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Journal of Clinical Microbiology, May 2005, p. 2220-2223, Vol. 43, No. 5
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.5.2220-2223.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Significance of Transiently Positive Enzyme-Linked Immunosorbent Assay Results in Detection of Helicobacter pylori in Stool Samples from Children
Thomas D. Haggerty,1*
Sharon Perry,1
Luz Sanchez,1
Guillermo Perez-Perez,2 and
Julie Parsonnet1,3
Division of Infectious Diseases and Geographic Medicine, Department of Medicine, Stanford University, Stanford, California,1
Division of Infectious Diseases, Departments of Medicine and Microbiology, New York University School of Medicine, New York, New York,2
Division of Epidemiology, Department of Health Research and Policy, Stanford University, Stanford, California3
Received 12 November 2004/
Returned for modification 1 January 2005/
Accepted 18 January 2005

ABSTRACT
In young children, the significance of stool samples transiently
positive for
Helicobacter pylori antigen is unknown. As part
of a larger prospective study on enteric infections, stool samples
were obtained from 323 children at two time points 3 months
apart and tested for
H. pylori antigen using a commercially
available enzyme-linked immunosorbent assay (ELISA) test. Seminested
PCR for a
Helicobacter-specific 16S rRNA gene was performed
on all 26 pairs reverting from positive to negative (transient
positives), all 4 persistent antigen-positive pairs, and 10
randomly selected persistent antigen-negative pairs.
Helicobacter species were amplified from the first stool samples of 15/26
(58%) of the transient positives and 1 (25%) of 4 persistent
positives. No
Helicobacter species were amplified from the 10
persistent negatives. Among the 15 amplicons from transient-positive
stool,
H. pylori was sequenced and identified from 12 (80%;
95% confidence interval, 52% to 96%) and other
Helicobacter spp. were identified from three (
Helicobacter canis,
Helicobacter winghamensis, and MIT 99-5504). Four of the 15 remained positive
by PCR for the second (antigen-negative) stool sample, including
all 3 initially identified as non-
H. pylori. Helicobacter bilis was amplified from the second sample of a persistent positive.
Two of eight transient positives from whom serum was available
had accompanying transient elevations in anti-
H. pylori antibodies.
Transiently positive stool ELISAs for
H. pylori are common and
represent
H. pylori in the majority of cases where sequences
can be obtained. A not-insignificant percentage of antigen-positive
stools, however, may represent other
Helicobacter species.

INTRODUCTION
Helicobacter pylori chronically colonizes the human stomach.
Little is know about the transmission of
H. pylori, although
most cases appear to be acquired in childhood. Because the date
of acquisition of
H. pylori is rarely known, most clinical and
epidemiological studies have focused on chronic infection. Some
data do indicate, however, that transient
H. pylori infection
can occur. For example, among patients who have been infected
experimentally (
10,
14) or endoscopically (
12), the majority
with reported follow-up had spontaneous resolution of infection.
In children, transiently positive breath, serum, and stool antigen
tests for
H. pylori infections have been reported (
17,
21,
24).
The validity of these findings, however, has been uncertain
due to the lack of standardization of these diagnostic tests
in young children.
In a cohort study designed to identify risk factors for H. pylori transmission, we collected two sequential stool samples from 323 children. Among these children, 26 reversions (i.e., transient stool positives) were discovered. We conducted a study to determine whether these transiently positive stool samples represent true transient infections or false-positive stool antigen tests.

