INTRODUCTION

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Helicobacter pylori infection occurs all over the world, and more than 50% of adults are infected with this pathogen (21). The prevalence of the infection in the pediatric population varies greatly depending on the socioeconomic status of the country concerned. In developed countries only a minority of children are colonized with H. pylori (11), whereas in developing countries, in general, there is a higher prevalence of colonization (5, 8), so that the majority of young adults are chronically infected (25, 28). Once established, the pathogen may reside in the gastric mucosa for years, possibly for the whole life of the host (31). In some subjects, the long-term colonization can lead to the development of severe gastroduodenal diseases such as duodenal ulcer and gastric adenocarcinoma (6, 11, 24).

Even though most H. pylori infections are clinically silent, the organism is associated with substantial morbidity and mortality (6, 8, 10, 11, 21, 24). The reason for such a clinically diverse outcome of infection remains uncertain but may include host and environmental factors (20) as well as differences in the prevalence or expression of bacterial virulence factors (3, 7). During the past decade, the products of several H. pylori genes have been used as markers for different clinical outcomes (6, 7). A cytotoxin that may damage epithelial cells by inducing the formation of vacuoles, frequently associated with duodenal ulcer or gastric adenocarcinoma etiopathogenesis (3), is encoded by vacA. Even though all H. pylori strains possess the gene, only about 40% of strains in the United States express HeLa cell vacuolating activity in vitro (33). Analysis of vacA from different strains has shown that the gene differs in its signal sequence, which could be s1a, s1b, s1c, or s2, and in its midregion sequence, which could be, at least, m1 or m2 (2, 35).

Although H. pylori is cosmopolitan, little is known about the geographic distribution of specific H. pylori strains, especially in developing countries (34). Also, data concerning the association between vacA genotypes and disease in children in both developed and developing countries are scarce, and few or no patients with a duodenal ulcer were included in the previous studies (1, 9, 16). Thus, the relationship between ulcerogenesis and specific vacA alleles could not be assessed. Furthermore, we are not aware of any study with a pediatric population that includes the subtyping of the s1 allele of the gene.

Classification of vacA mosaicism may be important for determination of whether distinct H. pylori vacA genotypes can be used to predict the clinical outcomes of H. pylori infection. It may also be important to provide an understanding of the distribution of different strains of the bacterium in the world. This study was undertaken to investigate vacA mosaicism in H. pylori strains obtained from children with and without a duodenal ulcer in Brazil.

	MATERIALS AND METHODS

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This study was approved by the Ethics Committee of the University Hospital, Federal University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil. Informed consent to participate in the study was obtained from the children and from their parents or guardians.

Study group. Sixty-five nonconsecutive H. pylori-positive children (mean age, 10.2 years; age range, 1 to 17 years) from the state of Minas Gerais in Brazil who underwent upper gastrointestinal endoscopy for investigation of gastric complaints were included in this study: a group of 24 patients (8 girls and 16 boys; mean age, 11.7 years; age range, 1 to 17 years) with an endoscopically documented duodenal ulcer and 41 patients (25 girls and 16 boys; mean age, 9.3 years; age range, 1 to 15 years) without a gastric or duodenal ulcer at endoscopy. No subject had received antimicrobial drugs, H2-receptor antagonists, acid pump inhibitors, nonsteroidal anti-inflammatory drugs, or any medication for at least 30 days before the endoscopy.

H. pylori isolation. Diagnosis of H. pylori infection was performed by culture. Two fragments from each region of the stomach were ground separately in a tissue homogenizer and were plated onto freshly prepared Belo Horizonte medium (29). The plates were incubated at 37°C in a microaerophilic atmosphere (Anaerocult C; Merck, Darmstadt, Germany) and were examined after 3 and 7 days of incubation. H. pylori was identified by macroscopic and microscopic morphologies, preformed urease test, and positive oxidase and catalase reactions (30), as well as by the presence of ureA, as described in a subsequent section. A pool of colonies obtained from each patient was transferred to a microcentrifuge tube that contained 500 µl of sterile distilled water. After centrifugation at 6,000 × g for 5 min, the supernatant was discarded and the pellet was stored at 80°C for DNA extraction.

