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1.  Serotypes, virulence genes and intimin types of Shiga toxin (verocytotoxin)-producing Escherichia coli isolates from minced beef in Lugo (Spain) from 1995 through 2003 
BMC Microbiology  2007;7:13.
Shiga toxin-producing Escherichia coli (STEC) have emerged as pathogens that can cause food-borne infections and severe and potentially fatal illnesses in humans, such as haemorrhagic colitis (HC) and haemolytic uraemic syndrome (HUS). In Spain, like in many other countries, STEC strains have been frequently isolated from ruminants, and represent a significant cause of sporadic cases of human infection. In view of the lack of data on STEC isolated from food in Spain, the objectives of this study were to determine the level of microbiological contamination and the prevalence of STEC O157:H7 and non-O157 in a large sampling of minced beef collected from 30 local stores in Lugo city between 1995 and 2003. Also to establish if those STEC isolated from food possessed the same virulence profiles as STEC strains causing human infections.
STEC were detected in 95 (12%) of the 785 minced beef samples tested. STEC O157:H7 was isolated from eight (1.0%) samples and non-O157 STEC from 90 (11%) samples. Ninety-six STEC isolates were further characterized by PCR and serotyping. PCR showed that 28 (29%) isolates carried stx1 genes, 49 (51%) possessed stx2 genes, and 19 (20%) both stx1 and stx2. Enterohemolysin (ehxA) and intimin (eae) virulence genes were detected in 43 (45%) and in 25 (26%) of the isolates, respectively. Typing of the eae variants detected four types: γ1 (nine isolates), β1 (eight isolates), ε1 (three isolates), and θ (two isolates). The majority (68%) of STEC isolates belonged to serotypes previously detected in human STEC and 38% to serotypes associated with STEC isolated from patients with HUS. Ten new serotypes not previously described in raw beef products were also detected. The highly virulent seropathotypes O26:H11 stx1 eae-β1, O157:H7 stx1stx2 eae-γ1 and O157:H7 stx2eae-γ1, which are the most frequently observed among STEC causing human infections in Spain, were detected in 10 of the 96 STEC isolates. Furthermore, phage typing of STEC O157:H7 isolates showed that the majority (seven of eight isolates) belonged to the main phage types previously detected in STEC O157:H7 strains associated with severe human illnesses.
The results of this study do not differ greatly from those reported in other countries with regard to prevalence of O157 and non-O157 STEC in minced beef. As we suspected, serotypes different from O157:H7 also play an important role in food contamination in Spain, including the highly virulent seropathotype O26:H11 stx1 eae-β1. Thus, our data confirm minced beef in the city of Lugo as vehicles of highly pathogenic STEC. This requires that control measures to be introduced and implemented to increase the safety of minced beef.
PMCID: PMC1810539  PMID: 17331254
2.  Loop-Mediated Isothermal Amplification Assays for Detecting Shiga Toxin-Producing Escherichia coli in Ground Beef and Human Stools 
Shiga toxin-producing Escherichia coli (STEC), encompassing E. coli O157 and non-O157 STEC, is a significant cause of food-borne illnesses and deaths in the United States and worldwide. Shiga toxins (encoded by stx) and intimin (encoded by eae) are important virulence factors for STEC strains linked to severe human illnesses such as hemorrhagic colitis and hemolytic-uremic syndrome. In this study, the stx1, stx2, and eae genes were chosen as targets to design loop-mediated isothermal amplification (LAMP) assays for the rapid, specific, sensitive, and quantitative detection of STEC strains. The assay performances in pure culture and spiked ground beef and human stools were evaluated and compared with those of quantitative PCR (qPCR). No false-positive or false-negative results were observed among 90 bacterial strains used to evaluate assay specificity. The limits of detection for seven STEC strains of various serogroups (O26, O45, O103, O111, O121, O145, and O157) were approximately 1 to 20 CFU/reaction in pure culture and 103 to 104 CFU/g in spiked ground beef, which were comparable to the results of qPCR. Standard curves generated suggested good linear relationships between STEC cell numbers and LAMP turbidity signals. When applied in ground beef samples spiked with two low levels (1 to 2 and 10 to 20 CFU/25 g) of STEC cultures, the LAMP assays achieved accurate detection after 6 to 8 h enrichment. The assays also consistently detected STEC in human stool specimens spiked with 103 or 104 CFU/0.5 g stool after 4 h enrichment, while qPCR required 4 to 6 h. In conclusion, the LAMP assays developed in this study may facilitate rapid and reliable identification of STEC contaminations in high-risk food commodities and also facilitate prompt diagnosis of STEC infections in clinical laboratories.
PMCID: PMC3256711  PMID: 22031701
3.  Detection of Shiga toxin-producing Escherichia coli (STEC) O157:H7, O26, O45, O103, O111, O121, and O145, and Salmonella in retail raw ground beef using the DuPont™ BAX® system 
Shiga toxin-producing Escherichia coli (STEC) and Salmonella are food-borne pathogens commonly associated with beef, and reliable methods are needed to determine their prevalence in beef and to ensure food safety. Retail ground beef was tested for the presence of E. coli O157:H7, STEC serogroups O26, O45, O103, O111, O121, and O145, and Salmonella using the DuPont™ BAX® system method. Ground beef (325 g) samples were enriched in 1.5 L of TSB with 2 mg/L novobiocin at 42°C for 18 h, and then evaluated using the BAX® System real-time PCR assays for E. coli O157:H7 and STEC suite, and the BAX® System standard PCR assays for E. coli O157:H7 MP and Salmonella. Samples positive for STEC target genes by the BAX® System assays were subjected to immunomagnetic separation (IMS) and plating onto modified Rainbow Agar O157. Enrichments that were PCR positive for Salmonella were inoculated into RV broth, incubated for 18 h at 42°C, and then plated onto XLT-4 agar. Presumptive positive STEC and Salmonella colonies were confirmed using the BAX® System assays. Results of the BAX® System STEC assays showed 20/308 (6.5%) of samples positive for both the Shiga toxin (stx) and intimin (eae) genes; 4 (1.3%) for stx, eae, and O26; 1 (0.3%) for stx, eae, and O45; 3 (1%) for stx, eae, and O103; and 1 (0.3%) for stx, eae, and O145. There were also 3 samples positive for stx, eae, and more than one STEC serogroup. Three (1.0%) of the samples were positive using the BAX® System real-time E. coli O157:H7 assay, and 28 (9.1%) were positive using the BAX® System Salmonella assay. STEC O103 and E. coli O157:H7 were isolated from 2/6 and 2/3 PCR positive samples, respectively. Salmonella isolates were recovered and confirmed from 27 of the 28 Salmonella PCR positive samples, and a portion of the isolates were serotyped and antibiotic resistance profiles determined. Results demonstrate that the BAX® System assays are effective for detecting STEC and Salmonella in beef.
