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Journal of Clinical Microbiology, December 2004, p. 5849-5853, Vol. 42, No. 12
0095-1137/04/$08.00+0 DOI: 10.1128/JCM.42.12.5849-5853.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Evaluation of Multiplex PCRs for Diagnosis of Infection with Diarrheagenic Escherichia coli and Shigella spp.
K. R. S. Aranda,1
U. Fagundes-Neto,2 and
I. C. A. Scaletsky1*
Departamento de Microbiologia, Imunologia, e Parasitologia,1
Departamento de Pediatria, Escola Paulista de Medicina, Universidade Federal de São Paulo, São Paulo, Brazil2
Received 13 January 2004/
Returned for modification 3 March 2004/
Accepted 3 August 2004

ABSTRACT
We have developed two multiplex PCR assays that detect typical
and atypical enteropathogenic
Escherichia coli (EPEC) isolates,
enteroaggregative
E. coli (EAEC) isolates, enterotoxigenic
E. coli (ETEC) isolates, enteroinvasive
E. coli (EIEC) isolates,
Shiga toxin-producing
E. coli (STEC) isolates, and
Shigella spp. The targets selected for each group were
eae and
bfpA for
EPEC isolates, the target of probe CVD432 for EAEC isolates,
the genes encoding heat-labile and heat-stable toxins for ETEC
isolates,
stx1 and
stx2 for STEC isolates, and
ipaH for EIEC
isolates and
Shigella spp. These PCRs were specific and sensitive
for rapid detection of target isolates in stools. Among 150
stool specimens from the acute diarrhea tested, 9 samples (6%)
had atypical EPEC, 9 (6%) had typical EPEC, 7 (4.7%) had EAEC,
3 (2%) had EIEC, 3 (2%) had
Shigella spp., and 1 (0.7%) had
an O26 STEC strain; we also detected mixed infections, 2 (1.3%)
with EAEC and
Shigella spp., 1 (0.7%) with atypical and typical
EPEC strains, and another with atypical EPEC and EAEC strains.
One of the multiplex PCRs directly applied to 36 stool specimens
correctly identified 100% of EPEC and EAEC isolates.