DNA extraction. DNA isolation was performed as described previously by Fox et al. (14). In brief, a buffer solution that contained 8.0% sucrose, 50 mM Tris-HCl (pH 8.0), 50 mM EDTA, and 0.1% Triton X-100 was added to the bacterial pellets. Lysozyme (from chicken egg white; Calbiochem, La Jolla, Calif.) was added to a final concentration of 3 mg/ml, and the solution was incubated for 12 min at 37°C. Sodium dodecyl sulfate and RNase A (Sigma Chemical Co., St. Louis, Mo.) were added to final concentrations of 1.0% and 0.5 mg/ml, respectively, and the mixture was incubated for 1 h at 37°C. Pronase (Sigma) and proteinase K (Life Technologies, Gaithersburg, Md.) were added to final concentrations of 0.8 and 0.5 mg/ml, respectively, and samples were incubated overnight at 37°C. A 1:10 volume of 5% (wt/vol) cetyltrimethylammonium bromide (Sigma)-0.7 M NaCl solution was added, and the mixture was gently rocked and incubated at 56°C for 10 min. DNA was extracted in an equal volume of a phenol (Life Technologies)-chloroform mixture (1:1) and was precipitated overnight at 20°C in the presence of 0.3 M sodium acetate (Sigma) and 3:1 volumes of absolute ethanol. The DNA was then pelleted by centrifugation at 12,000 × g for 30 min and was allowed to dry in air. The pellets were suspended in sterile distilled water, and the DNA was quantified by measuring the optical density at 260 nm and used for investigation of the presence of the ureA and vacA s and m alleles.

ureA detection. The genomic DNA from the bacterial samples was amplified with synthetic oligonucleotide primers (Table 1) and by the methodology described by Clayton et al. (10). An Escherichia coli strain (human isolate) was used as a negative control, H. pylori strain ATCC 49503 was used as a positive control, and distilled water was used as internal-reaction negative control.

vacA detection. PCR amplification of the vacA signal sequences and midregions was performed by using the oligonucleotide primers (Table 1) described by Atherton et al. (2). The strains were initially characterized as type s1 or s2 and type m1 or m2. All H. pylori strains with the s1 allele were further characterized into s1a or s1b variants (2).

PCR product detection. All amplified PCR products were resolved in agarose gels, stained with ethidium bromide, and detected under a short-wavelength UV light source. Standards of 50 or 100 bp (Life Technologies) were used as molecular size markers.

Statistical methods. Statistical analysis was performed by the two-tailed ² test with Yates' correction or by Fisher's exact test. The significance was set at a P value of <0.05.

	RESULTS

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More than one H. pylori strain was detected in 10 (15.4%) of the 65 patients studied, 5 of whom had a duodenal ulcer (Table 2). None of these patients were included in the analysis of the relationship between gastric disorders and specific vacA genotypes.

Among the 55 patients infected with H. pylori strains with nonmixed vacA allelic types, the s1 signal sequence was found in strains isolated from all 19 patients with a duodenal ulcer and from 21 (58.3%) children without an ulcer. The s2 allele was found only in strains isolated from patients without an ulcer (n = 15; 41.7%). There was a strong correlation between the presence of the s1 allele and the presence of a duodenal ulcer (P = 0.003; odds ratio [OR] = noncalculable).

When s1 strains were subtyped into s1a and s1b variants, only s1b strains were mostly found; one (2.5%) strain, however, isolated from a patient without an ulcer, did not react with primers designed to detect s1a and s1b types.

In regard to the middle region, the m1 allele was found in 34 (61.8%) patients: 16 (84.2%) with an ulcer and 18 (50.0%) without an ulcer. The m2 vacA allelic type was identified in 20 (36.4%) strains: 3 (15.8%) isolated from ulcer patients and 17 (47.2%) from children without an ulcer. A significant association between the presence of the m1 allele and duodenal ulceration was observed (P = 0.028; OR = 5.33, 95% confidence interval [CI] 1.15 to 27.95). One strain was identified as an m1-m2 (m1,2) hybrid, as described below.

