We previously reported that inactivation of rdxA and/or frxA converted Helicobacter pylori from metronidazole sensitive to metronidazole resistant. To examine the individual roles of rdxA and frxA in the development of metronidazole resistance in H. pylori, we examined the status of rdxA and frxA from 12 pairs of metronidazolesensitive and -resistant H. pylori isolates obtained following unsuccessful therapy containing metronidazole. Arbitrary primed fingerprinting analyses revealed that the genotypes of 11 sensitive and resistant pairs of strains were essentially identical. Amino acid sequence identities of RdxA and FrxA from the 14 metronidazolesensitive isolates ranged from 92 to 98% and 95 to 98%, respectively, compared to that of H. pylori J99 (MIC, 1 g/ml). All strains with high-level metronidazole resistance (MICs, 128 g/ml) contained premature truncation of both RdxA and FrxA caused by nonsense and/or frameshift mutations. Strains with intermediate resistance to metronidazole (MICs, 32 to 64 g/ml) contained a single premature truncation and/or altered RdxA and FrxA caused by nonsense, frameshift, and unique missense mutations. The low-level metronidazoleresistant strains (MICs, 8 g/ml) contained unique missense mutations in FrxA but no specific changes in RdxA. The results demonstrate that alterations in both the rdxA and frxA genes are required for moderate and high-level metronidazole resistance and that metronidazole resistance that develops during anti-H. pylori therapy containing metronidazole is most likely to involve a single sensitive strain infection rather than a coinfection with a metronidazole-resistant strain.Over 50% of the world population is infected with the important human pathogen Helicobacter pylori. The primary impediments to successful H. pylori treatment are lack of compliance with drug regimens and the presence of antibioticresistant H. pylori (10). Metronidazole was a critical ingredient of the first successful therapy for H. pylori and remains a major component of newer proton pump inhibitor-containing quadruple therapies (2, 12). The background prevalence of metronidazole resistance is high in the United States (20 to 45% of isolates) and is increased in those who have used the drug for other indications (23). Effective therapy for H. pylori infection requires two or more antimicrobial agents often given with antisecretory drugs to reduce gastric acidity (2, 12). Metronidazole resistance decreases the effectiveness of metronidazolecontaining anti-H. pylori therapies (2, 5).The antimicrobial action of metronidazole is dependent on reductive activation of metronidazole by the redox system of the target cell. Theoretically, any redox system possessing a reduction potential that is more negative than that of metronidazole in the cell would donate its electrons preferentially to metronidazole and lead to reductive activation (7, 17). Goodwin et al. reported that oxygen-insensitive NADPH nitroreductase (rdxA) was a putative metronidazole nitroreductase-encoding gene involved in metro...

Tetracycline is an important component of combination therapies for Helicobacter pylori eradication. Twentynine tetracycline-resistant isolates requiring MICs ranging from 4 to 16 g/ml were isolated from Korean (22 of 460) and Japanese (7 of 105) patients. Interestingly, all of the 29 tetracycline-resistant isolates exhibited crossresistance to metronidazole, and the cross-resistance was transferred to tetracycline-sensitive H. pylori strains.Helicobacter pylori infection is one of the most common infections worldwide and is etiologically associated with chronic gastritis, duodenal ulcers, gastric ulcers, gastric adenocarcinoma, and primary gastric lymphoma (3,12,13). Clinical experience has demonstrated that the eradication of H. pylori from infected patients is not easy and that the difficulty is mostly due to the lack of patient compliance with drug regimens and the development of antibiotic-resistant H. pylori (4). Tetracyclines are a family of broad-spectrum antibiotics that have been widely used for the treatment of bacterial infections since the 1950s. Extensive and widespread therapeutic use of tetracyclines in human and veterinary medicine and their use as growth promoters in animal feeds have resulted in tetracycline resistance in almost all bacterial genera, which has reduced the therapeutic usefulness of the tetracyclines (2,14,15,16). For H. pylori, two cases of tetracycline-resistant isolates have been reported (6, 9), but no further information is currently available.In order to understand the prevalence of antibiotic resistance among clinical H. pylori isolates, we have performed surveillance of antibiotic resistance using H. pylori obtained from Korean (n ϭ 460) and Japanese (n ϭ 105) patients since 1994. The patients from Japan had not received any previous H. pylori therapies (i.e., pretreatment isolates), but the patients from Korea received dual or triple anti-H. pylori therapies (i.e., posttreatment isolates). H. pylori strains were routinely culture on brain heart infusion (BHI) (Difco, Detroit, Mich.) agar plates and maintained as described previously (7). MIC measurements for metronidazole, clarithromycin, tetracycline, and amoxicillin were performed as described earlier (7). The resistance breakpoints used for metronidazole and clarithromycin were MICs of Ͼ8 (11) and Ͼ1 g/ml (8), respectively. Since the resistance breakpoints for tetracycline and amoxicillin are not established for H. pylori, we defined resistance as MICs of Ͼ2 g/ml for tetracycline and Ͼ8 g/ml for amoxicillin (9).All the tetracycline-resistant isolates (7 of 105; 6.7%) from the Japanese patients and 20 of the tetracycline-resistant isolates (22 of 460; 4.9%) from the Korean patients remained stable during stability tests (Table 1). However, two Korean resistant strains reverted to sensitive, as was previously shown for other antibiotic resistances in H. pylori (6). The proportion of metronidazole-resistant H. pylori isolates was higher for Korean patients (40.4%) than for Japanese patients (23.8%), whereas clarithromyc...

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A novel rheometer plate was used to measure radial pressure profiles during cone-and-plate and parallel-plate shearing flows of a concentrated colloidal dispersion of poly͑methyl methacryalate͒ spheres suspended in dioctyl phthalate. There is a long history of using suspensions of this type as a model rheological system. The measured pressure profile can be used to calculate N 1 and N 2 , and also provides a check on the flow field in the rheometer. At shear rates just below onset of shear thickening, our measurements show that N 1 is positive as predicted by Stokesian dynamics simulations of model Brownian hard spheres, but we are unable to determine the sign of N 2. After the onset of thickening, we find that in both flow geometries the pressure increases sharply with radial position. This is in striking contrast to the pressure profiles ordinarily observed for viscoelastic liquids ͑with the exception of certain liquid crystal polymers͒, for which the pressure decreases with radial position. Under these conditions, the apparent values of N 1 and N 2 are both negative with ͉N 2 ͉ Ͼ ͉N 1 ͉, as predicted by the Stokesian dynamics simulations. However, the flow in the cone-and-plate rheometer may not be viscometric after the onset of shear thickening.