TEXT
Five categories of
Escherichia coli have been well associated
with diarrhea in several epidemiological studies (
9): enteropathogenic
E. coli (EPEC), enteroaggregative
E. coli (EAEC), enterotoxigenic
E. coli (ETEC), enteroinvasive
E. coli (EIEC), and Shiga toxin-producing
E. coli (STEC). The virulence mechanisms that characterize these
categories of
E. coli are genetically encoded by chromosomal,
plasmid, and bacteriophage DNAs and are represented by the following
genes:
eae (attaching and effacing lesions),
bfpA (localized
adherence), the gene encoding enteroaggregative adherence,
ipaH (enteroinvasive mechanism), the genes encoding heat-labile toxin
(LT) and heat-stable toxin (ST), and
stx1 and
stx2 (Shiga toxins).
To correctly identify diarrheagenic
E. coli strains, these organisms
must be differentiated from nonpathogenic members of the normal
flora. Serotypic markers correlate, sometimes very closely,
with specific categories of diarrheagenic
E. coli; however,
these markers are rarely sufficient in and of themselves to
reliably identify a strain as diarrheagenic. Thus, the detection
of diarrheagenic
E. coli has focused increasingly on the identification
of certain characteristics which themselves determine the virulence
of these organisms. This identification process may include
HEp-2 cell adherence, DNA hybridization, and PCR assays to detect
the presence of specific virulence traits or the genes encoding
these traits. The first two types of assays require special
expertise, employ cell culture and radioactive material, and
are time-consuming.
We developed two multiplex PCR assays to detect the five categories of diarrheagenic E. coli organisms and Shigella spp. and assessed the direct application of those assays to human diarrheal stool samples.
The targets selected for each category were eae and bfpA for EPEC isolates, the target of probe CVD432 for EAEC isolates, the LT and ST genes for ETEC isolates, stx1 and stx2 for STEC isolates, and ipaH for EIEC isolates and Shigella spp. For each target gene, a different pair of primers was selected from the literature (Table 1). Multiplex PCR assay 1 utilizes three primer pairs and detects the presence of eae, bfpA, and the target of CVD432, generating amplification products of 917, 326, and 630 bp, respectively. Detection of eae confirms the presence of typical and/or atypical EPEC strains, while testing for bfpA confirms the presence of the bundle-forming pilus major subunit that is found only in typical EPEC strains (4, 5, 12). To include the identification of EAEC strains in the multiplex PCR, we selected a primer pair complementary to the EAEC probe sequence that detects 90% of EAEC strains (15). PCR assay 2 uses five primer pairs and detects the presence of the LT and ST genes, stx1, stx2, and ipaH, generating PCR products of distinct sizes which are easily distinguished after agarose gel electrophoresis. The primers detect the genes encoding LT and porcine and human ST in order to detect all types of ETEC in a single multiplex reaction (18). The stx1 and stx2 primers were designed to amplify Stx1 and all Stx2 variants (11). The ipaH sequences are present at multiple sites on both the large invasive plasmid and the chromosomes in Shigella spp. and EIEC strains (16).
All strains examined by PCR were grown on MacConkey agar plates
at 37°C. DNA was extracted from bacteria by resuspending
one bacterial colony in 50 µl of deionized water, boiling
the suspension for 5 min, and centrifuging it at 10,000
x g for 1 min. The supernatant was then used as the DNA template
for PCR. Having confirmed the specificity of each primer set
by monoplex PCR, we combined the primer sets in different ratios
and tested the reference strains with several PCR cycling protocols.
The optimized protocol was carried out with a 50-µl mixture
containing 10 mM Tris-HCl (pH 8.3), 50 mM KCl, 1.5 mM MgCl
2,
a 2 mM concentration of each deoxynucleoside triphosphate, 1.5
U of AccuPrime
Taq DNA polymerase (Invitrogen), 2 µl of
the DNA template, and the PCR primers. The optimal concentration
of each primer pair in the reaction mixture was determined empirically.
Each primer pair concentration was varied independently until
the PCR products exhibited equal intensities on 2% agarose gels
when a DNA mixture of the five prototype
E. coli strains was
used as the PCR template. The concentration for each primer
pair used in the final reactions is given in Table
1. The PCR
mixtures were then subjected to the following cycling conditions:
for assay 1, 50°C (2 min, 1 cycle); 95°C (5 min, 1 cycle);
40 cycles of 95°C (40 s), 58°C (1 min), and 72°C
(2 min); and a final extension step at 72°C (7 min, 1 cycle);
and for assay 2, 50°C (2 min, 1 cycle); 95°C (5 min,
1 cycle); 40 cycles of 95°C (45 s), 50°C (1 min), and
72°C (1 min); and 72°C (7 min, 1 cycle) in a thermal
cycler (model system 2400; Perkin-Elmer Corporation, Norwalk,
Conn.). PCR products (10 µl) were visualized after electrophoresis
in 2% agarose gels in Tris-borate-EDTA buffer and ethidium bromide
staining. In all experiments, the DNA mixture from the prototype
EPEC E2348/69, EAEC O42, ETEC H10407, EIEC EDL1284, and STEC
EDL931 strains served as the positive control (
9), while
E. coli K-12 DH5

