A set of novel multiplex Taqman real-time PCRs for the detection of diarrhoeagenic Escherichia coli and its use in determining the prevalence of EPEC and EAEC in a university hospital
© Hardegen et al. 2010
Received: 4 November 2009
Accepted: 22 January 2010
Published: 22 January 2010
Accurate measurement of the incidence of diarrhoeagenic E. coli in patients with diarrhoea is hindered by the current methods of detection and varies from country to country. In order to improve the diagnosis of diarrhoeagenic E. coli (DEC), we developed a set of multiplex TaqMan real-time PCRs designed to detect the respective pathogens from an overnight stool culture.
Over the period Jan. 2006 to Dec. 2006 all stool specimens (n = 1981) received were investigated for EPEC and EAEC.
Of these, 371 specimens had no growth of Enterobacteriaceae. Of the remaining 1610 specimens 144 (8,9%) were positive for EPEC and 78 (4,8%) positive for EAEC. Among the EPEC positive stool specimens 28 (19,4%) were received from the tropical diseases unit, 49 (34%) from the paediatric dept. and 67 (46,5%) from the remainder of the wards. The EAEC were distributed as follows: 39 (50%) - tropical diseases, 19 (24,4%) -paediatrics and 20 (25,6%) other wards. Proportionately more EAEC and EPEC were found in children less than 3 years of age than other age groups. In only 22,2% of the detected EPEC and 23% of EAEC was the investigation requested by hospital staff.
This is, to our knowledge, the first study using a multiplex TaqMan PCR for the successful detection of diarrhoeagenic E. coli. In conclusion, due to the high prevalence of DEC detected, investigation of EPEC and EAEC should be recommended as a routine diagnostic test for patients with infectious diarrhoea.
Infectious diarrhoea is a common complaint among patients seeking medical advice and, despite progress in diagnosis and treatment, remains one of the leading causes of morbidity and mortality worldwide [1, 2].
The spectrum of pathogens responsible for such infections varies with age and geographical location. A viral pathogenesis is probably the main cause of diarrhoea in industrialised countries [1, 3], however systematic surveys have shown that diarrhoeagenic E. coli (DEC) are a common cause of diarrhoea in both developing and developed countries. These pathogens, especially EAEC, may routinely be underestimated as a cause of diarrhoea due to under-representation of requests and difficulty recognising these pathogens in the laboratory [4, 5].
To date six categories of DEC have been defined on the basis of specific virulence properties [6, 7]. Enterotoxigenic E. coli (ETEC) cause diarrhoea via secretion of heat stable (ST) and/or heat labile (LT) enterotoxin. Enteroinvasive E. coli (EIEC) strains are closely related to Shigella spp. and the responsible genes are carried on the pINV plasmid. Shiga-toxin producing or enterohaemorrhagic E. coli (STEC/EHEC) cause both a non-bloody diarrhoea as well as a haemorrhagic colitis which may trigger haemolytic uraemic syndrome (HUS). The virulence properties defining EHEC are the Shiga toxins 1 and 2 (Stx1, Stx2) and, when present, the pathogenicity island LEE ("locus of enterocyte effacement"). Enteropathogenic E. coli (EPEC) show localised adherence to the small intestine and cause a histopathological "attaching and effacing (A/E) lesion" mediated through virulence factors encoded within the LEE. Strains additionally containing the EPEC adherence factor (EAF) plasmid have been termed "typical" EPEC, whereas strains that lack the EAF plasmid are referred to as "atypical" EPEC. Enteroaggregative E. coli (EAEC or EAggEC) was first described in 1987  and has been associated with acute diarrhoea in children, HIV positive individuals, and as a cause of travellers' diarrhoea . Moreover it has been linked to persistent diarrhoea and even growth retardation in children . Numerous findings suggest that there is an inflammatory process underlying the infection [11, 12]. The PCR-target used in this report is a sequence on the plasmid pCVD432 on which the genes coding for the fimbrial structure (aggregative adherence fimbria I) and further virulence factors are located . The sixth category is the diffusely adherent E. coli (DAEC) in which the bacteria, unlike EAEC do not adhere to the host cells in microcolonies but rather diffusely over the cell surface. No PCR has been developed yet to detect DAEC strains.