MATERIALS AND METHODS
Study participants.
As part of a large cohort study on transmission of enteric infections
(
20), households with an index case of gastroenteritis, with
at least one additional participating member, were recruited
through cooperating community health care settings as well as
community outreach. An index case was defined as a case of diarrhea
with or without vomiting characterized by at least five stools
per day and lasting no more than 14 days. There was no age restriction
for index cases. Episodes of possible noninfectious etiology,
such as pregnancy, poisoning, or drug effects, were excluded.
This source population is predominately low-income and Hispanic,
including a large percentage of foreign-born people. A brief
telephone interview confirming study eligibility was followed
by a home visit within 14 days of the index case of gastroenteritis
reported in the home. Visits were conducted by trained research
staff fluent in the primary language of the home. After providing
informed consent, a structured questionnaire was administered
regarding household demographics, socioeconomic markers, risk
factors for
H. pylori infection, household composition, and
family relationships. Blood samples were obtained from all consenting
household members. Because of concerns about the accuracy of
H. pylori serologic testing in young children, as well as resistance
to phlebotomy in this age group, stool samples were obtained
in children 2 years and younger as well as from older children
when consent was not provided for a blood draw. Approximately
12 weeks later, a second follow-up visit was conducted to collect
samples and collect logs kept by the subjects regarding household
episodes of gastroenteritis.
Stool ELISA.
Stool samples collected at home visits were transported directly back to the laboratory and stored at 20°C until processed. When samples were not available at the home visits, shipping supplies were left with the subjects and samples were sent by overnight mail. The Meridian (Cincinnati, OH) Premier Platinum HpSA enzyme-linked immunosorbent assay (ELISA) was used to detect the presence of H. pylori antigens in stool; the protocol was followed as directed. Briefly, stool samples were diluted, added to antibody-coated microwells, and incubated. H. pylori-specific polyclonal antibodies conjugated to horseradish peroxidase were added, incubated, and washed before peroxidase was added; a visible yellow reaction indicated the presence of H. pylori.
Stool pairs from children less than 18 years of age were categorized as persistently positive (both stools positive), persistently negative (both stools negative), transiently positive (first positive, second negative), or converting (first negative, second positive). Because no third sample was obtained from this last group, we cannot determine whether these represented true conversions or transiently positive stools. Thus, this group is excluded from further discussion.
DNA isolation, seminested PCR, and sequencing.
DNA was isolated from paired stool samples of all persistently and transiently positive stool pairs along with 10 randomly selected persistently negative samples using the QIAGEN (Valencia, CA) QIAmp DNA stool mini kit. The given protocol was followed, and the final elution volume was 200 µl.
PCR confirmation was done with seminested Helicobacter species-specific 16S rRNA gene (4), starting with primers C97, 5'-GCT ATG ACG GGT ATC C-3' (276 to 291 forward), and C05, 5'-ACT TCA CCC CAG TCG CTG-3' (1478 to 1495 reverse). Ten microliters of the isolated DNA was added to a 100-µl reaction mixture containing 2 U of Taq polymerase, a 0.25 µM concentration of each primer, 10 µl of 10x buffer, 0.2 mM deoxynucleotide triphosphates, and water. The second reaction was identical to the first with the exceptions of the primer pair being C97 and C98, 5'-GAT TTT ACC CCT ACA CCA-3' (381 to 398 reverse), and the template being 2 µl of a 1/100 dilution of the product from the first reaction. After visualization, PCR amplicons were extracted from agarose using the QIAGEN (Valencia, CA) QIAquick gel extraction kit. The protocol was followed as directed, and elution volumes were dependent on DNA concentrations (determined by gel electrophoresis visualization) such that the final volume of 22 µl contained 80 ng. After addition of primer, sequencing was performed using the Amersham (Piscataway, NJ) ET Terminator and ABI (New York, NY) Big Dye sequencing chemistry system and an ABI 3100 sequencer.
Serum ELISA.
Blood samples were centrifuged, and serum was stored at 4°C for later analysis. An in-house serum ELISA was used to evaluate H. pylori status as previously described (16). Briefly, high-binding microwell plates were coated with antigen isolated from multiple H. pylori strains. Dilute serum samples were added to the wells in triplicate and incubated before addition of a peroxidase-bound secondary immunoglobulin G (IgG; Sigma-Aldrich, St. Louis, MO), another incubation, and finally addition of a substrate showing H. pylori status. This ELISA has sensitivity and specificity of 86% and 100%, respectively, in children (20); it has not been specifically tested for validity in children less than 2 years old.
IgM titers were determined for nine available serum pairs. To assess the serum IgM responses to H. pylori whole-cell antigen preparation, an ELISA based on an extensively characterized pool of sonicates from five U.S. strains was used (18, 19). For the IgM ELISA, sera diluted 1:20 were added in duplicate and incubated before addition of a secondary IgM (Biosource, Camarillo, CA). Positive samples were those with optical densities greater than the mean plus 2 standard deviations of persistently seronegative samples tested.

