Notes Occurrence of Resistance Mutation and Clonal Expansion inHelicobacter pyloriMultiple‐Strain Infection: A Potential Risk in Clarithromycin‐Based Therapy

In a previous study, a patient was shown by means of DNA fingerprinting to be infected with different Helicobacter pylori strains before and after clarithromycin treatment. The strain isolated before treatment was susceptible (ClaS), whereas the strain isolated after 3 months of treatment was resistant (ClaR). Eighty H. pylori colonies from primary isolates were analyzed by DNA fingerprinting and restriction enzyme digestion of the 23S rRNA product of polymerase chain reaction in order to distinguish between ClaS and ClaR strains. One of the 40 colonies from isolates recovered before treatment showed a DNA fingerprint similar to that of the 40 from after treatment. However, a notable difference was that this isolate was not ClaR before treatment. Two ClaS H. pylori strains were present in the patient before treatment. The underrepresented strain gained a resistance mutation in the 23S rRNA gene and underwent clonal expansion during treatment. Recrudescence of the H. pylori infection therefore was the result.