was the negative control.
The two multiplex PCRs were further validated with 270 additional reference strains. One hundred EPEC and 50 EAEC reference strains were identified in a previous case-control study by their reactivity with the eae, EPEC adherence factor, and EAEC probes (4, 14). Fifty ETEC, 20 EIEC, and 50 Shigella species reference strains were also identified by DNA hybridization by other laboratories. The strains were subjected to both multiplex PCRs, and the results were compared with those obtained by monoplex PCR. Both multiplex PCR assays showed 100% specificity in identifying the reference strains; most importantly, nonspecific bands were not visualized. The same results were seen when DNA from the reference E. coli strains was mixed and used in the multiplex PCR assays (Fig. 1 and 2).
To demonstrate the diagnostic usefulness of both multiplex PCR
assays, we examined bacterial colonies isolated from stool specimens
obtained from 150 children less than 5 years old who had been
assisted in the emergency room of Hospital São Paulo,
which provides public medical assistance to children of low
socioeconomic status in the city of São Paulo, Brazil.
Every fecal specimen was examined by standard methods for the
presence of
Shigella spp.,
Salmonella spp.,
Giardia lamblia,
Yersinia enterocolitica,
Campylobacter spp.,
Cryptosporidium spp., and rotavirus. Four separate lactose-fermenting colonies
and two non-lactose-fermenting colonies from each patient were
cultivated in commercial test systems (PROBAC do Brasil, São
Paulo) for biochemical confirmation of species or genus. All
isolates of
E. coli and
Shigella spp. were screened by colony
hybridization with EPEC adherence factor (
8), the
E. coli attaching
and effacing gene encoding intimin (
eae) (
6), diffuse adherence
factors (
daaC and AIDA-I) (
2,
3), EAEC adherence factor (
1),
enterotoxins LT and ST (
7), the enteroinvasiveness factor (Inv)
(
17), and Shiga toxin probes (
10). These probes were labeled
with [