When the methodology recently proposed by Atherton et al. (4) was used to amplify m variants of vacA, our previous results were confirmed for all except one strain. The latter strain was previously identified as m1 and was obtained from a patient without an ulcer but was found to be an m1,2 hybrid. All samples which had previously given final PCR products for both m1 and m2 alleles were confirmed to be mixed.

Among the 55 nonmixed H. pylori isolates, vacA homologs that contained three of the possible combinations of the signal sequence and middle region types (s1-m1, s1-m2, and s2-m2) were identified. Strains with the s1-m1 genotype were found in 34 (61.8%) children: 16 (84.2%) of 19 with ulcers and 18 (50.0%) of 36 without ulcers (P = 0.028; OR = 5.33, 95% CI = 1.18 to 32.52). When the frequency of occurrence of the s1-m2 genotype was compared to those of the other genotypes, no difference was observed among duodenal ulcer patients and children without an ulcer (P = 0.33, OR = 3.19; 95% CI = 0.32 to 40.76). All strains with the s2-m2 combination (15 of 55; 27.3%) were obtained from patients without an ulcer. The s2-m1 combination was not detected.

When the relationship between vacA alleles and age was analyzed for the 36 children without an ulcer but infected with nonmixed H. pylori strains, strains of both the s1 (14.3% for patients up to 8 years of age and 85.7% for older children) and the m1 (11.1% for patients up to the age of 8 years and 88.9% for older children) allelic types were more frequently detected in children older than 8 years (P = 0.012; OR = 9.0, and 95% CI = 1.48 to 64.71 and P = 0.013, OR = 10.0, and 95% CI = 1.48 to 107.47, respectively) (Table 3).

	DISCUSSION

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Particular vacA genotypes have been considered markers of pathogenesis for individual H. pylori strains, at least in the United States and in European countries, since in vitro production of cytotoxin, in vivo epithelial damage, and development of peptic ulcer disease or gastric adenocarcinoma are all related to vacA-specific genotypes, as demonstrated for adults (3, 12, 35). However, studies of H. pylori virulence factors have been difficult to interpret because observations reported for one geographic region or ethnic group have not always been confirmed in different places (15, 22, 26, 27, 36, 37). In Brazil, where the prevalence of H. pylori infection is high, such studies are still scanty, and this paper provides the first description of vacA genotyping of H. pylori strains from a pediatric population in Brazil.

Our findings demonstrate a strong association between the presence of the s1 allele and the presence of a duodenal ulcer in children and are in agreement with those reported for adults (2, 3, 12, 34). As the most likely explanation for the high prevalence of strains with the s1 genotype in adults with peptic ulcer, it has been proposed that the elevated toxicities of s1 strains might contribute to the development of ulcerations (2, 3). Previous descriptions of the different vacuolating activities of strains with different vacA genotypes (2) demonstrate that, in vitro, s1 strains secrete larger amounts of cytotoxin than s2 strains, which are known to be less virulent (2, 3, 13). In fact, we found s2 H. pylori strains only in the gastric mucosa of children without an ulcer. Our results also demonstrate an association between the presence of the m1 allele and the presence of a duodenal ulcer in children. A similar result was also reported for adults from Japan, where the prevalence of m1 strains is high (36). According to Atherton et al. (3), m1 H. pylori strains are more frequently associated with gastric epithelial injury, and the midregion type of vacA is the most important genotypic correlate of a high level of cytotoxin activity in vitro, which could explain the association that we observed.

Conversely, an association between duodenal ulcer in children and infection with toxigenic H. pylori strains, identified by the detection of cytotoxin activity in vitro or by vacA genotyping, was not observed by others (1, 9, 19). These results may be related both to the small number of children with duodenal ulcers evaluated and to the population studied. Among investigations that used PCR to detect the presence of vacA in children (1, 9, 16), none characterized s1 strains into their different variants, s1a, s1b, and s1c. The prevalence of subtype s1b (97.5%) among H. pylori strains isolated from the children who we studied is in contrast to that described among strains obtained from adults from some Eastern and Western developed countries, where subtype s1c and s1a strains, respectively, are more frequently found (34). Type s1b strains are also more common in countries with extensive cultural and socioeconomic relationships with Brazil in past centuries, such as Portugal and Spain (34), from which H. pylori may have spread to the Brazilian population. For these reasons, it should be considered that differences in the frequency of particular H. pylori genotypes could play a role in the incidence of H. pylori-associated diseases.