RESULTS
A total of 323 children provided stool samples at both visits
3 months apart. A total of 75% of these children were index
cases for their households. Children (56% male) ranged in age
from 10 days to 12 years (median, 1 year), with 84% under age
2. A total of 264 (86%) children had experienced an episode
of diarrhea with or without vomiting within 2 weeks of the first
home visit. The 323 children resided in 289 homes containing
a median of 6 members (range, 2 to 18). In 220 (76%) homes,
Spanish was reported as the primary language.
Of the 323 paired stool samples, 26 (8.1%) were transiently positive, 278 (86%) were persistently negative, and 4 pairs (1.2%) were persistently positive (Table 1). Conversions (negative to positive) were found in 15 (4.6%) pairs and are excluded from further consideration here. A total of 73% (22/30) of the positives (18/26 transiently positive and 4/4 persistent positive) resided with at least one other H. pylori-positive person; however, this did not differ significantly from persistent negatives (69%). A difference was noticed when comparing the number residing with at least one other person who reported symptoms consistent with infectious gastroenteritis: 20 of 30 (67%) positives (17/26 transiently positive and 3/4 persistent positive) compared to 49% of persistent negatives (P = 0.06).
Seminested PCR was conducted on all 26 transiently positive
sample pairs, all 4 persistently positive stool antigen pairs,
and on a random selection of 10 persistently negative stool
antigen pairs. Among the 26 transiently positive stools, sequenced
PCR products from the first visit sample confirmed the presence
of a
Helicobacter sp. in 15 (58%), and one sample was indeterminate
(Table
2). Twelve of the 15 16S rRNA gene sequences (80%; 95%
confidence interval, 52 to 96) were consistent with
H. pylori;
the remaining three were consistent with
Helicobacter canis,
Helicobacter winghamensis, and MIT 5504 (
2,
5,
11). In 4 of
the 15 initially antigen-positive samples, the second stool
sample was also positive for
Helicobacter sp., despite a negative
stool antigen test; three of the four second-visit amplicon-positive
samples came from subjects with non-pylori
Helicobacter sp.
sequenced from the first visit sample.
Among the four children with persistently positive stool antigen
tests, two each yielded a single amplicon at only one of the
two visits (25%). On sequencing, one child had
H. pylori in
the first stool sample only and the other child had
Helicobacter bilis in the second sample only. Thus, among 17 children with
positive sequences for
Helicobacter spp., 4 (23.5%; 95% confidence
interval, 6.8 to 49.9%) represented non-pylori species. All
10 pairs of samples from children with persistently negative
stools were negative for
Helicobacter amplicons.
Serum samples were available at both time points for 8 transient positives, 2 persistent positives, and 148 persistent negative pairs (including 5 of the 10 randomly selected for sequence analysis). Of the eight transients with serum, two (25%) also had transiently positive titers of anti-H. pylori IgG with a >2-fold reduction in titer between visits. Six of the eight (75%) were consistently seronegative without remarkable change in titer between visits. Both of the persistently positive stool antigen children were persistently seronegative without notable changes in titer, including the child with confirmed H. bilis. Of the 148 persistently negative children with serum results, 143 (97%) were seronegative at both visits and 5 had positive or borderline results at one (3 children) or both (2 children) visits.
Paired IgM results were also available for 9 (3%) of the 308 children, including 7 transients, 1 persistent positive, and 1 persistent negative stool antigen pair. Only one transient positive had a positive IgM at the first time point only; this child was positive for IgG also only at the first time point. The remaining eight children had negative IgM results at both time points.