PMCID: PMC4061970  PMID: 24995164
Shiga toxin-producing E. coli; Salmonella; ground beef; detection; PCR; O157:H7; non-O157 STEC
4.  Evaluation of CHROMagar STEC and STEC O104 Chromogenic Agar Media for Detection of Shiga Toxin-Producing Escherichia coli in Stool Specimens 
Journal of Clinical Microbiology  2013;51(3):894-900.
The performance of CHROMagar STEC and CHROMagar STEC O104 (CHROMagar Microbiology, Paris, France) media for the detection of Shiga toxin-producing Escherichia coli (STEC) was assessed with 329 stool specimens collected over 14 months from patients with suspected STEC infections (June 2011 to August 2012). The CHROMagar STEC medium, after an enrichment broth step, allowed the recovery of the STEC strain from 32 of the 39 (82.1%) Shiga toxin-positive stool specimens, whereas the standard procedure involving Drigalski agar allowed the recovery of only three additional STEC strains. The isolates that grew on CHROMagar STEC medium belonged to 15 serotypes, including the prevalent non-sorbitol-fermenting (NSF) O157:H7, O26:H11, and O104:H4 serotypes. The sensitivity, specificity, and positive and negative predictive values for the CHROMagar STEC medium were between 89.1% and 91.4%, 83.7% and 86.7%, 40% and 51.3%, and 98% and 98.8%, respectively, depending on whether or not stx-negative eae-positive E. coli was considered atypical enteropathogenic E. coli (EPEC) or STEC that had lost Shiga toxin genes during infection. In conclusion, the good performance of CHROMagar STEC agar medium, in particular, the high negative predictive value, and its capacity to identify NSF O157:H7 as well as common non-O157 STEC may be useful for clinical bacteriology, public health, and reference laboratories; it could be used in addition to a method targeting Shiga toxins (detection of stx genes by PCR, immunodetection of Shiga toxins in stool specimens, or Vero cell cytotoxicity assay) as an alternative to O157 culture medium. This combined approach should allow rapid visualization of both putative O157 and non-O157 STEC colonies for subsequent characterization, essential for real-time surveillance of STEC infections and investigations of outbreaks.
PMCID: PMC3592037  PMID: 23284030
5.  Detection of Shiga Toxin-Producing Escherichia coli Serotypes O26:H11, O103:H2, O111:H8, O145:H28, and O157:H7 in Raw-Milk Cheeses by Using Multiplex Real-Time PCR ▿  
Shiga toxin (Stx)-producing Escherichia coli (STEC) strains are a diverse group of food-borne pathogens with various levels of virulence for humans. In this study, we describe the use of a combination of multiple real-time PCR assays for the screening of 400 raw-milk cheeses for the five main pathogenic STEC serotypes (O26:H11, O103:H2, O111:H8, O145:H28, and O157:H7). The prevalences of samples positive for stx, intimin-encoding gene (eae), and at least one of the five O group genetic markers were 29.8%, 37.3%, and 55.3%, respectively. The H2, H7, H8, H11, and H28 fliC alleles were highly prevalent and could not be used as reliable targets for screening. Combinations of stx, eae variants, and O genetic markers, which are typical of the five targeted STEC serotypes, were detected by real-time PCR in 6.5% of the cheeses (26 samples) and included stx-wzxO26-eae-β1 (4.8%; 19 samples), stx-wzxO103-eae-ɛ (1.3%; five samples), stx-ihp1O145-eae-γ1 (0.8%; three samples), and stx-rfbEO157-eae-γ1 (0.3%; one sample). Twenty-eight immunomagnetic separation (IMS) assays performed on samples positive for these combinations allowed the recovery of seven eaeβ1-positive STEC O26:H11 isolates, whereas no STEC O103:H2, O145:H28, or O157:H7 strains could be isolated. Three stx-negative and eaeβ1-positive E. coli O26:[H11] strains were also isolated from cheeses by IMS. Colony hybridization allowed us to recover STEC from stx-positive samples for 15 out of 45 assays performed, highlighting the difficulties encountered in STEC isolation from dairy products. The STEC O26:H11 isolates shared the same virulence genetic profile as enterohemorrhagic E. coli (EHEC) O26:H11, i.e., they carried the virulence-associated genes EHEC-hlyA, katP, and espP, as well as genomic O islands 71 and 122. Except for one strain, they all contained the stx1 variant only, which was reported to be less frequently associated with human cases than stx2. Pulsed-field gel electrophoresis (PFGE) analysis showed that they displayed high genetic diversity; none of them had patterns identical to those of human O26:H11 strains investigated here.
PMCID: PMC3067316  PMID: 21239543
6.  Shiga toxin-producing Escherichia coli strains from bovines: association of adhesion with carriage of eae and other genes. 
Journal of Clinical Microbiology  1996;34(12):2980-2984.
Out of 174 bovine Shiga toxin-producing Escherichia coli (STEC) strains isolated from diarrheic calves in Germany and Belgium, 122 strains (70.1%) were selected because of their reactivity with the eae (E. coli attaching and effacing gene) probe ECW1-ECW2. One hundred seven of these eae-positive strains (87.7%) harbored stx1 genes, 13 strains (10.7%) had stx2 genes, and 2 strains (1.6%) had both stx genes. The strains displayed 17 different O types, the majority (97 strains) [79.5%]) belonging to O5 (5 strains), O26 (21 strains), O111 (13 strains) O118 (36 strains), O145 (9 strains), and O157 (13 strains). In the HEp-2 cell adhesion assay, 99 strains (81.1%) showed a localized adhesion, and 80 strains (65.6%) stimulated actin accumulation, as determined in the fluorescence actin staining test. None of the strains harbored genes coding for bundle-forming pili (bfpA), clearly differentiating them from enteropathogenic. E. cole. espB gene sequences were only detectable in 23 (18.9%) of the eae-positive bovine STEC strains. Three different PCRs were established, differentiating between eae sequences of enteropathogenic E. coli strain E2348/69 (O127:H6) and STEC strain EDL933 (O157: H7). Primers matching in the more heterologous downstream eae sequences gave amplicons in only 8 of the 17 O types (O84:H-, O103:H2, O111:H-, O111:H2, O119:H25, O128:H-, O145:H28, and O157:H-). Only 15 STEC strains, belonging to serotypes O111H:-, O111H:2, O145:H28, and O157:H-, gave amplicons in all three eae-specific PCRs. These data demonstrate that bovine STEC strains are a heterogeneous group of pathogenic bacteria, a lot of which share virulence markers with STEC strains causing infections in humans. However, in contrast to human STEC strains, bovine eae-positive STEC strains are mainly restricted to the stx1 genotype. The observation that espB sequences are not highly conserved might have consequences for the serological recognition of the ESPB protein in patients. Like in human STEC strains, eae-related sequences are closely associated with certain E. coli O groups; however, they are not serotype specific.
