecal samples were positive for E. coli. All the E. coli isolates were screened by PCR to detect virulence genes stx1 , stx2 , eaeA, and hlyA. Of these, 15 isolates were found positive for VTEC having one or more genes in different combinations.

CONCLUSION

Genes such as stx1 , stx2 , eaeA, and hlyA were prevalent in VTEC isolates from feces of deer. The presence of VTEC isolates having virulent genes may pose a threat to public health.

Prevalence of vero toxic Escherichia coli in fecal samples of domestic as well as wild ruminants in Mathura districts and Kanpur zoo

AIM

The present study was planned to reveal the prevalence of verocytotoxigenic Escherichia coli (VTEC) in fecal samples of domestic and wild ruminants in Mathura district and Kanpur zoo.

MATERIALS AND METHODS

A total of 240 fecal samples comprising 60 each of cattle, buffalo, sheep and deer from Mathura districts and Kanpur zoo were screened for the presence of E. coli and VTEC genes positive by polymerase chain reaction (PCR).

RESULT

Out of 240 fecal samples, 212 E. coli strains were obtained. All the E. coli isolates were screened by PCR to detect virulence genes stx1 , stx2 , eaeA and hlyA. Of these, 25 isolates were identified as VTEC. The prevalence of VTEC in cattle, buffalo, sheep and deer was found 13.4% (8/60), 13.4% (8/60), 6.67% (4/60) and 8.33% (5/60), respectively.

CONCLUSION

stx1 , stx2 , eaeA and hlyA genes were prevalent in VTEC isolates from feces of cattle, buffalo, sheep and deer population of Mathura districts and Kanpur zoo. The presence of VTEC isolates in this region may pose a threat to public health.

Identification of the newly identified subtilase cytotoxin-encoding gene (subAB2-2) among clinical Shiga toxin-producing Escherichia coli isolates

Subtilase cytotoxin (SubAB) is an important virulence factor of eae-negative Shiga toxin-producing Escherichia coli (STEC). Three variants of SubAB-encoding genes have been reported in the literature; however, the newly described subAB variant (subAB2-2) was found only in STEC strains from deer meat, sheep, and some wild animals. In this study, subAB variants were detected by PCR and DNA sequencing in 5 out of 12 (41.6%) eae-negative STEC strains isolated from patients. Most subAB-positive STEC strains (80%) harbored the subAB1 gene. The subAB2-2 gene was detected for the first time in the clinical STEC O128:H2 strain. Other virulence genes including stx1a, stx1c, stx2b, ehxA, and tia were also detected in this strain. The DNA sequence analyses of the subAB1 and subAB2-2 genes of the clinical STEC strains showed 99% and 100% identity to those of the reference strains 98NK2 and LM27558stx2, respectively. This is the first report on the detection of the subAB2-2 gene in a clinical STEC isolate.

Cytotoxic and apoptotic effects of recombinant subtilase cytotoxin variants of shiga toxin-producing Escherichia coli

In this study, the cytotoxicity of the recently described subtilase variant SubAB2-2 of Shiga toxin-producing Escherichia coli was determined and compared to the plasmid-encoded SubAB1 and the chromosome-encoded SubAB2-1 variant. The genes for the respective enzymatic active (A) subunits and binding (B) subunits of the subtilase toxins were amplified and cloned. The recombinant toxin subunits were expressed and purified. Their cytotoxicity on Vero cells was measured for the single A and B subunits, as well as for mixtures of both, to analyze whether hybrids with toxic activity can be identified. The results demonstrated that all three SubAB variants are toxic for Vero cells. However, the values for the 50% cytotoxic dose (CD50) differ for the individual variants. Highest cytotoxicity was shown for SubAB1. Moreover, hybrids of subunits from different subtilase toxins can be obtained which cause substantial cytotoxicity to Vero cells after mixing the A and B subunits prior to application to the cells, which is characteristic for binary toxins. Furthermore, higher concentrations of the enzymatic subunit SubA1 exhibited cytotoxic effects in the absence of the respective B1 subunit. A more detailed investigation in the human HeLa cell line revealed that SubA1 alone induced apoptosis, while the B1 subunit alone did not induce cell death.

Molecular analysis of subtilase cytotoxin genes of food-borne Shiga toxin-producing Escherichia coli reveals a new allelic subAB variant

Background

The open reading frames of subAB genes and their flanking regions of 18 food-borne Shiga toxin-producing E. coli (STEC) strains were analyzed.

Results

All but one subAB open reading frames (ORF) were complete in all STEC strains. The subAB₁ genes of nine STEC strains were located on large plasmids. The subAB₂ allele (here designated subAB_2-1), which was recently described by others to be present in the Subtilase-Encoding PAI (SE-PAI) was found in 6 STEC strains. A new chromosomal subAB₂ variant, designated subAB_2-2 was detected in 6 strains and was linked to a chromosomal gene hypothetically encoding an outer membrane efflux protein (OEP). Three STEC strains contained both subAB₂ variants. DNA analysis indicated sequence conservation in the plasmid-located alleles and sequence heterogeneity among the chromosomal subAB₂ genes.

Conclusions

The results of this study have shown that 18 subAB-PCR positive STEC strains contain complete subAB open reading frames. Furthermore, the new allelic variant subAB_2-2 was described, which can occur in addition to subAB_2-1 on a new chromosomal locus.