Since DEC cannot be diagnosed adequately by culture and biochemical criteria alone, identification of these strains is difficult. Many PCR techniques detecting various genes coding for virulence traits of the different categories of DEC have been reported [14–18]. We have developed three new multiplex TaqMan PCR assays, designed to be run in parallel, to detect ETEC, EIEC, EHEC, EPEC and EAEC simultaneously.
In addition to many studies from developing countries recent reports underline the importance of EPEC and EAEC in developed countries [5, 19–21]. In Germany very limited data are available. Therefore the aim of this study was to evaluate the local importance of EPEC and EAEC in a German University hospital in relation to the requesting clinical department and age group, using the newly developed real-time PCR as a diagnostic tool.
From January 2006 to December 2006 a total of 1981 faecal samples received for Salmonella, Shigella and Campylobacter spp. culture were included in the screening for EPEC and EAEC. Of these 371 specimens had no growth of Enterobacteriaceae hence 1610 samples were tested for EPEC and EAEC by use of the PCR method described below. Stool samples were obtained from most departments of the University Hospital of Düsseldorf, Germany. The patient age ranged from a few days to 98 years.
All faecal samples received were plated on MacConkey Agar and incubated for 18 h at 37°C. A bacterial suspension from all colonies was made by rinsing the entire plate with 2 ml of sterile saline. The bacterial suspension was diluted to MacFarland Standard 1,0 (approx. 3*108 CFU/ml) using distilled water. This suspension was heated at 95°C for 15 minutes. 2,5 μl (corresponding to approximately 7,5*105 CFU) were applied directly to the multiplex PCR. The remainder of the sample was stored at -20°C.
Three different multiplex TaqMan PCRs were designed and were all run using the same time and temperature settings on a Biorad iCycler system. A two step amplification profile was used as follows: 95°C for 10 min. followed by 45 cycles of 15 s. at 95°C and 60 s. at 60°C for annealing and elongation. The PCR contained 10 pmol of each primer, 2 pmol of the target probes, 5 pmol of the internal inhibition control probe (all from Eurogentec, Cologne, Germany), 100 copies of inhibition control plasmid, 2,5 μl sample, 12,5 μl of No ROX PCR Mastermix (Qiagen, Hilden, Germany) and was diluted with distilled water to a final volume of 25 μl.
Primers and probes for multiplex TaqMan PCR
Primer or probe name
Oligonucleotide sequence (5'→3')
Amplicon size/Accession nr.
GTT CTT GGC GAA CAG GCT TGT C
TTA AGC CAG CTA CCA TCC ACC C
Cy5 -AGT ACT GAC GTG CAG GTC GCC TGT TCG- BHQ-3
ACT GGA CTT CTT ATT RCC GTT CTA TG
CTA AGC GGG TAT TGT TAC CAG A
CCT AAA CGG GTA TTA TCA CCA GA
ROX -AAT CCT GAT CAA TGA AGA CGT TAT AGC CCA- BBQ
AGG TTT GAT ATT GAT GTC CTT GAG GA
TCA GCT AAT AAT GTA TAG AAA TCC GCT GTT
FAM -CAT GTT CCT GAG AGT GCA ATC CCA GAC ATT AC- TAMRA
CTG GTT TTG ATT CAA ATG TTC GTG
TCC TGA GGG AAA GGT GAA AAA GAC
ROX -TTG ATT TCT TCA TAT TAC CTC CGG ACA TGG CA- BHQ-2
AGC GGC GCA ACA TTT CAG
TTG GTC TCG GTC AGA TAT GTG ATT C
FAM -TCG AAG TCC CGG GCA GTC AAC ATA TAG A- TAMRA
GAA CTC AAA TCT TGC ACC ATT CA
CGT CCG TCC GAG AAC AAT TAA G
Cy5 -ATC CCC GAC ACC GTT TGT GAG TTT CAC T- BHQ-3
CTT CCA TCT GCC GGA CAC ATA
ATT AAT ACT GAA TTG TCA TCA TCA TGC AT
ROX -AAG GAA ACT CAT CAG ATG CCA TTC TGG CA- BHQ-2
GAC GTG GAC CTC ACT CTG AAC TG
TCC CCA CTC TGA CAC CAT CC
FAM -TAC TCC GGA AGC ACA TTG CTG ATT CGC- TAMRA
Drosophila simulans +
HEX -ATG CCT CTT CAC ATT GCT CCA CCT TTC CT- BHQ1