PMCID: PMC229445  PMID: 8940434
7.  Occurrence of Diarrheagenic Virulence Genes and Genetic Diversity in Escherichia coli Isolates from Fecal Material of Various Avian Hosts in British Columbia, Canada 
Contamination of surface water by fecal microorganisms originating from human and nonhuman sources is a public health concern. In the present study, Escherichia coli isolates (n = 412) from the feces of various avian host sources were screened for various virulence genes: stx1 and stx2 (Shiga toxin-producing E. coli [STEC]), eae (enteropathogenic E. coli [EPEC]), est-h, est-p, and elt (encoding heat-stable toxin [ST] variants STh and STp and heat-labile toxin [LT], respectively) (enterotoxigenic E. coli [ETEC]), and ipaH (enteroinvasive E. coli [EIEC]). None of the isolates were found to be positive for stx1, while 23% (n = 93) were positive for only stx2, representing STEC, and 15% (n = 63) were positive for only eae, representing EPEC. In addition, five strains obtained from pheasant were positive for both stx2 and eae and were confirmed as non-O157 by using an E. coli O157 rfb (rfbO157) TaqMan assay. Isolates positive for the virulence genes associated with ETEC and EIEC were not detected in any of the hosts. The repetitive element palindromic PCR (rep-PCR) fingerprint analysis identified 143 unique fingerprints, with an overall Shannon diversity index of 2.36. Multivariate analysis of variance (MANOVA) showed that the majority of the STEC and EPEC isolates were genotypically distinct from nonpathogenic E. coli and clustered independently. MANOVA analysis also revealed spatial variation among the E. coli isolates, since the majority of the isolates clustered according to the sampling locations. Although the presence of virulence genes alone cannot be used to determine the pathogenicity of strains, results from this study show that potentially pathogenic STEC and EPEC strains can be found in some of the avian hosts studied and may contaminate surface water and potentially impact human health.
PMCID: PMC3957651  PMID: 24441159
8.  Detection of Shiga toxin-producing Escherichia coli in ground beef using the GeneDisc real-time PCR system 
Escherichia coli O157:H7 and certain non-O157 Shiga toxin-producing Escherichia coli (STEC) serogroups have emerged as important public health threats. The development of methods for rapid and reliable detection of this heterogeneous group of pathogens has been challenging. GeneDisc real-time PCR assays were evaluated for detection of the stx1, stx2, eae, and ehxA genes and a gene that identifies the O157 serogroup followed by a second GeneDisc assay targeting serogroup-specific genes of STEC O26, O45, O91, O103, O111, O113, O121, O145, and O157. The ability to detect the STEC serogroups in ground beef samples artificially inoculated at a level of ca. 2–20 CFU/25 g and subjected to enrichment in mTSB or buffered peptone water (BPW) was similar. Following enrichment, all inoculated ground beef samples showed amplification of the correct set of target genes carried by each strain. Samples inoculated with STEC serogroups O26, O45, O103, O111, O121, O145, and O157 were subjected to immunomagnetic separation (IMS), and isolation was achieved by plating onto Rainbow agar O157. Colonies were confirmed by PCR assays targeting stx1, stx2, eae, and serogroup-specific genes. Thus, this work demonstrated that GeneDisc assays are rapid, sensitive, and reliable and can be used for screening ground beef and potentially other foods for STEC serogroups that are important food-borne pathogens worldwide.
PMCID: PMC3526733  PMID: 23267438
GeneDisc; real-time PCR; STEC; virulence genes; O-group; detection; isolation; E. coli O157:H7
9.  Evaluation of Performance and Potential Clinical Impact of ProSpecT Shiga Toxin Escherichia coli Microplate Assay for Detection of Shiga Toxin-Producing E. coli in Stool Samples 
Journal of Clinical Microbiology  2004;42(4):1652-1656.
Shiga toxin-producing Escherichia coli bacteria (STEC) are emerging pathogens capable of producing sporadic and epidemic diarrhea, hemorrhagic colitis, and potentially life-threatening hemolytic-uremic syndrome. Although the presence of E. coli O157 can be readily detected in stool by sorbitol-MacConkey agar culture (SMAC), STEC non-O157 serotypes cannot. In contrast to culture, testing for the presence of Shiga toxins 1 and 2 in stool detects both O157 and non-O157 STEC serotypes capable of causing disease. Over two consecutive summers, we evaluated the performance of the ProSpecT Shiga toxin E. coli Microplate assay (Alexon-Trend, Ramsey, Minn.), an enzyme immunoassay for the detection of Shiga toxins 1 and 2, on all stools submitted for culture of enteric pathogens, and the potential clinical impact of Shiga toxin detection. Twenty-nine stool specimens were STEC positive by ProSpecT assay. Twenty-seven of 29 STEC-positive isolates were confirmed by SMAC and serotyping or by a second enzyme immunoassay and PCR (positive predictive value, 93%). Thirteen of 27 confirmed Shiga toxin-producing strains were serotype O157. The remaining 14 strains represented 8 other serotypes. The ProSpecT assay was 100% sensitive and specific for detection of E. coli O157 in stool (7 of 7) compared to SMAC. In addition, the ProSpecT assay detected twice as many STEC as SMAC. Fifty-two percent of confirmed STEC-positive stools were nonbloody. Thus, in our population, screening strategies that test only visibly bloody stools for STEC would miss a majority of cases. Eleven (41%) STEC-positive patients were hospitalized, and eight (30%) developed severe disease (two developed hemolytic-uremic syndrome, and six developed hemorrhagic colitis). Prior to detection of STEC infection, seven (26%) and eight patients (30%) underwent unnecessary diagnostic procedures or received potentially deleterious empirical treatment, respectively. We propose that establishing a specific diagnosis of STEC may have prevented these potentially harmful interventions. We conclude that the ProSpecT assay is sensitive and specific for the detection of Shiga toxins 1 and 2 in stool and has potentially significant clinical impact for the individual patient and public health. Shiga toxin assays should be considered for routine use in settings where prevalence of STEC disease warrants testing.