DISCUSSION
In this study, we were able to detect
Helicobacter species in
15 (58%) of 26 antigen-positive stools from children who subsequently
had antigen-negative stool samples, including 12 (46%) confirmed
for presence of
H. pylori sp. Given the relatively low sensitivity
of stool PCR for
Helicobacter sp. using our methods (approximately
25% to 50%), we conclude that the great majority of transiently
positive stools are likely to represent transient infection
with
Helicobacter sp. (
7,
26). The lack of IgG seroconversion
or IgM seropositivity in a small subsample supports the transient
nature of these infections, although the validity of serology
in young children is not well established (
8,
15,
25).
Transient H. pylori infection may play an important role in the permanent acquisition of H. pylori. Transient H. pylori infections have previously been reported serologically (i.e., seroreversions), with breath tests, and with stool antigen testing. In our study, 26 (8%) of 323 had a transient Helicobacter infection at a relatively random date in their lives. Thus, it seems highly probable that childrenparticularly from more-crowded householdshave frequent and recurrent exposure to H. pylori that only occasionally progresses to persistent infection. In our study we have shown a trend (P = 0.15) for persistent positives and transients to reside with at least one H. pylori-infected symptomatic person. In both experimental (10) and accidental (12, 13) exposures of humans to the organism, H. pylori detection and associated inflammation were followed by spontaneous clearance within 14 days in a subset of subjects; in others the infections persisted. Epidemiological studies have also shown evidence of transient H. pylori infections in young children (17) and patients previously cured of H. pylori (9). In addition to humans, transient infections have also been reported in previously cured macaques (23). We speculate that exposure to H. pylori is common and that chronicity is established in the minority of cases. The reasons for persistence or lack thereof are unknown.
A not-insignificant proportion of Helicobacter spp. identified in the children's stools were non-pylori Helicobacter sp. Among 17 children with amplifiable DNA from stools (15 transient positives and 2 persistent positives), PCR with sequencing indicated infection with non-pylori species, including H. winghamensis, H. bilis, H. canis, and MIT 99-5504. These organisms are typically considered zoonoses (H. winghamensis and H. bilis in rodents, H. canis in dogs and cats, and MIT 99-5507 in rhesus monkeys with colitis), although rare reports of diarrhea (H. canis and H. winghamensis) or biliary disease (H. bilis) in humans have also been made (3, 4, 11). Among the children with non-pylori Helicobacter infections in our study, all four suffered from gastroenteritis in the 2 weeks prior to initial stool culture. These symptoms, however, were not appreciably different from those of other children in our study, the majority of whom were index cases of gastroenteritis. Given the subject selection criteria, the indistinct symptomatology of individual enteric pathogens, and the infrequency of each of the non-pylori species, we can draw no conclusions about these organisms' pathogenicities. It is even uncertain as to whether these organisms were responsible for the positive stool antigen tests. Although none of the stools from 10 children with persistently negative antigen tests yielded non-pylori amplicons, 3 of the children with transient positive antigen tests continued to yield Helicobacter even after their stools were no longer antigen positive. Thus, it remains possible that these Helicobacter species were coincidental, rather than causal, for initial antigen positivity. In a rodent model, however, Sjunnesson and colleagues found significant cross-reactivity in HpSA between the H. pylori and other Helicobacters, including H. bilis and H. canis (H. winghamensis and 99-5507 were not tested) (22). Thus, we believe between 6.8% and 50% of stool Helicobacters in children represent non-pylori species and that these may create false-positive antigen tests.
The proportion of confirmed H. pylori in transient-positive stools (46%) was higher than that observed in persistent-positive stools (12.5%). This would suggest that shedding of the organism diminishes with increasing chronicity of infection. We have previously shown that hypochlorhydria may enhance H. pylori shedding in stool (6). Others have shown that acute H. pylori induces hypochlorhydria in some subjects (1, 14). Thus, through its physiological effects, acute H. pylori infection may perpetuate transmission to other hosts, whether it becomes chronically established or not. Further studies will be needed to understand the discrepancies in these findings, particularly in light of limited data on stool antigen specificity in young children.
The prevalence of H. pylori is decreasing over time in the United States and other developed countries. Despite this decline, our data suggest that H. pylori is circulating at a high endemic rate among a lower-socioeconomic-status U.S. population. Of 30 children with H. pylori infection at first testing, 87% were no longer shedding the organism 3 months later. Thus, most infected hosts eliminate infection, at least some of the time. It is likely that large exposures, recurrent exposure, or exposure with the right confluence of cofactorsi.e., nutritional status, coinfections, and gastric acid levelare required to establish chronicity. Understanding the factors that determine success of H. pylori colonization will undoubtedly shed critical insights into prevention and control of this common pathogen.

ACKNOWLEDGMENTS
This study was supported by Public Health Service grant R01
AI42801-05 from the National Institute of Health.
We thank Shufang Yang for both her part in establishing the serum ELISA used in this study and for running all of the samples. Additionally, we owe a debt of gratitude to the SIFT (Stanford Infection and Family Transmission) team for their many long days and late nights collecting data and samples in the field.

FOOTNOTES
* Corresponding author. Mailing address: Division of Infectious Diseases and Geographic Medicine, Department of Medicine, Stanford University, 300 Pasteur Dr., Stanford, CA 94305-5107. Phone: (650) 725-7511. Fax: (650) 725-3584. E-mail:
tdhaggerty{at}stanford.edu.


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Journal of Clinical Microbiology, May 2005, p. 2220-2223, Vol. 43, No. 5
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.5.2220-2223.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
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