The susceptibilities to three antimicrobials of 195 Helicobacter pylori strains isolated from Mexican patients is reported; 80% of the strains were resistant to metronidazole, 24% were resistant to clarithromycin, and 18% presented a transient resistance to amoxicillin. Resistance to two or more antimicrobials increased significantly from 1995 to 1997.

Studies examining associations between Helicobacter pylori virulence markers and disease have concentrated on adults in developed countries. This study assessed adults and children in Mexico. Ninety patients were recruited, 56 adults (37 with active peptic ulceration and 19 with no ulcers) and 34 children (all with recurrent abdominal pain and no ulcers). H. pylori was cultured from gastric biopsy specimens, and vacA alleles and cagA were typed by use of polymerase chain reaction from multiple colony sweeps. Multiple vacA types were common in single-biopsy isolates and were more frequent in adults with ulcers (95%) than in adults without ulcers (37%; P<.001) or in children (52%; P<.01). vacA s1b and cagA+ strains were more frequent in adults than in children. vacA s1 and cagA+ strains had similar frequencies in adults with and without ulcers. In conclusion, infection with multiple H. pylori strains, defined by different vacA genotypes, is common in Mexico. Such mixed infection is associated with ulcer disease. Strain populations infecting Mexican adults and children differ.

Genetic diversity of the human gastric pathogen Helicobacter pylori in an individual host has been observed; whether this diversity represents diversification of a founding strain or a mixed infection with distinct strain populations is not clear. To examine this issue, we analyzed multiple single-colony isolates from two to four separate stomach biopsies of eight adult and four pediatric patients from a high-incidence Mexican population. Eleven of the 12 patients contained isolates with identical random amplified polymorphic DNA, amplified fragment length polymorphism, and vacA allele molecular footprints, whereas a single adult patient had two distinct profiles. Comparative genomic hybridization using whole-genome microarrays (array CGH) revealed variation in 24 to 67 genes in isolates from patients with similar molecular footprints. The one patient with distinct profiles contained two strain populations differing at 113 gene loci, including the cag pathogenicity island virulence genes. The two strain populations in this single host had different spatial distributions in the stomach and exhibited very limited genetic exchange. The total genetic divergence and pairwise genetic divergence between isolates from adults and isolates from children were not statistically different. We also analyzed isolates obtained 15 and 90 days after experimental infection of humans and found no evidence of genetic divergence, indicating that transmission to a new host does not induce rapid genetic changes in the bacterial population in the human stomach. Our data suggest that humans are infected with a population of closely related strains that vary at a small number of gene loci, that this population of strains may already be present when an infection is acquired, and that even during superinfection genetic exchange among distinct strains is rare.

Helicobacter pylori virulence determinants have not previously been studied in detail in Latin Americans with H. pylori infections. We characterized the vacA(vacuolating cytotoxin gene A) and cagA(cytotoxin-associated gene A) types of more than 400 single-colony isolates from 20 patients in Mexico City. For 17 patients H. pylori strains of two or more different vacAgenotypes were isolated from gastric biopsy specimens, indicating infection with two or more strains of H. pylori. The most frequent vacA genotype was s1b/m1. vacAdiversity was more marked than that described previously, in that isolates from seven patients had untypeable vacA midregions and isolates from nine patients had type s1 signal sequence coding regions which could not be further subtyped. Previously undescribedvacA type s2/m1 strains were found in five patients. All patients were infected with cagA-positive strains, but occasionally, these coexisted with small numbers ofcagA-negative strains. In conclusion, coinfection with multiple H. pylori strains is common in Mexico, andvacA in these strains is genetically more diverse than has been described in other populations.

Vacuolating cytotoxin (vacA) alleles of Helicobacter pylori vary, particularly in their mid region (which may be type m1 or m2) and their signal peptide coding region (type s1 or s2). We investigated nucleotide diversity among vacA alleles in strains from several locales in Asia, South America, and the USA. Phylogenetic analysis of vacA mid region sequences from 18 strains validated the division into two main groups (m1 and m2) and showed further significant divisions within these groups. Informative site analysis demonstrated one example of recombination between m1 and m2 alleles, and several examples of recombination among alleles within these groups. Recombination was not sufficiently extensive to destroy phylogenetic structure entirely. Synonymous nucleotide substitution rates were markedly different between regions of vacA, suggesting different evolutionary divergence times and implying horizontal transfer of genetic elements within vacA. Non-synonymous/ synonymous rate ratios were greater between m1 and m2 sequences than among m1 sequences, consistent with m1 and m2 alleles encoding functions fitting strains for slightly different ecological niches.The Gram-negative bacterium Helicobacter pylori is centrally involved in the pathogenesis of human peptic ulceration and is a major risk factor for the development of distal gastric adenocarcinoma and gastric lymphoma [4]. It exhibits an unusually high level of genetic diversity between strains [7], which has important implications for virulence, antibiotic resistance, and vaccine development. Implications of diversity for virulence are most obvious for the vacuolating cytotoxin gene, vacA, in that the various allelic types of vacA are differently associated with cytotoxicity and with disease state [1,2]. How genetic diversity in H. pylori is established and maintained remains unclear: recombination between strains appears important, but two studies using multilocus enzyme electrophoresis have given different assessments of [7,8]. Recently, Suerbaum et al. analyzed nucleotide sequence from parts of the two flagella genes flaA and flaB and from a small part of the vacuolating cytotoxin gene, vacA [24]: they found that recombination had been so frequent that all evidence of phylogenetic descent was obscured. We have studied vacA and have developed a pathogenically relevant PCR-based typing system based on vacA nucleotide sequence heterogeneity [1,2,26]. During the development of this system, we collected multiple sequence data from the two variable regions of vacA on which our typing system was based, the 0.7-kb mid region (for which we described two types, m1 and m2) and the region encoding the second half of the signal sequence (type s1a, s1b, s1c, or s2). Analysis of our sequence data provides contrasting results to those of Suerbaum et al. in that, although we demonstrate (by different methodology) that recombination has occurred between vacA alleles, we also show that this has not entirely disrupted the underlying phylogenetic relationship between se...