PMCID: PMC387566  PMID: 15071021
10.  Rapid and Sensitive Detection of Shiga Toxin-Producing Escherichia coli from Nonenriched Stool Specimens by Real-Time PCR in Comparison to Enzyme Immunoassay and Culture▿  
Journal of Clinical Microbiology  2009;47(7):2008-2012.
Shiga toxin (Stx)-producing Escherichia coli (STEC) bacteria are a frequent cause of food-borne gastroenteritis, hemorrhagic colitis, and hemolytic uremic syndrome. Because antimicrobial agents are generally contraindicated in patients infected with STEC, a sensitive and specific diagnostic test with rapid turnaround is essential. Current culture methods may fail to detect non-O157 STEC. We evaluated a Stx gene real-time PCR assay using hybridization probes and the LightCycler instrument with 204 prospectively collected stool specimens, which were also tested for Stx by enzyme immunoassay (EIA) (ProSpecT STEC; Remel, Lenexa, KS) and by culturing on chromogenic agar (Chromagar O157; BD BBL, Sparks, MD). In addition, 85 archived stool specimens previously tested for Stx (by EIA) and/or E. coli O157:H7 (by culture) were tested by PCR. Sample preparation for PCR included mixing the stool in sterile water and extraction of nucleic acid using the MagNA Pure LC instrument (Roche Diagnostics). The PCR assay had 100% sensitivity and specificity compared to EIA and culture for specimens collected prospectively (4 of 204 specimens were positive) and compared to culture and/or EIA for archival specimens (42 of 85 specimens were positive). Both the EIA and PCR produced positive results from a specimen containing an O103 serotype STEC in the prospective specimens, and the PCR test detected three positive specimens that contained nonviable STEC in the archived specimens. The PCR assay demonstrated 100% sensitivity and specificity compared to EIA and/or culture and more rapid turnaround than either EIA or culture.
PMCID: PMC2708480  PMID: 19439539
11.  Serotypes, Virulence Genes, and Intimin Types of Shiga Toxin (Verotoxin)-Producing Escherichia coli Isolates from Human Patients: Prevalence in Lugo, Spain, from 1992 through 1999 
Journal of Clinical Microbiology  2004;42(1):311-319.
We have analyzed the prevalence of Shiga toxin-producing Escherichia coli (STEC) in stool specimens of patients with diarrhea or other gastrointestinal alterations from the Xeral-Calde Hospital of Lugo City (Spain). STEC strains were detected in 126 (2.5%) of 5,054 cases investigated, with a progressive increase in the incidence from 0% in 1992 to 4.4% in 1999. STEC O157:H7 was isolated in 24 cases (0.5%), whereas non-O157 STEC strains were isolated from 87 patients (1.7%). STEC strains were (after Salmonella and Campylobacter strains) the third most frequently recovered enteropathogenic bacteria. A total of 126 human STEC isolates were characterized in this study. PCR showed that 43 (34%) isolates carried stx1 genes, 45 (36%) possessed stx2 genes and 38 (30%) carried both stx1 and stx2. A total of 88 (70%) isolates carried an ehxA enterohemolysin gene, and 70 (56%) isolates possessed an eae intimin gene (27 isolates with type γ1, 20 with type β1, 8 with type ζ, 5 with type γ2, and 3 with type ɛ). STEC isolates belonged to 41 O serogroups and 66 O:H serotypes, including 21 serotypes associated with hemolytic uremic syndrome and 30 new serotypes not previously reported among human STEC strains in other studies. Although the 126 STEC isolates belonged to 81 different seropathotypes (associations between serotypes and virulence genes), only four accounted for 31% of isolates. Seropathotype O157:H7 stx1 stx2 eae-γ1 ehxA was the most common (13 isolates) followed by O157:H7 stx2 eae-γ1 ehxA (11 isolates), O26:H11 stx1 eae-β1 ehxA (11 isolates), and O111:H- stx1 stx2 eae-γ2 ehxA (4 isolates). Our results suggest that STEC strains are a significant cause of human infections in Spain and confirm that in continental Europe, infections caused by STEC non-O157 strains are more common than those caused by O157:H7 isolates. The high prevalence of STEC strains (both O157:H7 and non-O157 strains) in human patients, and their association with serious complications, strongly supports the utilization of protocols for detection of all serotypes of STEC in Spanish clinical microbiology laboratories.
PMCID: PMC321739  PMID: 14715771
12.  stx1c Is the Most Common Shiga Toxin 1 Subtype among Shiga Toxin-Producing Escherichia coli Isolates from Sheep but Not among Isolates from Cattle 
Journal of Clinical Microbiology  2003;41(3):926-936.
Unlike Shiga toxin 2 (stx2) genes, most nucleotide sequences of Shiga toxin 1 (stx1) genes from Shiga toxin-producing Escherichia coli (STEC), Shigella dysenteriae, and several bacteriophages (H19B, 933J, and H30) are highly conserved. Consequently, there has been little incentive to investigate variants of stx1 among STEC isolates derived from human or animal sources. However stx1OX3, originally identified in an OX3:H8 isolate from a healthy sheep in Germany, differs from other stx1 subtypes by 43 nucleotides, resulting in changes to 12 amino acid residues, and has been renamed stx1c. In this study we describe the development of a PCR-restriction fragment length polymorphism (RFLP) assay that distinguishes stx1c from other stx1 subtypes. The PCR-RFLP assay was used to study 378 stx1-containing STEC isolates. Of these, 207 were isolated from sheep, 104 from cattle, 45 from humans, 11 from meat, 5 from swine, 5 from unknown sources, and 1 from a cattle water trough. Three hundred fifty-five of the 378 isolates (93.9%) also possessed at least one other associated virulence gene (ehxA, eaeA, and/or stx2); the combination stx1, stx2, and ehxA was the most common (175 of 355 [49.3%]), and 90 of 355 (25.4%) isolates possessed eaeA. One hundred thirty-six of 207 (65.7%) ovine isolates possessed stx1c alone and belonged to 41 serotypes. Seventy-one of 136 (52.2%) comprised the common ovine serotypes O5:H−, O128:H2, and O123:H−. Fifty-two of 207 isolates (25.1%) possessed an stx1 subtype; 27 (51.9%) of these belonged to serotype O91:H−. Nineteen of 207 isolates (9.2%) contained both stx1c and stx1 subtypes, and 14 belonged to serotype O75:H8. In marked contrast, 97 of 104 (93.3%) bovine isolates comprising 44 serotypes possessed an stx1 subtype, 6 isolates possessed stx1c, and the remaining isolate possessed both stx1c and stx1 subtypes. Ten of 11 (91%) isolates cultured from meat in New Zealand possessed stx1c (serotypes O5:H−, O75:H8/H40, O81:H26, O88:H25, O104:H−/H7, O123:H−/H10, and O128:H2); most of these serotypes are commonly recovered from the feces of healthy sheep. Serotypes containing stx1 recovered from cattle rarely were the same as those isolated from sheep. Although an stx1c subtype was never associated with the typical enterohemorrhagic E. coli serogroups O26, O103, O111, O113, and O157, 13 human isolates possessed stx1c. Of these, six isolates with serotype O128:H2 (from patients with diarrhea), four O5:H− isolates (from patients with hemolytic-uremic syndrome), and three isolates with serotypes O123:H− (diarrhea), OX3:H8 (hemolytic-uremic syndrome), and O81:H6 (unknown health status) represent serotypes that are commonly isolated from sheep.
