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AIM To explore the virulence and the potential pathogenicity of coccoid Helicobacter pylori (H. pylori) transformed from spiral form by exposure to antibiotic.METHODS Three strains of H. pylori, isolated from gastric biopsy specimens of confirmed peptic ulcer, were converted from spiral into coccoid from by exposure to metronidazole.Both spiral and coccoid form of H. pylori were tested for the urease activity, the adherence to Hep-2 cells and the vacuolating cytotoxicity to Hela cells, and the differences of the protein were analysed by SDS-PAGE and Western blot,The mutation of the genes including ureA, ureB,hpaA; vacA and cagA, related with virulence,was detected by means of PCR and PCR-SSCP.RESULTS In the coccoid H. pylori, the urease activity, the adherence to Hep-2 cells and the vacuolating cytotoxicity to Hela cells alldecreased. In strain F44, the rate and index of adherence reduced from 70.0% ± 5.3% to 33% ±5.1% and from 2.6 ±0.4 to 0.96 ±0.3 (P＜0.01),respectively. The invasion of coccoid H. pylori into Hep-2 cell could be seen under electronmicroscope. SDS-PAGE showed that the content of the protein with the molecular weight over Mr74 000 decreased, and the hybriditional signal in band Mr 125 000 weakened, while the band Mr 110000 and Mr63000 strengthened in coccoid H. pylori as shown in Western blot. The results of PCR were all positive, and PCR-SSCP indicated that there may exist the point mutation in gene hpaA or vacA.CONCLUSION The virulence and the proteins with molecular weight over Mr74 000 in coccoid H. pylori decrease, but no deletion exists in amplification fragments from ureA, ureB, hpaA,vacA and cagA genes, suggesting that coccoid H. pylori may have potential pathogenicity.