-
32P]dCTP with a random primer extension kit (Rediprime
DNA labeling system; Amersham). Colony hybridization assays
were performed as previously described elsewhere (
13). The identified
strains of
E. coli and
Shigella spp. were subjected to the multiplex
PCR assays, and the results were compared with those obtained
by DNA probe hybridization.
A total of 267 E. coli and 17 Shigella species strains isolated from 150 patients were subjected to both multiplex PCRs and DNA hybridization assays (Table 2). Thirty-six (24%) of the 150 patients had a potentially diarrheagenic strain of E. coli or Shigella spp. in their stool samples. Atypical and typical EPEC strains were isolated from nine children each. Seven children were infected with an EAEC strain, three children were infected with an EIEC strain, and one child was infected with an O26 STEC strain. Shigella spp. were isolated from three children. One child (patient 66) was infected with atypical and typical EPEC strains, one child (patient 118) was infected with atypical EPEC and EAEC strains, and two children were infected with EAEC and Shigella species (patients 55 and 131) strains. For patient 127, an STEC strain of serogroup O26 was detected, which was positive for the eae and stx1 genes. None of the other 114 patients with diarrhea had E. coli strains containing the target genes in their stools. There was agreement between results of the PCR multiplex and DNA hybridization assays for almost all strains. For patients 4 and 99, one more E. coli strain positive for the eae gene was detected by PCR assays, and, for patient 150, two more E. coli strains positive for the eae gene were detected by PCR assays. For patients 103 and 117, one more gene-positive strain was detected with the DNA probe.
To assess the sensitivity of the multiplex PCR assays, overnight
cultures of prototype strains were suspended in phosphate-buffered
saline at a MacFarland standard of 1, which is equivalent to
3
x 10
8 CFU of
E. coli, and were serially diluted 10-fold. DNA
was extracted from these samples and subjected to both multiplex
PCRs. The sensitivity of detection was 10
3 CFU per assay for
all target genes. The PCR assays were then used to detect diarrheagenic
E. coli strains directly from fecal samples spiked with different
concentrations of prototype strains. The prototype strains,
suspended in phosphate-buffered saline at different concentrations,
were used to spike 180 mg of stool specimen. The DNA was isolated
from the spiked stool specimens by use of the QIAamp stool mini
kit (QIAGEN) and subjected to both multiplex PCRs. The method
accurately detected the presence of all gene products in 180
mg of feces seeded with 3
x 10
8 CFU of a single strain and was
unable to detect prototype strains at concentrations less than
10
8 CFU. Also, the presence of several
E. coli strains together
(each of them at 3
x 10
8 CFU) in a fecal sample allowed visualization
of bands specific for each strain (data not shown). The detection
limit was found to be approximately 1.7
x 10
6 CFU per g of feces.
This method was then directly applied to 36 stool specimens to detect diarrheagenic E. coli and Shigella spp. These specimens were obtained from infants hospitalized in Hospital São Paulo with diarrhea or other gastrointestinal alterations. The stools were watery, and no mucus or blood was present. Most of the children were dehydrated and needed treatment with fluids and electrolytes. None of the 36 infants received antibiotics, and the duration of diarrhea was less than 1 week (median, 5 days), starting before hospitalization. Each fecal sample, obtained upon admission, was directly subjected to both multiplex PCRs and was analyzed by conventional assays, such as biochemical identification, serotyping, and DNA hybridization with specific DNA probes (Table 3). Seven (19.4%) of the 36 fecal specimens were multiplex PCR positive: 2 specimens were positive for typical EPEC isolates (O111 and O142 serogroups), 3 were positive for EAEC isolates, and 2 were positive for atypical EPEC isolates in assay 1 (Fig. 1). The same results were obtained by colony DNA hybridization with specific DNA probes. All fecal samples tested were negative by PCR assay 2.
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TABLE 3. Comparison of results with multiplex PCR, DNA probe hybridization, and conventional assays to detect diarrheagenic E. coli strains
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Both multiple PCR assays showed very high specificity when compared
to conventional methods for detecting the virulence genes. This
high specificity was demonstrated using several reference strains
as well as clinical isolates. There was complete agreement between
the results of single and multiplex PCRs for all reference strains
tested. In an epidemiological study, we compared standard methods,
including colony blot hybridization, with multiplex PCR assays
for the identification of diarrheagenic
E. coli and
Shigella spp. in the diarrhea of 150 children. There was total agreement
between the results of multiplex PCRs and DNA hybridization
for all tested isolates.
The PCR multiplex assays were also sensitive for the detection of diarrheagenic E. coli. Stool specimens spiked with 108 CFU from cultures of prototype E. coli strains generated a specific PCR product which was visible on an ethidium bromide-stained agarose gel. This corresponds to 106 bacteria per g of feces. Of course, many more strains should be tested before concluding that the PCR assays have 100% sensitivity.
Most importantly, the two multiplex PCR assays were also found to be effective for direct detection of EPEC, EAEC, ETEC, STEC, EIEC, and Shigella spp. in stool specimens from 36 patients with diarrhea. The specificities of both multiplex PCRs were evidenced by the absence of nonspecific PCR products in feces from children without any diarrheagenic E. coli or Shigella spp.
In conclusion, the two multiplex PCR assays presented in this paper correctly determined the presence of corresponding diarrheagenic E. coli and Shigella species virulence genes in all strains tested. Multiplex PCR assay 1 correctly identified 100% of EPEC and EAEC isolates directly in stool specimens. Although assay 1 cannot detect all EAEC isolates and assay 2 cannot distinguish EIEC from Shigella species or one Shigella species from another, these multiplex PCR assays offer a practical possibility for rapid identification of diarrheagenic E. coli and Shigella spp. and could be used in the routine diagnostic laboratory.

ACKNOWLEDGMENTS
We thank Beatriz E. C. Guth and Kinue Irino for the gift of
ETEC reference strains. We also thank Elaine G. Rodrigues and
Laura Quinn Leverton for critical reading of the manuscript
and helpful suggestions.
This work was supported by Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

FOOTNOTES
* Corresponding author. Mailing address: Departamento de Microbiologia, Imunologia e Parasitologia, Universidade Federal de São Paulo, Escola Paulista de Medicina, Rua Botucatu, 862, 04023-062 São Paulo, SP, Brazil. Phone: 55-11-55764537. Fax: 55-11-55716504. E-mail:
scaletsky{at}ecb.epm.br.


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Journal of Clinical Microbiology, December 2004, p. 5849-5853, Vol. 42, No. 12
0095-1137/04/$08.00+0 DOI: 10.1128/JCM.42.12.5849-5853.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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