PMCID: PMC150265  PMID: 12624011
13.  Rapid Detection of Shiga Toxin-Producing Bacteria in Feces by Multiplex PCR with Molecular Beacons on the Smart Cycler 
Journal of Clinical Microbiology  2002;40(4):1436-1440.
We have developed a rapid (1-h) real-time fluorescence-based PCR assay with the Smart Cycler thermal cycler (Cepheid, Sunnyvale, Calif.) for the detection of Shiga toxin-producing Escherichia coli (STEC), as well as other Shiga toxin-producing bacteria. Based on multiple-sequence alignments, we have designed two pairs of PCR primers that efficiently amplify all variants of the Shiga toxin genes stx1 and stx2, respectively. These primer pairs were combined for use in a multiplex assay. Two molecular beacons bearing different fluorophores were used as internal probes specific for each amplicon. Assays performed with purified genomic DNA from a variety of STEC strains (n = 23) from diverse geographic locations showed analytical sensitivities of about 10 genome copies per PCR. Non-STEC strains (n = 20) were also tested, and no amplification was observed. The PCR results correlated perfectly with the phenotypic characterization of toxin production in both STEC and non-STEC strains, thereby confirming the specificity of the assay. The assay was validated by testing 38 fecal samples obtained from 27 patients. Of these samples, 26 were PCR positive for stx1 and/or stx2. Compared with the culture results, both the sensitivity and the negative predictive value were 100%. The specificity was 92%, and the positive predictive value was 96%. Moreover, this assay detected STEC from a sample in which the STEC concentration was at the limit of detection of the conventional culture methods and from a sample in which STEC was not detected by the conventional culture methods. This real-time PCR assay is simple, rapid, sensitive, and specific and allows detection of all Shiga toxin-producing bacteria directly from fecal samples, irrespective of their serotypes.
PMCID: PMC140333  PMID: 11923369
14.  A Poly-N-Acetylglucosamine−Shiga Toxin Broad-Spectrum Conjugate Vaccine for Shiga Toxin-Producing Escherichia coli 
mBio  2014;5(2):e00974-14.
Many pathogens produce the β-(1−6)-linked poly-N-acetylglucosamine (PNAG) surface polysaccharide that is being developed as a broadly protective antimicrobial vaccine. However, it is unknown whether systemically injected PNAG vaccines or antibodies would provide protective immunity against pathogens confined to the gastrointestinal tract such as Shiga toxin (Stx)-producing Escherichia coli (STEC), an important group of gastrointestinal (GI) pathogens for which effective immunotherapeutics are lacking. To ascertain whether systemic IgG antibody to PNAG impacts this infectious situation, a vaccine consisting of a synthetic nonamer of nonacetylated PNAG, 9GlcNH2, conjugated to the Shiga toxin 1b subunit (9GlcNH2-Stx1b) was produced. Rabbit antibodies raised to the conjugate vaccine were tested for bacterial killing and toxin neutralization in vitro and protection against infection in infant mice. Cell surface PNAG was detected on all 9 STEC isolates tested, representing 6 STEC serogroups, including E. coli O157:H7. Antibody to the 9GlcNH2-Stx1b conjugate neutralized Stx1 potently and Stx2 modestly. For O157:H7 and O104:H4 STEC strains, antibodies elicited by the 9GlcNH2-Stx1b conjugate possessed opsonic killing and bactericidal activity. Following intraperitoneal injection, antibodies to both PNAG and Stx were needed for infant mouse protection against O157 STEC. These antibodies also mediated protection against the Stx2-producing O104:H4 strain that was the cause of a recent outbreak in Germany, although sufficient doses of antibody to PNAG alone were protective against this strain in infant mice. Our observations suggest that vaccination against both PNAG and Stx, using a construct such as the 9GlcNH2-Stx1b conjugate vaccine, would be protective against a broad range of STEC serogroups.
The presence of poly-N-acetylglucosamine (PNAG) on many pathogens presents an opportunity to target this one structure with a multispecies vaccine. Whether antibodies to PNAG can protect against pathogens confined to the gastrointestinal tract is not known. As Shiga toxin (Stx)-producing Escherichia coli (STEC) bacteria are serious causes of infection whose virulence is dependent on elaboration of Stx, we prepared a vaccine containing a synthetic nonamer of PNAG (9GlcNH2) conjugated to Shiga toxin 1b subunit (9GlcNH2-Stx1b) to evaluate bacterial killing, toxin neutralization, and protective efficacy in infant mice. All nine (100%) clinical strains of STEC from different serogroups expressed PNAG. Vaccine-induced antibody mediated in vitro killing of STEC and neutralization of both Stx1 and Stx2. Passive administration of antibody to the conjugate showed protection requiring immunity to both PNAG and Stx for O157 strains, although for an O104 strain, antibody to PNAG alone was protective. Immunity to PNAG may contribute to protection against STEC infections.
PMCID: PMC3977355  PMID: 24667709
15.  Distribution of Virulence Genes and Their Association of Serotypes in Pathogenic Escherichia coli Isolates From Diarrheal Patients in Korea 
To characterise the genetic and serological diversity of pathogenic Escherichia coli, we tested 111 E coli strains isolated from diarrhoeal patients in Korea between 2003 and 2006.