PCR-based detection methods, especially from faecal samples are prone to inhibition of amplification . As an internal control (IC) specimens were spiked with synthesised nucleotide sequences using an unrelated probe sequence from the retrotransposon Ninja from Drosophila simulans (AB110070) in the pCR II-TOPO Vector (Invitrogen, Karlsruhe, Germany) flanked by the respective primers used in the PCR . A serial dilution of targeted template in a MacFarland standard 1,0 apathogenic E. coli suspension revealed reliable fluorescence signals to a mean number of 1,5 bacterial genomes per 2,5 μl for all targets. No cross reactivity with various bacteria (Streptococcus, Staphylococcus, Enterococcus, Pseudomonas, Klebsiella, Proteus, Citrobacter, Salmonella, Yersinia, Campylobacter, Aeromonas caviae, Clostridium difficile and Enterobacter spp.) other than the known positivity with Shigella dysenteriae using the EHEC PCR was detected.
The data were divided into three groups depending on the requesting department: the department for tropical diseases, paediatrics and a last group of all the remaining departments. The department for tropical diseases group consisted of 150 samples only but had a relatively high positivity rate with 28 EPEC positive and 39 EAEC positive samples. 18,7% of all specimens tested within this group were positive for EPEC and 26% for EAEC. Most patients seen in this department have a travel history and, with the exception of only 4 patients, all were between 18 and 65 years of age. From the paediatric wards 458 specimens were tested and 49 EPEC positive and 19 EAEC positive samples were detected showing an prevalence of 10,7% for EPEC and 4,1% for EAEC. From the remaining 1002 samples from all other departments 67 (6,7%) were tested positive for EPEC and 20 (2%) positive for EAEC. Table 2 shows the results.
Number of stool samples tested according to department and positivity rate for EPEC and EAEC
In our hospital all stool specimens from children aged up to one year are tested for EPEC routinely, however it was noted that requests for DEC are very rarely made for patients above this age. Therefore we looked at the number of DEC cases found by routine investigations in comparison to the total number of cases found by systematic screening of all specimens received. Table 2 shows the findings for EPEC and EAEC. 15 EPEC positive samples from children below one year of age were detected by routine screening and of the positive results for EPEC or EAEC in the group of samples from children between one and five years of age none was detected by routine request. In the age group between 5 and 18 years of age only one (EPEC positive) specimen was actually requested by the paediatric department and 1 (EAEC positive) specimen from the department for tropical diseases. Between 18 and 65 years a relatively large number of positive results was found in screening all received stool specimens, but only 16 (21,6%) EPEC positive and 15 (29,4%) EAEC positive specimens were detected due to a specific request from the respective department. Furthermore, except for three positive EPEC specimens and one positive EAEC specimen, all of the specimens were sent by the department for tropical diseases. In the group above 65 years in only two of the specimens that were positive for EAEC was the investigation routinely requested, again by the department for tropical diseases.
Detecting DEC relies greatly on PCR techniques and many reports are available on procedures used to identify all categories of DEC [13–18, 26]. Most of these techniques however, are based on standard PCR. We have developed a novel multiplex real-time PCR which, with three reactions run parallel to one another, identifies the 5 major categories of DEC simultaneously. To our knowledge this is the first reported use of a TaqMan PCR, which provides both a convenient diagnostic tool and avoids the risk of cross contamination due to post amplification handling.