Among the 65 patients who we studied, a duodenal ulcer was more frequently observed in children older than 8 years of age (P = 0.027; OR = 6.35; 95% CI = 1.23 to 61.71). s1-m1 was the predominant genotype found in ulcer patients older than 8 years of age (15 of 18; 83.3%) when the 55 children with nonmixed H. pylori infection were considered (Table 3). When the group of children without duodenal ulcer was analyzed, we demonstrated for the first time that the frequencies of occurrence of strains with the s1 allele and of midregion type m1 increase with age, with such strains being more frequently found in children older than 8 years of age. In regard to the s1 allele, Alarcon et al. (1) also observed an increase in the prevalence of the s1 allele, with strains with the s1 allele found in about 30% of infected children (ages, 3 to 20 years) and 70.0% of infected adults (ages, 21 to 60 years).

Klein et al. (18) have shown that the initial infection with H. pylori could be less established in the gastric mucosa of young children, who acquire, lose, and reacquire the infection. We hypothesize that infants may acquire a multiplicity of different H. pylori strains and that the prevalence of strains of a particular genotype, such as s1-m1, could be a consequence of bacterial factors that facilitate colonization with some kinds of strains. Further studies on the competition or interaction among H. pylori isolates are still needed to improve our understanding of this bacterium-gastric mucosa relationship.

Many studies have demonstrated that adult patients may carry more than one H. pylori strain (17, 32, 34). Infections with multiple strains have been reported in adults from the Iberian peninsula, Mexico, and Central and South America, infections with multiple strains may be associated with the high prevalence of H. pylori infection in these populations (23, 34). In Brazil, where the prevalence of H. pylori infection reaches rates of about 80% in symptomatic and asymptomatic children older than 12 years of age (8, 25), infection with more than one H. pylori strain was found in 15.4% of patients, a frequency that is lower than that reported for adults from Portugal (44.5%), Mexico (65.0%), and other countries in South America (23.8%). Certainly, the risk of infection with multiple strains is not related to age only but is related to several other host characteristics, as well as bacterial and environmental factors, such as living conditions and habits of hygiene (25).

Even though some studies have demonstrated that the s2-m1 combination, although rare, can be found (12, 23), we did not find strains of this genotype among the strains that we studied.

Although mutation could explain the finding of combinations of signal sequence and midregion alleles in vacA, frequent recombination between strains could also generate high levels of diversity in H. pylori. It could explain, for example, the finding of an m1,2 hybrid and also of a strain with an untypeable s1 allele (negative reactions for s1a and s1b). A new s1 subtype, named s1c and considered a specific recombinant of s1a and s1b, was recently reported by van Doorn et al. (35). We tested one strain that was positive for both s1a and s1b variants and the s1 untypeable strain with primers specific for s1c, but we obtained negative results. Thus, these strains could be different ones or the isolate positive for both s1a and s1b could have a mixture of alleles. Further studies are being performed to make these findings clearer.

We conclude that, different from the results reported for adults in the United States and other Western developed countries, vacA s1b variants are the most prevalent types of H. pylori strains isolated from Brazilian children. The strong association between vacA s1 strains and peptic ulcer is complemented by the equally important finding that the vacA s2 allele occurred only in strains isolated from patients without an ulcer. These observations confirm the potential importance of vacA genotyping for assessment of whether H. pylori strains with specific alleles are associated with particular diseases. Thus, vacA detection might be helpful for determination of which children are at the highest risk for severe clinical outcomes, such as duodenal ulcer or gastric carcinoma, and, eventually, to define strategies for the treatment or prevention of H. pylori infection.