The isolates were tested through polymerase chain reaction (PCR) and slide agglutination method for the detection of virulence genes and serotypes, respectively. To compare the expression of Shiga toxin (stx)-1 and stx2 genes, real-time quantitative reverse-transcriptase PCR and rapid exprssion assay, reversed-passive latex agglutination, were performed.
Forty-nine Shiga toxin-producing E coli (STEC) strains and 62 non-STEC strains, including 20 enteropathogenic E coli, 20 enterotoxigenic E coli, 20 enteroaggregative E coli, and 2 enteroinvasive E coli were randomly chosen from the strains isolated from diarrhoeal patients in Korea between 2003 and 2006. PCR analysis indicated that locus of enterocyte effacement pathogenicity island, that is, eaeA, espADB, and tir genes were present in STEC, enteropathogenic E coli, and enteroinvasive E coli. Quorum sensing-related gene luxS was detected in most of pathogenic E coli strains. Major serotypes of the STEC strains were O157 (26%) and O26 (20%), whereas the non-STEC strains possessed various serotypes. Especially, all the strains with serotype O157 carried stx2 and the tested virulence factors. Of the STEC strains, the data of real-time quantitative reverse-transcriptase PCR and reversed-passive latex agglutination tests showed that messenger RNA- and protein expression of stx2 gene were higher than those of stx1 gene.
Our results provide the epidemiological information regarding the trend of STEC and non-STEC infections in the general population and show the fundamental data in association of serotypes with virulence genes in diarrhoeagenic E coli strains from Korea.
PMCID: PMC3766895  PMID: 24159437
diarrhoeal patients; pathogenic Escherichia coli; serotypes; virulence factors
16.  Characterization of Shiga toxin-producing Escherichia coli (STEC) in feces of healthy and diarrheic calves in Urmia region, Iran 
Background and Objectives
Shiga toxin-producing Escherichia coli (STEC) have emerged as human pathogens and contamination of foods of animal origin has been a major public health concern. The aim of the present study was to determine the dissemination of STEC in healthy and diarrheic calves in Urmia region which is located in West Azerbaijan province, Iran.
Materials and Methods
In the current study, a total of 124 Escherichia coli isolates from clinically healthy (n = 73) and diarrheic calves (51) belonging to 6 different farms located in West Azerbaijan province, Iran, were screened by the polymerase chain reaction (PCR) assay for the presence of virulence genes characteristic for STEC, that is, Shiga-toxin producing gene(s) (stx1, stx2), intimin (eaeA) and enterohemolysin (hlyA).
STEC isolates were recovered from 21.92% (16/73) in healthy calves, and 19.6% (10/51) in diarrheic calves. Overall, PCR results showed that 6 (23.1%) isolates carried stx1 gene, 7 (26.92%) possessed stx2 gene while 13 isolates (50%) gave positive amplicon both for stx1 and stx2 genes. All stx positive isolates were assayed further to detect eaeA and hlyA sequences. Seven out of the 26 (26.92%) Shiga toxin gene positive isolates were positive for the eaeA gene, and 15 (57.69%) were positive for the hlyA gene. Both virulence genes (eaeA and hlyA) in the same isolate were observed in 5 (19.23%) of the stx + isolates. In total, diverse virulence gene profiles were detected, from which isolates with the genetic profile stx1 stx2 hlyA was the most prevalent. In addition, eaeA gene was more evident in isolates from diarrheic calves than in healthy calves.
There was no significant difference in detecting STEC isolates between healthy and diarrheic calves. It seems that calves to be the reservoir of STEC within the herds and calf management may represent specific control points for reducing STEC spread within dairy units.
PMCID: PMC3434643  PMID: 22973471
Shiga toxin; E. coli; calves; Iran
17.  Development of a multiplex loop-mediated isothermal amplification assay to detect shiga toxin-producing Escherichia coli in cattle 
Journal of Veterinary Science  2014;15(2):317-325.
A multiplex loop-mediated isothermal amplification (mLAMP) assay was developed for simultaneous detection of the stx1 and stx2 genes and applied for detection of shiga toxin-producing Escherichia coli (STEC) in cattle farm samples. Two target genes were distinguished based on Tm values of 85.03 ± 0.54℃ for stx1 and 87.47 ± 0.35℃ for stx2. The mLAMP assay was specific (100% inclusivity and exclusivity), sensitive (with a detection limit as low as 10 fg/µL), and quantifiable (R2 = 0.9313). The efficacy and sensitivity were measured to evaluate applicability of the mLAMP assay to cattle farm samples. A total of 12 (12/253; 4.7%) and 17 (17/253; 6.7%) STEC O157, and 11 (11/236; 4.7%) non-O157 STEC strains were isolated from cattle farm samples by conventional selective culture, immunomagnetic separation, and PCR-based culture methods, respectively. The coinciding multiplex PCR and mLAMP results for the types of shiga toxin revealed the value of the mLAMP assay in terms of accuracy and rapidity for characterizing shiga toxin genes. Furthermore, the high detection rate of specific genes from enrichment broth samples indicates the potential utility of this assay as a screening method for detecting STEC in cattle farm samples.
PMCID: PMC4087235  PMID: 24675834
cattle farm; E. coli O157; LAMP; shiga toxin; stx
18.  Development of a Multiplex PCR Assay for Detection of Shiga Toxin-Producing Escherichia coli, Enterohemorrhagic E. coli, and Enteropathogenic E. coli Strains 
Escherichia coli O157:H7 and other pathogenic E. coli strains are enteric pathogens associated with food safety threats and which remain a significant cause of morbidity and mortality worldwide. In the current study, we investigated whether enterohemorrhagic E. coli (EHEC), Shiga toxin-producing E. coli (STEC), and enteropathogenic E. coli (EPEC) strains can be rapidly and specifically differentiated with multiplex PCR (mPCR) utilizing selected biomarkers associated with each strain’s respective virulence genotype. Primers were designed to amplify multiple intimin (eae) and long polar fimbriae (lpfA) variants, the bundle-forming pilus gene bfpA, and the Shiga toxin-encoding genes stx1 and stx2. We demonstrated consistent amplification of genes specific to the prototype EHEC O157:H7 EDL933 (lpfA1-3, lpfA2-2, stx1, stx2, and eae-γ) and EPEC O127:H6 E2348/69 (eae-α, lpfA1-1, and bfpA) strains using the optimized mPCR protocol with purified genomic DNA (gDNA). A screen of gDNA from isolates in a diarrheagenic E. coli collection revealed that the mPCR assay was successful in predicting the correct pathotype of EPEC and EHEC clones grouped in the distinctive phylogenetic disease clusters EPEC1 and EHEC1, and was able to differentiate EHEC1 from EHEC2 clusters. The assay detection threshold was 2 × 104 CFU per PCR reaction for EHEC and EPEC. mPCR was also used to screen Argentinean clinical samples from hemolytic uremic syndrome and diarrheal patients, resulting in 91% sensitivity and 84% specificity when compared to established molecular diagnostic procedures. In conclusion, our mPCR methodology permitted differentiation of EPEC, STEC and EHEC strains from other pathogenic E. coli; therefore, the assay becomes an additional tool for rapid diagnosis of these organisms.