By rinsing all the colonies from the agar plate with saline rather than picking representative colonies as has been described in the literature, we were able to increase the sensitivity of the PCR. This method is quick, simple and inexpensive. The real-time PCR was able to detect 1,5 genome equivalents per assay.
As reported by the Robert Koch Institute, Berlin, Germany, of all reportable intestinal infections in 2006 in Germany, DEC, excepting EHEC, was the fifth largest group of pathogens after Norovirus, Rotavirus, Salmonella and Campylobacter spp. In our study reported here, the prevalence of EPEC (8,9%) and EAEC (4,8%) exceeded those for salmonella and campylobacter, each 2,4%. It is possible that the incidence of EPEC and EAEC throughout Germany is far higher than that reported. This discrepancy may very well be due to the low level of routine requests for DEC as we observed in the study period in this hospital. Unfortunately no data can be provided here to compare the incidence of DEC to that of viral pathogens since in this study stool specimens were not systematically tested for viral pathogens. However, in the paediatric population (the age group under 18 years in this study), many specimens were additionally tested for viruses and out of 40 EPEC positive samples additionally tested for viruses, 16 were additionally positive for rotavirus, norovirus or adenovirus. 4 specimens positive for EPEC were additionally positive for Salmonella spp. (1) or Campylobacter spp. (3). This leaves EPEC as the sole identified pathogen in 50% of all cases. EAEC was the only pathogen detected in 45% of the samples in this age group. In nine cases the specimen was positive for rotavirus, norovirus or adenovirus as well, whereas in three samples Campylobacter spp. was found as a co-infection and one specimen was tested positive for Salmonella spp. Even though this study provides no systematic data on viral testing we must assume that viral infections outnumber the cases of diarrhoea due to DEC in children. It is possible that more stool samples were submitted for viral detection only and thus the prevalence for EPEC and EAEC found in this study may still be an underestimation in this age group.
A strong association between EPEC and diarrhoea in children has been reported , yet we also found a relatively high prevalence for EPEC in older patients.
A large proportion of EPEC (and EAEC) infections may well be associated with travel, since a high number of positive specimens within the group between 18 and 65 years were obtained from the department for tropical diseases. Concerning the association of atypical EPEC with diarrhoea findings are contradictory [27–29], but it continues to be the most prevalent pathotype of EPEC found in industrialised countries [27, 29, 30], which we also noted in our study. A subset of the heterogeneous group of atypical EPEC has been found in patients with bloody diarrhoea, (in some cases leading to HUS). They resembled EHEC according to their serotype, virulence profile and multilocus sequence types, leading to the assumption that these strains might be EHEC that have at some stage lost the shiga toxin gene during infection . Unusual in this study is the detection of EAF without the eae gene as most often eae is found alone (atypical EPEC) or with EAF. It is questionable whether strains positive for EAF but not for eae should be classified as EPEC and whether they actually are pathogenic. Further investigation is required as to whether this is due to a sequence variation within the intimin gene resulting in a failure of amplification or whether perhaps other bacterial strains have acquired the EAF plasmid.
One limitation of this study is the inability of the method to distinguish between those patients with a double infection with two or more DEC and those patients who are infected with a single bacterium carrying more than one virulence gene. Because we use a pool of bacteria from an overnight culture, it is not possible to trace the result back to a single bacterial clone. In this study we found 17 specimens which were positive for both EPEC and EAEC. In no case was the distinction between a single bacterium carrying both factors or a double infection clinically relevant. Nevertheless, it would be interesting to determine the frequency of multiple gene carriage by a single clone and we are currently prospectively attempting to trace back to the gene-carrying clone(s).
In our study we find the prevalence for EPEC and EAEC to be unexpectedly high in Düsseldorf, and we conclude that all patients with diarrhoeal disease should be routinely tested for these pathogens, especially children below the age of five, returning travellers and in those specimens in which no other pathogen can be identified. We also demonstrate the use of a novel multiplex PCR for the detection of diarrhoeagenic E. coli from stool specimens.
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