Occurrence of resistance mutation and clonal expansion in Helicobacter pylori multiple-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 cla rithromycin treatment. The strain isolated before treatment was susceptible (ClaS), whereas the strain isolated after 3 months of treatment was resist ant (ClaR). Eighty H. pylori colonies from primary isolates were analyzed b y DNA fingerprinting and restriction enzyme digestion of the 23S rRNA produ ct of polymerase chain reaction in order to distinguish between ClaS and Cl aR strains. One of the 40 colonies from isolates recovered before treatment showed a DNA fingerprint similar to that of the 40 from after treatment. H owever, a notable difference was that this isolate was not ClaR before trea tment. Two ClaS H. pylori strains were present in the patient before treatm ent. The underrepresented strain gained a resistance mutation in the 23S rR NA gene and underwent clonal expansion during treatment. Recrudescence of t he H. pylori infection therefore was the result.