PMCID: PMC3417533  PMID: 22919600
Shiga toxin-producing E. coli; enterohemorrhagic E. coli; enteropathogenic E. coli; E. coli O157; diagnostics
19.  Identification of Human-Pathogenic Strains of Shiga Toxin-Producing Escherichia coli from Food by a Combination of Serotyping and Molecular Typing of Shiga Toxin Genes▿  
Applied and Environmental Microbiology  2007;73(15):4769-4775.
We examined 219 Shiga toxin-producing Escherichia coli (STEC) strains from meat, milk, and cheese samples collected in Germany between 2005 and 2006. All strains were investigated for their serotypes and for genetic variants of Shiga toxins 1 and 2 (Stx1 and Stx2). stx1 or variant genes were detected in 88 (40.2%) strains and stx2 and variants in 177 (80.8%) strains. Typing of stx genes was performed by stx-specific PCRs and by analysis of restriction fragment length polymorphisms (RFLP) of PCR products. Major genotypes of the Stx1 (stx1, stx1c, and stx1d) and the Stx2 (stx2, stx2d, stx2-O118, stx2e, and stx2g) families were detected, and multiple types of stx genes coexisted frequently in STEC strains. Only 1.8% of the STEC strains from food belonged to the classical enterohemorrhagic E. coli (EHEC) types O26:H11, O103:H2, and O157:H7, and only 5.0% of the STEC strains from food were positive for the eae gene, which is a virulence trait of classical EHEC. In contrast, 95 (43.4%) of the food-borne STEC strains carried stx2 and/or mucus-activatable stx2d genes, an indicator for potential high virulence of STEC for humans. Most of these strains belonged to serotypes associated with severe illness in humans, such as O22:H8, O91:H21, O113:H21, O174:H2, and O174:H21. stx2 and stx2d STEC strains were found frequently in milk and beef products. Other stx types were associated more frequently with pork (stx2e), lamb, and wildlife meat (stx1c). The combination of serotyping and stx genotyping was found useful for identification and for assignment of food-borne STEC to groups with potential lower and higher levels of virulence for humans.
PMCID: PMC1951031  PMID: 17557838
20.  Comparison of Three Different Methods for Detection of Shiga Toxin-Producing Escherichia coli in a Tertiary Pediatric Care Center 
Journal of Clinical Microbiology  2013;51(2):481-486.
Shiga toxin-producing Escherichia coli (STEC) is a well-known cause of sporadic and epidemic food-borne gastroenteritis. A low infectious dose, approximately 10 microorganisms, is sufficient to cause disease that may lead to hemolytic-uremic syndrome. The objective of this study was to compare the performances of an in-house real-time PCR, a commercial enzyme immunoassay (EIA) (Premier EHEC; Meridian Bioscience), and culture on sorbitol MacConkey agar for the detection of STEC in a tertiary care pediatric hospital. Of 632 stool samples tested, 21 were positive for STEC. All were detected by PCR, 6 were detected by EIA, and only 5 O157 STEC isolates were identified by culture. Among the 15 specimens falsely negative by EIA, there were 9 Stx1, 2 Stx2, and 4 Stx1 and Stx2 STEC isolates. The latter group included 2 O157 STEC isolates that would have been missed if only EIA had been performed. To our knowledge, this is the first prospective study performed in a pediatric hospital which demonstrates the superiority of PCR over EIA for the detection of STEC. We conclude that PCR is specific and more sensitive than EIA. PCR should be considered for routine use in clinical settings where molecular detection facilities are available. Its lower limit of detection, equivalent to the infectious dose, is an obvious advantage for patient care and public health surveillance.
PMCID: PMC3553904  PMID: 23175264
21.  Prevalence and Characterization of Shiga Toxin-Producing Escherichia coli in Swine Feces Recovered in the National Animal Health Monitoring System's Swine 2000 Study 
Applied and Environmental Microbiology  2004;70(12):7173-7178.
A study was conducted to determine the prevalence of Shiga toxin-producing Escherichia coli (STEC) in swine feces in the United States as part of the National Animal Health Monitoring System's Swine 2000 study. Fecal samples collected from swine operations from 13 of the top 17 swine-producing states were tested for the presence of STEC. After enrichment of swine fecal samples in tryptic soy broth, the samples were tested for the presence of stx1 and stx2 by use of the TaqMan E. coli STX1 and STX2 PCR assays. Enrichments of samples positive for stx1 and/or stx2 were plated, and colony hybridization was performed using digoxigenin-labeled probes complementary to the stx1 and stx2 genes. Positive colonies were picked and confirmed by PCR for the presence of the stx1, stx2, or stx2e genes, and the isolates were serotyped. Out of 687 fecal samples tested using the TaqMan assays, 70% (484 of 687) were positive for Shiga toxin genes, and 54% (370 of 687), 64% (436 of 687), and 38% (261 of 687) were positive for stx1, stx2, and both toxin genes, respectively. Out of 219 isolates that were characterized, 29 (13%) produced stx1, 14 (6%) produced stx2, and 176 (80%) produced stx2e. Twenty-three fecal samples contained at least two STEC strains that had different serotypes but that had the same toxin genes or included a strain that possessed stx1 in addition to a strain that possessed stx2 or stx2e. The STEC isolates belonged to various serogroups, including O2, O5, O7, O8, O9, OX10, O11, O15, OX18, O20, O57, O65, O68, O69, O78, O91, O96, O100, O101, O120, O121, O152, O159, O160, O163, and O untypeable. It is noteworthy that no isolates of serogroup O157 were recovered. Results of this study indicate that swine in the United States harbor STEC that can potentially cause human illness.
PMCID: PMC535163  PMID: 15574914
22.  Prevalence and Characterization of Non-O157 Shiga Toxin-Producing Escherichia coli Isolates from Commercial Ground Beef in the United States▿ †  
Escherichia coli O157:H7 is a Shiga toxin (stx)-producing E. coli (STEC) strain that has been classified as an adulterant in U.S. beef. However, numerous other non-O157 STEC strains are associated with diseases of various severities and have become an increasing concern to the beef industry, regulatory officials, and the public. This study reports on the prevalence and characterization of non-O157 STEC in commercial ground beef samples (n = 4,133) obtained from numerous manufacturers across the United States over a period of 24 months. All samples were screened by DNA amplification for the presence of Shiga toxin genes, which were present in 1,006 (24.3%) of the samples. Then, culture isolation of an STEC isolate from all samples that contained stx1 and/or stx2 was attempted. Of the 1,006 positive ground beef samples screened for stx, 300 (7.3% of the total of 4,133) were confirmed to have at least one strain of STEC present by culture isolation. In total, 338 unique STEC isolates were recovered from the 300 samples that yielded an STEC isolate. All unique STEC isolates were serotyped and were characterized for the presence of known virulence factors. These included Shiga toxin subtypes, intimin subtypes, and accessory virulence factors related to adherence (saa, iha, lifA), toxicity (cnf, subA, astA), iron acquisition (chuA), and the presence of the large 60-MDa virulence plasmid (espP, etpD, toxB, katP, toxB). The isolates were also characterized by use of a pathogenicity molecular risk assessment (MRA; based on the presence of various O-island nle genes). Results of this characterization identified 10 STEC isolates (0.24% of the 4,133 total) that may be considered a significant food safety threat, defined by the presence of eae, subA, and nle genes.
PMCID: PMC3067332  PMID: 21257806
23.  Detection of Chlamydia trachomatis by Isothermal Ramification Amplification Method: a Feasibility Study 
Journal of Clinical Microbiology  2002;40(1):128-132.
Chlamydia trachomatis is the leading cause of sexually transmitted disease in the United States. Effective screening for this agent can facilitate prompt treatment and prevent its sequelae. The recent introduction of liquid-based cytology has made possible the simultaneous screening of cervical intraepithelial lesions and detection of C. trachomatis in a single collection vial. In this study we determined whether cytological fluid could support DNA-based amplification for the detection of C. trachomatis. Three methods were compared, including ramification amplification (RAM), real-time PCR with molecular beacon, and Abbott’s ligase chain reaction (LCx). RAM is a novel, recently introduced, isothermal DNA amplification technique that utilizes a circular probe for target detection and achieves exponential amplification through the mechanism of primer extension, strand displacement, and ramification. Our results show that RAM can detect as few as 10 C. trachomatis elementary bodies in less than 2 h, comparable to results with real-time PCR. Thirty clinical specimens collected in PreservCyt solution were tested by LCx, real-time PCR, and RAM. Among 30 specimens, 15 were positive by PCR and LCx and 14 were positive by RAM. One specimen missed by RAM had an inadequate amount of residual cellular material. Our results show that nucleic acid amplification methods can serve to detect C. trachomatis and presumably other sexually transmitted agents in cytological fluid and that the RAM assay can be an alternative to PCR and LCx because of its simplicity and isothermal amplification.
PMCID: PMC120135  PMID: 11773105
24.  Real-Time Fluorescence PCR Assays for Detection and Characterization of Shiga Toxin, Intimin, and Enterohemolysin Genes from Shiga Toxin-Producing Escherichia coli 
Journal of Clinical Microbiology  2002;40(7):2555-2565.
PCR assays have proved useful for detecting and characterizing Shiga toxin-producing Escherichia coli (STEC). Recent advances in PCR technology have facilitated the development of real-time fluorescence PCR assays with greatly reduced amplification times and improved methods for the detection of amplified target sequences. We developed and evaluated two such assays for the LightCycler instrument: one that simultaneously detects the genes for Shiga toxins 1 and 2 (stx1 and stx2) and another that simultaneously detects the genes for intimin (eae) and enterohemolysin (E-hly). Amplification and sequence-specific detection of the two target genes were completed within 60 min. Findings from the testing of 431 STEC isolates of human and animal origin, 73 isolates of E. coli negative for stx genes, and 118 isolates of other bacterial species with the LightCycler PCR (LC-PCR) assays were compared with those obtained by conventional block cycler PCR analysis. The sensitivities and specificities of the LC-PCR assays were each 100% for the stx1, eae, and E-hly genes and 96 and 100%, respectively, for the stx2 gene. No stx2 genes were detected from 10 stx2f-positive isolates because of significant nucleotide differences in their primer annealing regions. Melting curve analyses of the amplified Shiga toxin genes revealed sequence variation within each of the tested genes that correlated with described and novel gene variants. The performance characteristics of the LC-PCR assays, such as their speed, detection method, and the potential subtyping information available from melting curve analyses, make them attractive alternatives to block cycler PCR assays for detecting and characterizing STEC strains.
PMCID: PMC120605  PMID: 12089277
25.  Glutamate Decarboxylase Genes as a Prescreening Marker for Detection of Pathogenic Escherichia coli Groups 
The enzyme glutamate decarboxylase (GAD) is prevalent in Escherichia coli but few strains in the various pathogenic E. coli groups have been tested for GAD. Using PCR primers that amplify a 670-bp segment from the gadA and gadB genes encoding GAD, we examined the distribution of the gadAB genes among enteric bacteria. Analysis of 173 pathogenic E. coli strains, including 125 enterohemorrhagic E. coli isolates of the O157:H7 serotype and its phenotypic variants and 48 isolates of enteropathogenic E. coli, enterotoxigenic E. coli, enteroinvasive E. coli, and other Shiga toxin-producing E. coli (STEC) serotypes, showed that gadAB genes were present in all these strains. Among the 22 non-E. coli isolates tested, only the 6 Shigella spp. carried gadAB. Analysis of naturally contaminated water and food samples using a gadAB-specific DNA probe that was labeled with digoxigenin showed that a gadAB-based assay is as reliable as standard methods that enumerate E. coli organisms on the basis of lactose fermentation. The presence of few E. coli cells initially seeded into produce rinsates could be detected by PCR to gadA/B genes after overnight enrichment. A multiplex PCR assay using the gadAB primers in combination with primers to Shiga toxin (Stx) genes stx1 and stx2 was effective in detecting STEC from the enrichment medium after seeding produce rinsate samples with as few as 2 CFU. The gadAB primers may be multiplexed with primers to other trait virulence markers to specifically identify other pathogenic E. coli groups.
PMCID: PMC92988  PMID: 11425729

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